Return to Home Page

Further Reading....

Lefohn A. S. and Benedict H. M. (1982) Development of a mathematical index that describes ozone concentration, frequency, and duration. Atmospheric Environment. 16:2529-2532.

After carefully reviewing the existing mathematical approaches that have been utilized for describing pollutant exposures, the authors have developed a mathematical parameter which (a) utilizes readily available data, and (b) provides a description of concentration, frequency, and duration. The application of the parameter makes possible the utilization of key monitoring data for establishing (a) ozone trends patterns; (b) mathematical equations describing health effects; and (c) mathematical relationships describing crop yield reductions.

Note: This paper may have been the first to propose that the higher hourly average ozone concentrations should be given greater weight than the mid- and low-level values when assessing crop growth reduction. Earlier papers had discussed the possible importance of the higher concentrations for affecting injury (i.e., spots on plants) to vegetation. In collaboration with Harris Benedict, this paper lays out my initial thinking that led to the development of the mathematical formulation for the W126 cumulative exposure index.

Lefohn A. S. and Jones C. K. (1986) The characterization of ozone and sulfur dioxide air quality data for assessing possible vegetation effects. JAPCA. 36:1123-1129.

Since the 1960s, much effort has been devoted to collecting and formatting air quality data. This paper discusses (1) the availability of air quality data for assessing potential biological impacts associated with ozone and sulfur dioxide ambient exposures, (2) examples of how air quality data can be characterized for assessing vegetation effects, and (3) the limitations associated with some exposure parameters used for developing relevant vegetation dose-response yield reduction models. Data are presented showing that some ozone monitoring sites, not continuously affected by local urban sources, experience consecutive hourly ozone exposures 0.10 ppm in the late evening and early morning hours. These sites experience their maximum ozone concentrations either in the spring or summer months. Sites influenced by local rural sources experience their maximum ozone concentrations during the summer months. It is suggested that further research be performed to identify whether the sensitivity of a target organism at the time of exposure, as well as the pollutant concentration and chemical form that enters into the target organism, is as important in defining effects as air pollutant exposure.

Lefohn A. S. and Runeckles V. C. (1987) Establishing standards to protect vegetation - Ozone exposure/dose considerations. Atmospheric Environment. 21:561-568.

The establishment of appropriate standards to protect vegetation requires an understanding of the bridge between ambient air quality exposure and ultimate response. This paper discusses the ambient air quality-vegetation response system and suggests various approaches that could be used to identify an appropriate and simple ozone standard which could provide the needed degree of environmental protection. Repeated peak ozone concentrations appear to be responsible for affecting vegetation. Plants are sensitive to different hourly mean ozone distribution patterns, even though the seasonal mean may be the same. The application at all locations of a long-term ozone standard that averages hourly concentrations will not protect vegetation from repeated peaks. In establishing a secondary ozone standard, more effort should be made to develop a cumulative seasonal ozone standard that accommodates repeated exposure of vegetation to peak concentrations. Vegetation effects data derived from experiments applying ambient ozone exposures or regimes that mimic ambient conditions should be used as the primary data set to identify the hourly ozone distribution patterns that elicit adverse vegetation responses.

Note: This paper introduces the concept of the sigmoidally weighted cumulative exposure index. The actual mathematical formulation for the index is provided in the Lefohn et al. (1988) paper cited below.

Lefohn A. S., Laurence J. A. and Kohut R. J. (1988) A comparison of indices that describe the relationship between exposure to ozone and reduction in the yield of agricultural crops. Atmospheric Environment. 22:1229-1240.

The objective of this study is to compare the use of several indices of exposure in describing the relationship between O3 and reduction in agricultural crop yield. No attempt has been made to determine which exposure-response models best fit the data sets examined. Hourly mean O3 concentration data, based on 2-3 measurements per hour, were used to develop indices of exposure from soybean and winter wheat experiments conducted in open-top chambers at the Boyce Thompson Institute, Ithaca, New York NCLAN field site. The comparative efficacy of cumulative indices (i.e., number of occurrences equal to or above specific hourly mean concentrations, sum of all hourly mean concentrations equal to or above a selected level, and the weighted sum of all hourly mean concentrations) and means calculated over an experimental period to describe the relationship between exposure to O3 and reductions in the yield of agricultural crops was evaluated. None of the exposure indices consistently provided a best fit with the Weibull and linear models tested. The selection of the model appears to be important in determining the indices that best describe the relationship between exposure and response. The focus of selecting a model should be on fitting the data points as well as on adequately describing biological responses. The investigator should be careful to couple the model with data points derived from indices relevant to the length of exposure. While we have used a small number of data sets, our analysis indicates that exposure indices that weight peak concentrations differently than lower concentrations of an exposure regime can be used in the development of exposure-response functions. Because such indices may have merit from a regulatory perspective, we recommend that additional data sets be used in further analyses to explore the biological rationale for various indices of exposure and their use in exposure-response functions.

Note: This paper describes the mathematical formulation of the sigmoidally weighted cumulative exposure index, W126, and was the first paper to quantitatively show why the use of the 7-h seasonal mean was not an appropriate index to use as a standard to protect vegetation.

Lefohn A. S., Runeckles V. C., Krupa S. V. and Shadwick D. S. (1989) Important considerations for establishing a secondary ozone standard to protect vegetation. JAPCA. 39:1039-1045.

Air quality standards are established to prevent or minimize the risk of adverse effects from air pollution to human health, vegetation, and materials. In order to develop standards which provide an adequate measure of protection to vegetation, it is necessary to define, in as precise terms as possible, the relationship between ambient air quality and the potential for adverse effects on vegetation. Based on recent evidence published in the literature, as well as retrospective studies using data from the National Crop Loss Assessment Network (NCLAN), cumulative indices can be used to describe exposures of ozone for predicting agricultural crop effects. However, the mathematical form of the standard that may be proposed to protect crops does not necessarily have to be of the same form as that used in the statistical or process oriented mathematical models that relate ambient ozone exposures with vegetation effects. This paper discusses the limitations associated with applying a simple statistic that may take the place of a more biologically-meaningful exposure parameter. While the NCLAN data have been helpful in identifying indices that may be appropriate for establishing exposure-response relationships, the limitations associated with the NCLAN protocol need to be considered when attempting to apply these relationships in the establishment of a secondary national ambient air quality standard. The Weibull model derived from NCLAN experiments must demonstrate its generality and universal applicability. Furthermore, its predictive power must be tested using independent sets of field data.

Lefohn A. S., Shadwick D. S. and Mohnen V. A. (1990) The characterization of ozone concentrations at a select set of high-elevation sites in the eastern United States. Environ. Pollut. 67:147-178.

Hourly averaged data for ozone collected in 1986 and 1987 were analyzed and characterized for a select set of high-elevation sites in the eastern United States. Pressure-corrected adjustments may be necessary when comparing ozone concentrations measured at two different elevations. When unadjusted concentrations (i.e., in units of parts per million) were used, the Whiteface Mountain sites showed what appeared to be an ozone elevational gradient. A gradient was not observed for the two MCCP Shenandoah National Park sites (SH1 and SH2). When adjusted ozone values (i.e., in units of micrograms per cubic meter) were used, the elevational gradient reported for Whiteface Mountain was no longer observed. When unadjusted concentrations were used, in most cases, the high-elevation sites appeared to be receiving greater ozone exposure than the nearby, lower elevation sites. When adjusted ozone values were used, a consistent conclusion was not evident. On a regional basis for the period May through September 1987, when unadjusted concentrations were used, the high-elevation sites in the South appeared to experience higher cumulative ozone exposures than sites in the North. When adjusted ozone values were used, the geographic gradient was not strong. Assuming that target sensitivity remains nearly constant as elevation changes, adjusted concentrations should be taken into consideration when evaluating the relationship between ozone exposures at high-elevation sites and biological effects.

Lefohn A. S., Krupa S. V. and Winstanley D. (1990) Surface ozone exposure measured at clean locations around the world. Environ. Pollut. 63:189-224.

For assessing the effects of air pollution on vegetation, some researchers have used control chambers as the basis of comparison between crops and trees grown in contemporary polluted rural locations and those grown in a clean environment. There has been some concern whether the arbitrary ozone level of 0.025 ppm and below often used in charcoal-filtration chambers to simulate the natural background concentration of ozone is appropriate. Because of the many complex and man-made factors that influence ozone levels, it is difficult to determine natural background. To identify a range of ozone exposures that occur at "clean" sites, we have calculated ozone exposures observed at a number of "clean" monitoring sites located in the United States and Canada. We do not claim that these sites are totally free from human influence, but rather that the ozone concentrations observed at these "clean" sites may be appropriate to use by vegetation researchers in control chambers as pragmatic and defensible surrogates for natural background. For comparison, we have also calculated ozone exposures observed at 4 "clean" remote sites in the Northern and Southern Hemispheres and at two remote sites (Whiteface Mountain, NY and Hohenpeissenberg, FRG) that are considered to be more polluted. Exposure indices relevant for describing the relationship between ozone and vegetation effects were applied. For studying the effects of ozone on vegetation, the higher concentrations are of interest. For the four exposure indices used, the sigmoidally-weighted index appeared to best separate those sites that experienced frequent high concentration exposures from those that experienced few high concentrations. Although there was a consistent seasonal pattern for the National Oceanic and Atmospheric Administration (NOAA) Geophysical Monitoring for Climate Change (GMCC) sites indicating a Winter/Spring maximum, this was not the case for the other remote sites. Some sites in the continental United States and southern Canada experienced ozone exposures in the range between those values experienced at the South Pole and Mauna Loa NOAA GMCC sites. The 7-month average of the daily 7-h average ozone concentration at "clean" sites located in the continental United States and southern Canada ranged from 0.028 to 0.050 ppm. Our analysis indicates that seasonal 7-h average values of 0.025 ppm and below, used by some vegetation researchers as a reference point, may be too low and that estimates of crop losses and tree damage in many locations may have been too high. Our analysis indicates that a more appropriate reference point in North America might be between 0.030 and 0.045 ppm. We have observed that the subtle effects of changing distribution patterns of hourly average ozone concentrations may be obscured with the use of exposure indices such as the monthly average. Future assessments of the effects associated with ground-level ozone should involve the use of exposure indices sensitive to changes in the distribution patterns of hourly average ozone concentrations.

Lefohn A. S., Shadwick D. S., Feister U. and Mohnen V. A. (1992) Surface-level ozone: Climate change and evidence for trends. J. Air Waste Manag. Assoc. 42(2):136-144.

As a result of emissions of hydrocarbons, carbon monoxide, and nitrogen oxides from combustion processes, recent investigations indicate that the concentration of ozone in the Earth's atmosphere may be changing. Because ozone acts as a greenhouse gas, an increase in ozone concentration in the free troposphere may have climatic consequences. In the planetary boundary layer, increases in surface ozone may affect human health, the ecosystem, and the atmospheric chemical system. Using surface ozone measurements, this paper reviews the literature concerning (1) increases in baseline surface ozone concentrations from the mid-1800s to the present and (2) trends in ozone concentrations measured at the surface. The monthly average ozone concentrations measured at surface level in the last half of the nineteenth century appear to be lower than those currently measured at many rural locations in the world. The evidence is not conclusive that the surface ozone concentrations currently monitored at "clean" rural locations are approximately double those measured in the last half of the nineteenth century in Europe or North America. Although results for the past 10 to 25 years suggest that surface ozone levels in Europe may be rising, the evidence for increasing trends in surface ozone is not consistent among monitoring sites. The identification of trends is often a function of the period selected for analysis. A review of the limited number of available longer records from either Europe or North America suggests that it is difficult to detect any trends on a region-wide basis. For assessing trends in surface ozone concentrations, it is important that world-wide monitoring at remote locations be continued and expanded so that an adequate database becomes available.

Lefohn A. S. and Foley J. K. (1992) NCLAN results and their application to the standard-setting process: Protecting vegetation from surface ozone exposures. J. Air Waste Manag. Assoc. 42:1046-1052.

The current form of the standard is not appropriate for protecting vegetation from O3 exposures. As an alternative to the current form of the standard, it has been suggested in the literature that a maximum cumulative 3-month SUM06 O3 exposure index be used as the form of a secondary standard to protect agricultural crops. However, applying this index may result in inconsistent protection for vegetation. It appears that cumulative indices will have to be combined with other parameters to accurately quantify the occurrence of high hourly average concentrations. This paper describes the characterization of the hourly O3 exposures in selected National Crop Loss Assessment Network (NCLAN) experiments and discusses the application of the results to the standard-setting process. Our results indicated that, in most cases, the NCLAN experimental data we analyzed appeared to support the observation that the repeated occurrences of hourly average O3 concentrations of 0.10 ppm and higher result in adverse effects on vegetation. For the NCLAN experiments, the characterized distributions reflected the ability of the high hourly average concentrations to affect crop yield reduction. Prior to suggesting a new form of the secondary standard, it will be important to carefully characterize the specific regimes responsible for affecting vegetation and identify the important components of those regimes responsible for the effects. By applying this approach, it should be possible to limit the occurrence of inconsistent results when applying a new form of the secondary standard.

Note: This paper introduced the concept that the N100 (i.e., number of hourly average concentrations greater than or equal to 100 ppb) exposure index was required to improve the predictability of the cumulative exposure indices.

Lefohn, A.S. (ed.) (1992) Surface-level Ozone Exposures and Their Effects on Vegetation. Published by Lewis Publishers, Inc., Chelsea, MI. 366 pp.

Chapters include Introduction, Tropospheric Ozone: Formation and Fate, The Characterization of Ambient Ozone Exposures, Experimental Methodology for Studying the Effects of Ozone on Crops and Trees, Uptake of Ozone by Vegetation, Crop Responses to Ozone, Tree Responses to Ozone, and Ozone Standards and Their Relevance for Protecting Vegetation. Great book for graduate students and advanced undergraduates. The chapters are authored by leading authorities on the subject. The chapters are written for advanced undergraduates, graduate students, and others who are interested in this very important scientific field.

Note: Although the book was published in the early 1990s, I believe much of the information is still relevant for assessing the effects of ozone on vegetation.

Lefohn A. S. and Foley J. K. (1993) Establishing ozone standards to protect human health and vegetation: Exposure/dose-response considerations. J. Air Waste Manag. Assoc. 43:106-112.

For assessing the efficacy of a specific form of the National Ambient Air Quality Standard for O3, those exposure patterns that result in vegetation and human health effects must be identified. For vegetation, it has been found that the higher hourly average concentrations should be weighted more than the lower concentrations. Controlled human exposure work supports the suggestion that concentration may be more important than exposure duration and ventilation rates. It has been indicated in the literature that the current form of the federal O3 standard may not be appropriate for protecting vegetation and human health from O3 exposures. The proposed use of the cumulative index alone as a form of the standard may not provide sufficient protection to vegetation. An extended-period average index, such as a daily maximum 8-hour average concentration, may not be appropriate to protect human health because of the reduced ability to observe differences among hourly O3 concentrations exhibited within exposure regimes. For both vegetation and human health effects research, additional experimentation is required to identify differences in responses that occur when ambient-type exposure regimes are applied. Any standard promulgated to protect vegetation and human health from O3 exposures should consider combining cumulative exposure indices with other parameters so that those unique exposures that have the potential for eliciting an adverse effect can be adequately described.

Note: The paper discusses the importance of the higher hourly average concentrations for both human health and vegetation. In addition, it describes the patterns of typical ambient concentration exposure and the fact that "square-wave" exposures are not frequently observed. Most of the controlled human health laboratory experiments have applied constant concentration (i.e., square-wave) exposures.

Lefohn A. S., Foley J. K., Shadwick D. S. and Tilton B. E. (1993) Changes in diurnal patterns related to changes in ozone levels. J. Air Waste Manag. Assoc. 43:1472-1478.

Ozone is a ubiquitous air pollutant that affects both human health and vegetation. There is concern over the number of hours human populations are exposed, in nonattainment areas in the United States, to levels of O3 at which effects have been observed. As improvement in air quality is achieved, it is possible that O3 control strategies may produce distributions of 1-h O3 concentrations that result in different diurnal profiles that produce greater potential exposures to O3 at known effects levels for multiple hours of the day. These concerns have prompted new analysis of aerometric data. In this analysis, the change in the seasonally averaged diurnal pattern was investigated as changes in O3 levels occurred. For the data used in this analysis, 25 of the 36 sites that changed compliance status across years showed no statistically significant change in the shape of the average diurnal profile (averaged by O3 season). For 71% (10 out of 14) of the sites in southern California and Dallas-Fort Worth, Texas, that showed improvement in O3 levels (i.e., reductions in the number of exceedances over the years), but still remained in "nonattainment," a statistically significant change in the shape of the seasonally averaged diurnal profile occurred. Based on the results obtained in this study, the evaluation of diurnal patterns may be useful for identifying the influence of changes in emission levels versus meteorological variation on attainment status. Using data from the southern California and Dallas-Fort Worth sites, which showed improvements in O3 levels, changes were observed in the seasonally averaged diurnal profiles. On the other hand, for the sites moving between "attainment" and "nonattainment" status, such a change in shape was generally not observed and it was possible that meteorology played a more important role than changes in emission levels relative to attainment status.

Musselman R. C., McCool P. M. and Lefohn A. S. (1994) Ozone descriptors for an air quality standard to protect vegetation. J. Air Waste Manag. Assoc. 44:1383-1390.

Exposure of plants to ozone (O3) causes injury and reduced growth. Describing the form and function of the O3 exposure in relation to plant response is important in the regulatory process. Research has shown that plants show greater response to O3 as concentration is increases. The duration of the O3 exposure is also important in the ability of vegetation to maintain O3 repair mechanisms. The O3 entering the leaf is important in plant response, thus O3 fluxes aery more important than ambient concentrations. However, at this time an air quality standard useful for the regulatory process should be based on ambient O3 exposures. The selection of O3 exposure descriptors should incorporate factors pertinent to plant response. Research suggests that exposure descriptors which give greater weight to peak concentrations, and those which account for cumulative exposure, show the closest relationship to plant response. Ozone exposure summaries using concentration averages do not adequately relate land response with ambient exposures. Although the use of cumulative exposure indices may be preferable to seasonal means, it appears that the use of a single-parameter exposure index will not guarantee that the most important components of exposure have been captured. An appropriate alternative approach might use a combination of indices, such as a cumulative index and the number of hourly average concentrations above a threshold.

Lefohn A. S. and Manning W. J. (1995) Ozone exposures near wilderness areas in northern New England. Atmospheric Environment. 29:601-606.

Ozone (O3) is known to cause characteristic injury symptoms on a wide variety of plant species. In response to concern by Federal land managers, a comprehensive program was initiated in 1988 to assess the effects of O3 on vegetation in two Class I Wilderness areas in north central New Hampshire and one Class I Wilderness area in southern Vermont. To better quantify the possible risk associated with O3 exposures affecting vegetation in these Wilderness areas, hourly average O3 concentration data were characterized, using biologically based exposure indicators for a site located at Mt. Equinox, Vermont (549ma) and a site at Mt. Washington, New Hampshire (457ma). Mt. Equinox experienced more of a flat diurnal pattern than the Mt. Washington site. The higher amplitude for the Mt. Equinox diurnal patterns in comparison to the Mt. Washington site was indicative of the occurrence of higher hourly average concentrations, as well as the infrequent occurrence of hourly average concentrations below 20 pp b. The Mt. Equinox site experienced more occurrences of hourly average concentrations 80 and 100 ppb than the Mt. Washington site. Similarly, the SUM60 and W126 integrated exposure values for Mt. Equinox were greater than the values experienced at Mt. Washington. The lower elevation Mt. Washington site experienced a greater percentage of its O3 exposure during the daylight hours (0700-1859ha) than the Mt. Equinox site.

Altshuller A. P. and Lefohn A. S. (1996) Background ozone in the planetary boundary layer over the United States. J. Air Waste Manag. Assoc. 46:134-141.

Reliable estimates of background O3 in the planetary boundary layer are needed as part of the current review by the U.S. EPA of O3 health and welfare criteria and of the National Ambient Air Quality Standard for O3. Such estimates are especially necessary for comparing O3 concentrations at which vegetation effects occur to O3 concentrations reported to represent background levels. Some vegetation researchers have used the seasonal average of the daily 7-h (0900-1559h) average as the exposure parameter in exposure-response models. The 7-h (0900-1559h) seasonal mean reference point for O3 was assumed to be 0.025 ppm. Ozone aerometric data are presented from the monitoring sites in the United States which experience some of the lowest maximum hourly average concentrations, as identified in the U.S. EPA AIRS database. Criteria are enumerated and discussed for determining whether O3 concentrations at a given site can be considered to be "background" O3. Using several techniques,the current O3 background at inland sites in the United States and Canada for the daylight 7-h (0900-1559h) seasonal (April-October) average concentrations usually occurred within the range of 35 plus or minus 10 ppb. For coastal sites, the corresponding O3 concentrations are somewhat lower, occurring within the range of 25 plus or minus 10 ppb for locations in the northern hemisphere, but with most O3 concentrations at the coastal sites in the range of 30 plus or minus 5 ppb. These ranges suggest that the background O3 is somewhat dependent on a number of conditions such as the nature of upwind flow, lack of pollution sources, and terrain conditions including deposition with respect to forest or agricultural areas.

Note: The authors are discussing the range of 7-hour seasonal average concentration values and the range for hourly average concentrations values would be higher.

Lefohn A. S., Jackson W., Shadwick D. S. and Knudsen H. P. (1997) Effect of Surface Ozone Exposures on Vegetation Grown in the Southern Appalachian Mountains: Identification of Possible Areas of Concern. Atmospheric Environment. 31(11):1695-1708.

The results described in this paper are derived from an analysis, for the 8-year period 1983-1990, that combined experimental exposure-response effects data for deciduous and coniferous seedlings and/or trees with characterized O3 ambient exposure data for a local area and soil moisture to identify areas that may be at risk in the Southern Appalachian Mountains. Results from seedling and tree experiments operated in open-top chambers were used to characterize O3 exposure regimes that resulted in growth loss under controlled conditions. Available O3 monitoring data were characterized for the states of Alabama, Georgia, South Carolina, North Carolina, West Virginia, Tennessee, Kentucky, and Virginia, using the W126 biologically based cumulative exposure index. As a part of the analysis, both the occurrences of hourly average O3 concentrations 0.10 ppm and the soil moisture conditions in the geographic area were considered. Combining exposure information with moisture availability and experimental exposure-response data, the extreme northern and southern portions of the Southern Appalachian area were identified as having the greatest potential for possible vegetation effects. The study was based mostly on results from individual tree seedlings grown in chambers and pots and additional research is needed to identify what differences in effects might be observed if exposures were similar to those experienced in forests. Furthermore, we recommend future investigations to verify the location and presence of specific vegetation species and amounts and whether actual growth losses occurred in those areas of concern that have been identified in this study.

Note: This paper applied the W126 cumulative exposure index coupled with the N100 index and a soil moisture index to predict vegetation effects. Recent work
using actual field data published by Davis and Orendovici (2006) confirmed a statistically significant relationship using the combination of the W126 and N100 indices and the following parameters: plant species, Palmer Drought Severity Index, and the interaction of the W126 exposure index and the N100 index.

Lefohn A.S. (1997) Science, Uncertainty, and EPA's New Ozone Standards. Environmental Science & Technology. 31(6):280A-284A.

Although the EPA devoted considerable time and effort to reviewing and summarizing the relevant science concerning human health effects and vegetation in the peer-reviewed literature, there are still areas of uncertainty associated with the data that form the scientific basis of the recommendations for both standards. These uncertainties have ramifications for human health and vegetation, and may influence whether geographic areas reach attainment. The paper addresses the following: (1) How closely did the controlled human health experiments performed in the laboratory mimic exposures experienced in the real world? (2) Can the human health standard be attained? (3) What is the realistic range of natural background O3 concentrations that occur under ambient conditions? (4) Did the EPA overestimate its human health risk assessment by using too low a value for natural background? (5) How closely did the controlled vegetation experiments mimic exposures experienced in the real world? (6) Is the form of the proposed secondary standard adequate? (7) Is there an alternative form of the secondary standard that would be more appropriate? and (8) In which directions should future human health and vegetation research be focused?

Lefohn A. S. (1997) A New Ozone Standard in the United States. Atmospheric Environment. 31(22):3851-3852.

For surface ozone, EPA will be phasing out and replacing the 1-hour primary standard (maximum hourly average of 0.12 ppm) with a new 8-hour standard, assessed over rolling 3-year periods, designed to protect against longer exposure periods. Determination of whether violations of the new standard have occurred will be a much more complex affair than before. The 4th highest 8-hour average daily maximum concentration will be calculated for each year and averaged across an annually-rolling 3-year period, then rounded to the nearest 0.01 ppm. If this value exceeds 0.08 ppm, then it is deemed to be in violation of both the new 'primary' (protection of public health) and 'secondary' (protection of vegetation) standards. Because background ozone levels are closer to 0.06 ppm than 0.04 ppm, the 'law of diminishing returns' dictates that it will be much more difficult to achieve the legal limit of 0.08 ppm than EPA predicts. Results from our most recent analyses indicated that, while it may prove relatively 'easy' to reduce the very highest ozone concentrations, reductions at the 0.08 ppm level will continue to be much harder to achieve. The empirical evidence suggests for most sites that presently violate the 8-hour ozone standard, attainment of the new standard may prove elusive.

Note: The author is concerned, based on empirical evidence, that areas that "appear" to achieve attainment under cool, wet meteorological conditions will "fall out" of attainment during hot, dry meteorological conditions. In other words, the attainment status for many areas will not remain stable.

Oltmans S. J., Lefohn A. S., Scheel H. E., Harris J. M., Levy H. II, Galbally I. E. , Brunke E. G., Meyer C. P., Lathrop J. A., Johnson B. J., Shadwick D. S., Cuevas E., Schmidlin F.J ., Tarasick D. W., Claude H., Kerr J. B., Uchino O., and Mohnen V. (1998) Trends of Ozone in the Troposphere. Geophysical Research Letters. 25:139-142.

For many years, researchers have believed that surface ozone was increasing everywhere at a specific percent per year. Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. For example, at two stations in Europe, ozone amounts increased rapidly into the middle 1980s, but have increased less rapidly (or in some places not at all) since then.

Lefohn A. S., Shadwick D. S. and Ziman S. D. (1998) The Difficult Challenge of Attaining EPA's New Ozone Standard. Environmental Science & Technology. 32(11):276A-282A.

Using information from the EPA's air quality database, our research indicates that, for the period 1993-1995, more than 50% of the areas that would violate the new 8-hour ozone standard were influenced by mid-level hourly average concentrations (i.e., 0.06-0.09 ppm). Using data from monitoring sites that experienced statistically significant declines in ozone levels, our analysis indicates that there was less reduction of the hourly average concentrations in the mid-level than of hourly average concentrations above 0.09 ppm. Similar results were obtained when the 8-hour daily maximum values in the mid-level region were compared with the higher 8-hour values. A preliminary evaluation of the data indicates that the slowing of mid-level concentration reductions in comparison with the rate of decline of the higher values appears to be independent of both VOC and NOx reductions. Our analysis indicates that as control strategies are implemented, those violating sites that experience high daily maximum 8-hour average concentrations will realize faster declines than those violating sites that experience daily maximum 8-hour average concentrations above, but near the 8-hour 0.08 ppm standard. For most sites that violate the new 8-hour primary standard, attainment of the new 8-hour standard may be difficult, and in some cases, impractical, to achieve.

Note: The authors are concerned, based on empirical evidence, that areas that "appear" to achieve attainment under cool, wet meteorological conditions will "fall out" of attainment during hot, dry meteorological conditions. In other words, the attainment status for many areas will not remain stable.

Lefohn A. S., Husar J. D., and Husar R. B. (1999) Estimating Historical Anthropogenic Global Sulfur Emission Patterns for the Period 1850-1990. Atmospheric Environment. 33(21):3435-3444.

It is important to establish a reliable regional emission inventory of sulfur as a function of time when assessing the possible effects of global change and acid rain. The paper describes the development of a database of annual estimates of national sulfur emissions from 1850 to 1990. A common methodology was applied across all years and countries allowing for global totals to be produced by adding estimates from all countries. The emission estimates were based on net production (i.e., production plus imports minus exports), sulfur content, and sulfur retention for each country's production activities. Fine temporal resolution clearly shows emission changes associated with specific historical events (e.g., wars, depressions, etc.) on a regional, national, or global basis. The spatial pattern of emissions shows that the US, the USSR, and China were the main sulfur emitters (i.e., approximately 50% of the total) in the world in 1990. The USSR and the US appear to have stabilized their sulfur emissions over the past 20 years, and the recent increases in global sulfur emissions are linked to the rapid increases in emissions from China. Sulfur emissions have been reduced in some cases by switching from high- to low-sulfur coals. Flue gas desulfurization (FGD) has apparently made important contributions to emission reductions in only a few countries, such as Germany.

Lefohn A. S. (2000) Developing Realistic Air Pollution Exposure/Dose Criteria for Ecological Risk Assessments. In: Integrated Assessment of Ecosystem Health. K. Scow, G. Fogg, D. Hinton, M. Johnson, (eds.). Published by Lewis Publishers, CRC Press, Boca Raton, FL. pp. 307-320.

There is a need for flexible problem-solving approaches that can link ecological measurements and data with the decision-making needs of environmental managers. Increasingly, ecological risk assessment is being suggested as a way to address this wide array of ecological problems. This paper discusses the ambient exposure characterization component associated with the analysis phase of risk assessment methodology. Using surface ozone (O3) as an example, specific guidance is provided on future research directions that are needed to assist scientists and policymakers in improving the quality of data that are available for quantifying this phase of the risk analysis.

Massman W. J., Musselman R. C., and Lefohn A. S. (2000) A Conceptual Ozone Dose-Response Model to Develop a Standard to Protect Vegetation. Atmospheric Environment. 34(5):745-759.

In this paper, we use physical reasoning based on (i) plant defenses and (ii) general resistance concepts of dry deposition to derive a suggested general form of a dose-base standard. The dose-based standard is then related to the more traditional exposure-based standard.

Note: The authors stress the importance of detoxification processes and that flux-based models that ignore plant detoxification processes will overestimate plant effects.

Lefohn A. S. and D. S. Shadwick. (2000) Differences in Trending Estimates in the United States Using Several Ozone Metrics. Proceedings of the 93rd Annual Meeting of the Air & Waste Management Association, Salt Lake City, Utah. Air & Waste Management Association, Pittsburgh, PA.

For assessing changes over time for ozone exposure in the United States, we compared the 1-hour and two 8-hour metrics (2nd highest and 4th highest daily maximum values averaged over 3 years) for two periods (1980-1997 and 1988-1997). In addition, to explore changes in the distribution frequency of hourly average concentrations, we compared the 1-hour and 8-hour metrics with the W126 exposure index, a metric that is sensitive to the distribution of mid- and high-level hourly average concentrations. When strong trending did not occur, considerable variation of agreement occurred among the metrics.

Lefohn A. S., Oltmans S. J. , Dann T. , and Singh H. B. (2001) Present-day variability of background ozone in the lower troposphere. J. Geophys. Res., 106 (D9):9945-9958.

There is a substantial background of ozone present in the lower troposphere in the Northern Hemisphere that has both a stratospheric and photochemical tropospheric origin. Levels of hourly averaged ozone concentrations in the range 0.04 - 0.08 ppm are often measured as part of the "background ozone" burden. Stratospheric processes play a significant role in defining these background ozone concentrations. In order to better understand the frequency, spatial, and temporal characteristics of this background ozone burden, we have analyzed hourly average ozone concentrations greater than or equal to 0.05 and 0.06 ppm that were experienced during the photochemically quiescent months in the winter and spring at several rural sites across southern Canada, the northern United States, and northern Europe. Our results were mostly consistent and indicated that hourly average ozone concentrations greater than or equal to 0.05 and 0.06 ppm occur frequently during the winter and spring months. Most occurrences were during April and May but sometimes as late as June. In some, but not all, of the cases that were studied, a plausible explanation for the higher ozone values was the presence of upper tropospheric and stratospheric air that was transported down to the surface.

Note: This paper describes the importance of stratospheric processes and how they play an important role during the springtime at many monitoring sites located at both high and low elevations. Emprical data have been published over the years documenting the importance of the stratosphere in affecting surface ozone levels. Although models have been exercised challenging the importance of the stratosphere in affecting surface ozone levels, empirical data collected at policy-relevant background monitoring sites in North America confirm the importance of the stratospheric processes.

Pinto J. P., Lefohn A. S. , and Shadwick D. S. (2004) Spatial variability of PM2.5 in urban areas in the United States. J. Air & Waste Management Association. 54:440-449

Epidemiologic time-series studies typically use either daily 24-hour PM concentrations averaged across several monitors in a city or data obtained at a 'central monitoring site' to relate to human health effects. If 24-hour average concentrations differ substantially across an urban area, exposure misclassification could be an important consideration when a limited number of ambient PM monitors are used to represent population-average ambient exposures. Using the U.S. Environmental Protection Agency's Aerometric Information Retrieval System (AIRS) database for 1999 and 2000, the spatial variability of PM2.5 concentrations in 27 urban areas across the United States was characterized. We observed that the PM2.5 concentrations varied to differing degrees in the urban areas examined. Even within urban areas in which all site pairs were highly correlated, a variable degree of heterogeneity in PM2.5 concentrations was found. Our findings indicate that the potential for exposure misclassification errors in time-series epidemiologic studies exists. Exposure misclassification errors resulting from the neglect of spatial variability may contribute to uncertainties in the relative risk estimates resulting from epidemiologic investigations. In future epidemiologic studies, it is important that the spatial variation in ambient PM2.5 concentrations within a study area be taken into consideration so as to reduce some sources of exposure misclassification.

Cooper O. R., Stohl A., Hübler G., Hsie E.Y., Parrish D. D., Tuck A. F., Kiladis G. N., Oltmans S. J., Johnson B. J., Shapiro M., Moody J. L., and Lefohn A. S. (2005) Direct transport of mid-latitude stratospheric ozone into the lower troposphere and marine boundary layer of the tropical Pacific Ocean. J. Geophys. Res., 110, D23310, doi:10.1029/2005JD005783.

The detailed survey of mid-latitude stratospheric intrusions penetrating into the northern hemisphere tropics was one goal of the Pacific Sub-Tropical Jet Study 2004, conducted from Honolulu, Hawaii during Jan. 19-29 and Feb. 28 - Mar. 15. Using the NOAA GIV jet aircraft, instrumented with dropsondes and a 1-second resolution ozone instrument, we targeted an intrusion above Hawaii on February 29. The data describe the strongest tropospheric ozone enhancements ever measured above Hawaii (in comparison to a 22 year ozonesonde record) and illustrate the mixing of stratospheric ozone into the mid-troposphere as a result of convection triggered by the advection of relatively cold mid-latitude air into the tropics. Measurements from the GIV and Mauna Loa Observatory (3.4 km) show enhanced ozone in the lower troposphere indicating the remnants of the intrusion reached these levels. This conclusion is supported by a study using a stratospheric ozone tracer generated by the FLEXPART Lagrangian particle dispersion model. This paper also describes a similar intrusion that enhanced ozone at Mauna Loa on March 10, as well as Honolulu, which is located in the marine boundary layer. GIV flights in and out of Honolulu measured enhanced ozone associated with this event on several occasions. The Mar. 10 event transported an estimated 1.75 Tg of ozone into the tropical troposphere and we suggest that stratospheric intrusions that break away from the polar jet stream as they advect into the tropics are more effective at transporting ozone into the troposphere than intrusions that remain close to the polar jet stream in mid-latitudes. Analysis of the dynamic conditions indicates the frequency of stratospheric intrusions was not anomalous during Jan.-Mar. 2004. While the March 10 event was by itself an extreme event, strong stratospheric intrusions can be expected to influence the tropical lower troposphere in any year.

Musselman R. C., Lefohn A. S., Massman W. J., and Heath, R. L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.

Early studies of plant response to ozone (O3) utilized concentration-based metrics, primarily by summarizing the commonly monitored hourly average datasets. Research with the O3 concentration parameter led to the recognition that both peak concentrations and cumulative effects are important when relating plant response to O3. The US and Canada currently use O3 concentration-based (exposure-based) parameters for ambient air quality standards for protecting vegetation; the European countries use exposure-based critical levels to relate O3 to vegetation response. Because plant response is thought to be more closely related to O3 absorbed into leaf tissue, recent research has been focused on flux-based O3 parameters. Even though flux-based indices may appear to be more biologically relevant than concentration-based indices, there are limitations associated with their use. The current set of flux-based indices assumes that the plant has no defense mechanism to detoxify O3. This is a serious limitation. In this paper, we review the literature on exposure- and flux-based indices for predicting plant response. Both exposure- and flux-based metrics may overestimate plant response. At this time, flux-based models that take into consideration detoxification mechanisms (referred to as effective flux) provide the best approach to relate O3 to plant response. However, because there is considerable uncertainty in quantifying the various defense mechanisms, effective flux at this time is difficult to quantify. Without adequate effective-flux based models, exposure-based O3 metrics appear to be the only practical measure for use in relating ambient air quality standards to vegetation response.

Note: This paper is a critical review of the science dealing with the development and application of exposure- and flux-based models in predicting vegetation effects. The work was derived from the authors' participation in the writing of several of the vegetation sections in the EPA's Ozone Criteria Document that was published in 2006.

Oltmans S. J., Lefohn A. S., Harris J. M., Galbally I., Scheel H. E., Bodeker G., Brunke E., Claude H., Tarasick D., Johnson B.J., Simmonds P., Shadwick D., Anlauf K., Hayden K., Schmidlin F., Fujimoto T., Akagi K., Meyer C., Nichol S., Davies J., Redondas A., and Cuevas E. (2006) Long-term changes in tropospheric ozone. Atmospheric Environment. 40:3156-3173.

Tropospheric ozone changes are investigated using a selected network of surface and ozonesonde sites to give a broad geographic picture of long-term variations. The picture of long-term tropospheric ozone changes is a varied one in terms of both the sign and magnitude of trends and in the possible causes for the changes. At mid latitudes of the S.H. three time series of 20 years in length agree in showing increases that are strongest in the austral spring (August–October). Profile measurements show this increase extending through the mid troposphere but not into the highest levels of the troposphere. In the N.H. in the Arctic a period of declining ozone in the troposphere through the 1980s into the mid-1990s has reversed and the overall change is small. The decadal-scale variations in the troposphere in this region are related in part to changes in the lowermost stratosphere. At mid latitudes in the N.H., continental Europe and Japan showed significant increases in the 1970s and 1980s. Over North America rises in the 1970s are less than those seen in Europe and Japan, suggesting significant regional differences. In all three of these mid latitude, continental regions tropospheric ozone amounts appear to have leveled off or in some cases declined in the more recent decades. Over the North Atlantic three widely separated sites show significant increases since the late-1990s that may have peaked in recent years.

Note: This paper is an important contribution to the literature because it, similar to the Oltmans et al. (1998) paper cited above, describes the long-term trends in tropospheric ozone that are occurring at many remote locations in the world. This is of particular interest to those concerned about global climate change, long-range transport, and natural and anthropogenic perturbations on meteorological processes.

Musselman R. C. and Lefohn A. S. (2007) The use of critical levels for determining plant response to ozone in Europe and in North America. Short Communication. Proceedings: Impacts of Air Pollution and Climate Change on Forest Ecosystems. TheScientificWorldJOURNAL 7(S1), 15–21. ISSN 1537-744X; DOI 10.1100/tsw.2007.24.

Critical levels to determine plant response to ozone (O3) have been used in Europe since the 1980s, utilizing the concentration-based AOT40 to relate plant response to ambient O3 exposure. More recently, there has been progress in Europe toward utilizing flux-based critical levels, because plant response is more closely related to O3 uptake than to the amount of O3 in ambient air. Flux-based critical levels are plant species specific; data for parameterization of flux-based critical levels models are lacking for most plant species. Although flux-based critical levels are now being used for a limited number of agricultural crops and tree species where data are available, the use of flux-based critical levels is limited by the lack of adequate consideration and incorporation of plant internal detoxification mechanisms in flux modeling. Critical levels have not been used in North America; however, recent interest in the US and Canada for using critical loads for nitrogen and sulfur has generated interest in using critical levels for O3. A major obstacle for utilization of critical levels in North America is that ambient air quality standards for O3 in the US and Canada are concentration-based and are not specific to individual plant species. Cumulative exposure-based metrics, particularly when implemented with a quantification of peak concentrations and environmental variables such as a drought index, are currently the most useful to relate O3 to vegetation response. Because data are unavailable to quantify detoxification potential of vegetation, effective flux models are not available to determine plant response to O3.

Hazucha M. J. and Lefohn A. S. (2007) Nonlinearity in human health response to ozone: Experimental laboratory considerations. Atmospheric Environment. 41:4559-4570.

Results from controlled laboratory exposures of human volunteers indicate that higher ozone (O3) hourly average concentrations elicit a greater effect on hour-by-hour physiologic response (i.e., forced expiratory volume in 1 s [FEV1]) than lower hourly average values, which implies a nonlinear dose-response relationship. The current 8-h average human health O3 standard is not adequate for describing this nonlinear FEV1 hour-by-hour pattern of response. Consequently, it is recommended that physiologically consistent sigmoidally shaped dose-response models based on controlled human laboratory data be integrated into the air quality standard-setting process. The sigmoidally shaped model is continuous, does not require the identification of a population threshold concentration, and deals with plateau considerations at the high end of the distribution of exposures. For developing a consistent standard to protect human health, it is important to identify those ambient-type concentration patterns that elicit adverse human health effects. Such a standard should be ultimately based not only on spirometric response but other potentially important health impairment endpoints. Because of the paucity of experimental results that utilize ambient-type concentration regimes, additional studies are needed to create a database that uses realistic ambient-type exposures (i.e., variable concentration regimes) for human laboratory studies. The ambient-type concentration patterns that elicit an adverse health effect can be subsequently integrated into a form and level of a protective standard.

Note: This paper discusses the observation that the absolute value of the high hourly average concentrations (e.g., hourly average concentrations greater than or equal to 100 ppb) affect the dynamic FEV1 responses more than the mid- or lower-level concentrations, resulting in a nonlinear relationship between dose and FEV1 response. The result of this observation is that the 8-hour average concentration is not an adequate exposure metric to use as a standard to protect human health.

Oltmans S. J., Lefohn A. S., Harris J. M., and Shadwick D. (2008) Background ozone levels of air entering the west coast of the U.S. and assessment of longer-term changes. Atmospheric Environment. 42:6020-6038.

An analysis of surface ozone measurements at a west coast site in northern California (Trinidad Head) demonstrates that this location is well situated to sample air entering the west coast of the U.S. from the Pacific Ocean. During the seasonal maximum in the spring, this location regularly observes hourly average ozone mixing ratios greater than or equal to 50 ppbv in air that is uninfluenced by the North American continent. Mean daytime values in the spring exceed 40 ppbv. A location in southern California (Channel Islands National Park) demonstrates many of the characteristics during the spring as Trinidad Head in terms of air flow patterns and ozone amounts suggesting that background levels of ozone entering southern California from the Pacific Ocean are similar to those in northern California. Two inland locations (Yreka and Lassen Volcanic National Park) in northern California with surface ozone data records of 20 years or more are more difficult to interpret because of possible influences of local or regional changes. They show differing results for the long-term trend during the spring. The 10-year ozone vertical profile measurements obtained with weekly ozonesondes at Trinidad Head show no significant longer-term change in tropospheric ozone.

Note: This paper provides a quantitative estimate based on actual data of policy-relevant background ozone concentrations for the west coast of the United States.

Lefohn A. S., Shadwick D., and Oltmans S. J. (2008). Characterizing long-term changes in surface ozone levels in the United States (1980-2005). Atmospheric Environment. 42:8252-8262.

Using statistical trending on a site-by-site basis of the (1) health-based annual 2nd highest 1-hour average concentration and annual 4th highest daily maximum 8-hour average concentration and (2) vegetation-based annual seasonally corrected 24-hour W126 cumulative exposure index, we have investigated temporal and spatial statistically significant changes that occurred in surface O3 in the United States for the periods 1980-2005 and 1990-2005 and explored whether differences in trending occur depending upon the selection of the exposure metric. Using the trending results, the analyses quantitatively explore the evidence for the higher hourly average O3 concentrations decreasing faster than the mid- and lower-values. Most of the monitoring sites analyzed in our study experienced decreasing or no trends. Few monitoring sites experienced increasing trends. For those monitoring sites with declining O3 levels, an initial pattern of rapid decrease in the higher hourly average concentrations, followed by a much slower decrease in mid-level concentrations was observed. In some cases, we observed shifts from the lower hourly average O3 concentrations to the mid-level values. On a site-by-site basis, the majority of monitoring sites (1) changed from negative trend to no trend, (2) continued a negative trend, or (3) remained in the no trend status, when comparing trends for the 1980-2005 to the 1990-2005 time periods. For all three exposure metrics, approximately 60% of the monitoring sites shifted from negative trending to no trending status. It appeared that all regions of the United States were equally affected by the shift in status. The greatest statistically significant decreases in the 2nd highest 1-hour average concentrations and the annual 4th highest daily maximum 8-hour average concentration for the two temporal periods occurred in southern California. Monitoring sites in other portions of the United States experienced lesser decreases than this geographic area. In contrast to the two exposure indices, the vegetation-based 24-hour W126 O3 cumulative index for 1980-2005 experienced significant declines in the midwestern states and the northeastern United States as well as in southern California. For the 1990-2005 period, monitoring sites in southern California and the northeastern United States experienced the greatest decreases in the W126 exposure metric. Testing for statistically significant changes in the number of hourly average concentrations within specified concentration intervals identified specific months that experienced shifts in the distribution of the hourly average concentrations. We observed that a statistically significant trend at a specific monitoring site, using one exposure index, did not necessarily result in a similar trend using the other two indices. Because different trending patterns were observed when applying the various exposure indices, a careful selection of O3 exposure metrics is required when assessing trends for specific purposes, such as human health, vegetation, and climate change effects.

Heath R. L., Lefohn A. S., and Musselman R. C. (2009). Temporal processes that contribute to nonlinearity in vegetation responses to ozone exposure and dose. Atmospheric Environment. 43:2919-2928.

Ozone interacts with plant tissue through distinct temporal processes. Sequentially, plants are exposed to ambient O3 that (1) moves through the leaf boundary layer, (2) is taken up into plant tissue primarily through stomata, and (3) undergoes chemical interaction within plant tissue, first by initiating alterations and then as part of plant detoxification and repair. In this paper, we discuss the linkage of the temporal variability of apoplastic ascorbate with the diurnal variability of defense mechanisms in plants and compare this variability with daily maximum O3 concentration and diurnal uptake and entry of O3 into the plant through stomata. We describe the quantitative evidence on temporal variability in concentration and uptake and find that the time incidence for maximum defense does not necessarily match diurnal patterns for maximum O3 concentration or maximum uptake. We suggest that the observed out-of-phase association of the diurnal patterns for the above three processes produces a nonlinear relationship that results in a greater response from the higher hourly average O3 concentrations than from the lower or mid-level values. The fact that these out-of-phase processes affect the relationship between O3 exposure/dose and vegetation effects ultimately impact the ability of flux-based indices to predict vegetation effects accurately for purposes of standard setting and critical levels. Based on the quantitative aspect of temporal variability identified in this paper, we suggest that the inclusion of a diurnal pattern for detoxification in effective flux-based models would improve the predictive characteristics of the models. While much of the current information has been obtained using high O3 exposures, future research results derived from laboratory biochemical experiments that use short but elevated O3 exposures should be combined with experimental results that use ambient-type exposures over longer periods of time. It is anticipated that improved understanding will come from future research focused on diurnal variability in plant defense mechanisms and their relationship to the diurnal variability in ambient O3 concentration and stomatal conductance. This should result in more reliable O3 exposure standards and critical levels.

Note: This paper describes how uptake of ozone into the plant, ozone exposures, and defense processes relate temporally to one another. The three processes are more than likely out of phase with one another. It is the out-of-phase relationship that explains why the higher hourly average concentrations of ozone are more important than the mid- and lower-levels. We believe that when a mathematical term for defense is added to the flux equations, the models should be able to match the observations observed in the field and the laboratory that the higher ozone concentrations should be weighted greater than the mid- and lower-level values. Many of the flux models today predict just the opposite.

Oltmans, S.J., Lefohn, A.S., Harris, J.M., Tarasick, DW., Thompson, AM., Wernli, H., Johnson, B.J., Novelli, P.C., Montzka, S.A., Ray, J.D., Patrick, L.C., Sweeney, C., Jefferson, A., Dann, T., Davies, J., Shapiro, M., Holben, B.N. (2010). Enhanced ozone over western North America from biomass burning in Eurasia during April 2008 as seen in surface and profile observations. Atmospheric Environment. 44:4497-4509.

During April 2008, as part of the International Polar Year (IPY), a number of ground based and aircraft campaigns were carried out in the North American Arctic region (e.g., ARCTAS, ARCPAC). The widespread presence during this period of biomass burning effluent, both gaseous and particulate, has been reported. Unusually high ozone readings for this time of year were recorded at surface ozone monitoring sites from northern Alaska to northern California. At Barrow, Alaska, the northernmost point in the United States, the highest April ozone readings recorded at the surface (hourly average values >55 ppbv) in 36 years of observation were measured on April 19, 2008. At Denali National Park in central Alaska, an hourly average of 79 ppbv was recorded during an 8-hr period in which the average was over 75 ppbv, exceeding the ozone ambient air quality standard threshold value in the U.S. Elevated ozone (>60 ppbv) persisted almost continuously from April 19-23 at the monitoring site during this event. At a coastal site in northern California (Trinidad Head), hourly ozone readings were >50 ppbv almost continuously for a 35-hr period from April 18-20. At several sites in northern California, located to the east of Trinidad Head, numerous occurrences of ozone readings exceeding 60 ppbv were recorded during April 2008. Ozone profiles from an extensive series of balloon soundings showed lower tropospheric features at ~1-6 km with enhanced ozone during the times of elevated ozone amounts at surface sites in western Canada and the U.S. Based on extensive trajectory calculations, biomass burning in regions of southern Russia was identified as the likely source of the observed ozone enhancements. Ancillary measurements of atmospheric constituents and optical properties (aerosol optical thickness) supported the presence of a burning plume at several locations. At two coastal sites (Trinidad Head and Vancouver Island), profiles of a large suite of gases were measured from airborne flask samples taken during probable encounters with burning plumes. These profiles aided in characterizing the vertical thickness of the plumes, as well as confirming that the plumes reaching the west coast of North America were associated with biomass burning events.

Note: This paper describes the importance of Eurasian biomass burning on affecting the surface ozone concentrations at monitoring sites along the western US and Canada, as well as Montana, Wyoming, and North Dakota. Enhanced ozone concentrations were observed during the period when Eurasian biomass burning products were transferred to North America. This paper, our second dealing with policy-relevant background (PRB), provides a quantitative estimate, based on actual data, of policy-relevant background ozone concentrations for the western United States when PRB conditions are prevalent.

Lefohn, A.S., Hazucha, M.J., Shadwick, D., Adams, W.C. (2010). An Alternative Form and Level of the Human Health Ozone Standard. Inhalation Toxicology. 22:999-1011.

Controlled human laboratory studies have shown that there is a disproportionately greater pulmonary function response from higher hourly average ozone (O3) concentrations than from lower hourly average values and thus, a nonlinear relationship exists between O3 dose and pulmonary function (FEV1) response. The nonlinear dose-response relationship affects the efficacy of the current 8-h O3 standard to describe adequately the observed spirometric response to typical diurnal O3 exposure patterns. We have reanalyzed data from five controlled human response to O3 health laboratory experiments as reported by Hazucha et al. (1992), Adams (2003, 2006a, 2006b), and Schelegle et al. (2009). These investigators exposed subjects to multi-hour variable/stepwise O3 concentration profiles that mimicked typical diurnal patterns of ambient O3 concentrations. Our findings indicate a common response pattern across most of the studies that provides valuable information for the development of a lung function (FEV1)–based alternate form for the O3 standard. Based on our reanalysis of the realistic exposure profiles used in these experiments, we suggest that an alternative form of the human health standard, similar to the proposed secondary (i.e., vegetation) standard form, be considered. The suggested form is an adjusted 5-h cumulative concentration weighted O3 exposure index, which addresses both the delay associated with the onset of response (FEV1 decrement) and the nonlinearity of response (i.e., the greater effect of higher concentrations over the mid- and low-range values) on an hourly basis.

Note: The paper, while bringing focus to an alternative form and level of the human health standard, goes beyond this focus by detailing the common response observed across most of the studies described, which composes of induction, response, and recovery (i.e., reversal) phases. The importance of better characterizing hourly average policy-relevant background ozone concentrations is discussed in relationship to applying realistic control concentrations in the human laboratory studies. The relevance of applying enhanced unrealistic constant-concentration exposure regimes in experimental studies is discussed. The paper is an important contribution to the science of the quantification of dose-response and its effects on target organisms because it details the importance of the timing of defense processes, relevant control ambient-type exposures, and the three phases of response. While the stressant applied is ozone, the observations described in the paper may be applicable to other stressants. There is a close similarity with the observations reported in our paper for human health with those reported in Heath et al. (2009) for vegetation (please see above for the citation) - the importance of the weighting of the peak exposures and the timing of the defense mechanisms as exposures occur.


Lefohn, A.S., Shadwick, D., Oltmans, S.J. (2010). Characterizing Changes of Surface Ozone Levels in Metropolitan and Rural Areas in the United States for 1980-2008 and 1994-2008. Atmospheric Environment. 44:5199-5210.

In this analysis, we characterize urban and rural ozone (O3) trends across the US for the periods 1980-2008 (29 years) and 1994-2008 (15 years) using three exposure metrics, which summarize daily O3 concentrations to reflect different ways O3 may affect human health and vegetation. We observe that a statistically significant trend at a specific monitoring site, using one exposure metric, does not necessarily result in a similar trend using the other two metrics. The two most common trends among the monitoring sites are either a continuation of negative trending over the 29-year period or a shift from negative to no trend status, indicating a leveling off of the trending. Very few sites exhibit statistically significant increases in the exposure indices. In characterizing the statistically significant changes in the distribution of hourly average O3, we observe subtle statistically significant changes in the lower part of the distribution (i.e., below 50 ppb) that are not necessarily captured by the trending patterns associated with the three exposure metrics. Using multisite data from 12 metropolitan cities, we find that as the frequency of higher hourly average concentrations is reduced, the lower hourly average concentrations also move upward toward the mid-level values. The change in the number of the hourly average concentrations in the lower range is consistent with decreased NO scavenging. We recommend assessing possible subtle shifts in O3 concentrations by characterizing changes in the distribution of hourly average concentrations by month. Identifying statistically significant monthly changes in the mid- and low-level hourly average concentrations may provide important information for assessing changes in physical processes associated with global climate change, long-range transport, and the efficacy of models used for emission and risk reductions. Our results indicate that it is important to investigate the change in the trending pattern with time (e.g., moving 15-year trending) in order to assess how year-to-year variability may influence the trend calculation.

Note: The paper, while discussing the trends at both rural and urban monitoring sites for the three effects-related exposure metrics, explores the subtle shifts over time in the hourly average concentrations within the distribution. As NO is reduced, one would anticipate a shift from the lower part of the distribution toward the mid-range of the hourly average concentrations. Such is what is observed at some monitoring sites. Similarly, as the O3 precursors are reduced, one should see a shift from the highest hourly average concentrations toward the mid-range. Such a shift occurs. Thus, both ends of the distribution shift toward the mid-range concentrations. Oltmans et al. (1998 and 2006) and others report a "flattening" effect, where surface O3 trends have begun to disappear in the latter years. In most cases, the linear trending approach misses the "flattening" effect. To quantify the "flattening" effect, we have characterized the change in trending pattern using moving 15-year trends. This paper provides examples of the use of the moving 15-year trending.


Lefohn, A.S., Wernli, H., Shadwick, D., Limbach, S., Oltmans, S.J., Shapiro, M. (2011). The Importance of Stratospheric-Tropospheric Transport in Affecting Surface Ozone Concentrations in the Western and Northern Tier of the United States. Atmospheric Environment. 45:4845-4857.

Stratospheric-tropospheric exchange (STE) processes contribute at both high and low-elevation monitoring sites to background ozone (O3) concentrations. This study addresses the importance of stratospheric intrusions contributing to enhanced hourly average surface O3 concentrations (i.e., greater than or equal to 50 ppb) at 12 O3 monitoring stations in the western and northern tier of the US for 2006, 2007, and 2008. The Lagrangian Analysis Tool (LAGRANTO) trajectory model identified specific days when stratosphere-to-troposphere transport was optimal to elevate surface O3 levels. The coincidences between the number of days with a daily maximum hourly average O3 concentration greater than or equal to 50 ppb and stratosphere-to-troposphere transport to surface (STT-S >0) were quantified. The high-elevation site at Yellowstone National Park (NP) in Wyoming exhibited the most coincidences (i.e., more than 19 days a month) during the spring and summer for hourly average O3 concentrations greater than or equal to 50 ppb with STT-S >0 of the 12 monitoring sites. At this site, the daily maximum hourly springtime average O3 concentrations were usually in the 60-70 ppb range. The maximum daily 8-h average concentrations mostly ranged from 50 to 65 ppb. At many of the lower-elevation sites, there was a preference for O3 enhancements to be coincident with STT-S >0 during the springtime, although summertime occurrences were sometimes observed. When statistically significant coincidences occurred, the daily maximum hourly average concentrations were mostly in the 50-65 ppb range and the daily maximum 8-h average concentrations were usually in the 50-62 ppb range. For many cases, the coincidences between the enhancements and the STT-S events occurred over a continuous multiday period. Our analysis provides an important step in better understanding the variability of natural background O3 concentrations. The study has provided insight into stratospheric intrusions, with emphasis on the combined role of quasi-isentropic large-scale advection and mesoscale boundary layer turbulence for stratospheric air influencing enhanced surface O3.

McDonald-Buller, E.C., Allen, D.T., Brown, N., Jacob, D.J., Jaffe, D., Kolb, C.E., Lefohn, A.S., Oltmans, S., Parrish, D.D., Yarwood, G., Zhang, L. (2011). Establishing Policy Relevant Background (PRB) Ozone Concentrations in the United States. Environmental Science & Technology. 45(22):9484-97.

Policy Relevant Background (PRB) ozone concentrations are defined by the United States (U.S.) Environmental Protection Agency (EPA) as those concentrations that would occur in the U.S. in the absence of anthropogenic emissions in continental North America (i.e., the U.S, Canada, and Mexico). Estimates of PRB ozone have had an important role historically in the EPA's human health and welfare risk analyses used in establishing National Ambient Air Quality Standards (NAAQS). The margin of safety for the protection of public health in the ozone rulemaking process has been established from human health risks calculated based on PRB ozone estimates. Sensitivity analyses conducted by the EPA have illustrated that changing estimates of PRB ozone concentrations have a progressively greater impact on estimates of mortality risk as more stringent standards are considered. As defined by the EPA, PRB ozone is a model construct, but it is informed by measurements at relatively remote monitoring sites (RRMS). This review examines the current understanding of PRB ozone, based on both model predictions and measurements at RRMS, and provides recommendations for improving the definition and determination of PRB ozone.

Lefohn, A.S., Wernli, H., Shadwick, D., Oltmans, S.J., Shapiro, M. (2012). Quantifying the Importance of Stratospheric-Tropospheric Transport on Surface Ozone Concentrations at High- and Low-Elevation Monitoring Sites in the United States. Atmospheric Environment. 62:646-656.

In this study, we quantify the frequency of stratosphere-troposphere exchange (STE) events that result in ozone (O3) concentration enhancements (i.e., hourly average concentrations greater than or equal to 50 ppb) observed at 39 high- and low-elevation monitoring sites in the US during the years 2007-2009. We employ a refined forward trajectory-based approach to address the relationship between stratospheric intrusions and enhancements in hourly average O3 concentrations. The model is applied to high-resolution European Center for Medium-Range Weather Forecasting (ECMWF) analyses to identify specific days when the potential for stratosphere-to-troposphere transport (STT) exists to affect surface O3 levels. Our results indicate that STT down to the surface (STT-S) frequently contributes to enhanced surface O3 hourly averaged concentrations at sites across the US, with substantial year-to-year variability. The O3 concentrations associated with the STT-S events appear to be large enough to enhance the measured O3 concentrations during specific months of the year. Months with a statistically significant coincidence between enhanced O3 concentrations and STT-S occur most frequently at the high-elevation sites in the Intermountain West, as well as at the high-elevation sites in the West and East. These sites exhibit a preference for coincidences during the springtime and in some cases, the summer, fall, and late winter. Besides the high-elevation monitoring sites, low-elevation monitoring sites across the entire US experience enhanced O3 concentrations coincident with STT-S events.

Oltmans, S.J., Lefohn, A.S., Shadwick, D., Harris, J.M., Scheel, H.-E., Galbally, I., Tarasick, D.A., Johnson, B.J., Brunke, E., Claude, H., Zeng, G., Nichol, S., Schmidlin, F., Redondas, A., Cuevas, E., Nakano, T., Kawasato, T. (2013). Recent Tropospheric Ozone Changes - A Pattern Dominated by Slow or No Growth. Atmospheric Environment. 67:331-351.

Longer-term (i.e., 20-40 years) tropospheric ozone (O3) time series obtained from surface and ozonesonde observations have been analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time-series model, running 15-year trends, and changes in the distribution by month in the O3 mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (~30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10-15 years. It is uncertain if the flattening of the O3 change over Japan reflects the impact of O3 transported from continental East Asia in light of reported O3 increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and midlatitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and O3 has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on O3 precursors have likely been a factor in the leveling off or decline from earlier O3 increases.

Lefohn, A.S., Emery, C., Shadwick, D., Wernli, H., Jung, J., Oltmans, S.J., 2014. Estimates of Background Surface Ozone Concentrations in the United States Based on Model-Derived Source Apportionment. Atmospheric Environment. 84:275-288.

We analyze background surface ozone (O3) concentrations as estimated by coupled GEOS-Chem/CAMx models for 23 monitoring sites across the US at high- and low-elevation, rural and urban locations during 2006. Specifically, we consider hourly contributions from global tropospheric O3 entering North America, stratospheric O3 over North America, and natural O3 formed from continental biogenic, fire, and lightning sources, according to CAMx source apportionment calculations. Unlike historical modeled background definitions that reflect the absence of anthropogenic emissions, we define “Emissions-Influenced Background” (EIB), which includes chemical interactions with anthropogenic emissions and thus reflects “current” background levels at the sites analyzed. We further define global background O3 (GBO3) as the sum of the global tropospheric and stratospheric components and find that higher modeled GBO3 occurs during the spring at sites across the US. At many of the sites during the spring, fall, and winter months higher GBO3 is associated with more frequent stratosphere-to-troposphere transport to the surface (STT-S) events according to independent three-dimensional trajectories based on global meteorological analyses. Patterns of higher spring EIB O3 are followed by lower values during the summer, due to heightened chemical interaction with anthropogenic sources, which are then followed by rising EIB O3 during the fall and winter months. For some high-elevation western US sites, this seasonal pattern is less discernible due to relatively small anthropogenic contributions and the high EIB O3 estimated throughout the year. EIB O3 at all high-elevation sites contributes a significant proportion to total O3 throughout the year and throughout the observed total O3 frequency distribution, while EIB O3 at most urban sites contributes a major portion to total O3 during non-summer months and to the mid-range concentrations (30-50 ppb) of the frequency distribution.

Lefohn, A.S., Cooper, O.R., 2015. Introduction to the special issue on observations and source attribution of ozone in rural regions of the western United States. Atmospheric Environment, 109: 279-281.

Lefohn, A.S., Malley, C.S., Simon, H., Wells. B., Xu, X., Zhang, L., Wang, T., 2017. Responses of human health and vegetation exposure metrics to changes in ozone concentration distributions in the European Union, United States, and China. Atmospheric Environment 152: 123-145. doi:10.1016/j.atmosenv.2016.12.025.

The impacts of surface ozone (O3) on human health and vegetation have prompted O3 precursor emission reductions in the European Union (EU) and United States (US). In contrast, until recently, emissions have increased in East Asia and most strongly in China. As emissions change, the distribution of hourly O3 concentrations also changes, as do the values of exposure metrics. The distribution changes can result in the exposure metric trend patterns changing in a similar direction as trends in emissions (e.g., metrics increase as emissions increase) or, in some cases, in opposite directions. This study, using data from 481 sites (276 in the EU, 196 in the US, and 9 in China), investigates the response of 14 human health and vegetation O3 exposure metrics to changes in hourly O3 concentration distributions over time. At a majority of EU and US sites, there was a reduction in the frequency of both relatively high and low hourly average O3 concentrations. In contrast, for some sites in mainland China and Hong Kong, the middle of the distribution shifted upwards but the low end did not change and for other sites, the entire distribution shifted upwards. The responses of the 14 metrics to these changes at the EU, US, and Chinese sites were varied, and dependent on (1) the extent to which the metric was determined by relatively high, moderate, and low concentrations and (2) the relative magnitude of the shifts occurring within the O3 concentration distribution. For example, the majority of the EU and US sites experienced decreasing trends in the magnitude of those metrics associated with higher concentrations. For the sites in China, all of the metrics either increased or had no trends. In contrast, there were a greater number of sites that had no trend for those metrics determined by a combination of moderate and high O3 concentrations. A result of our analyses is that trends in mean or median concentrations did not appear to be well associated with some exposure metrics applicable for assessing human health or vegetation effects. The identification of shifting patterns in the O3 distribution and the resulting changes in O3 exposure metrics across regions with large emission increases and decreases is an important step in examining the linkage between emissions and exposure metric trends. The results provide insight into the utility of using specific exposure metrics for assessing emission control strategies.

Thank you for taking the time to learn a little bit about my world of science. Science is fun and full of the unknown. Albert Einstein was accurate when he described the joy of coming out of a dark tunnel to see some of nature's secrets. We many times travel down the wrong path in search of nature's secrets, but some are fortunate enough to find the correct path for a short moment and are privileged to get a quick glimpse of some of the beautiful scientific truths.

A complete listing of publications is available.

Home Page | News | Corporation | Maps | Publications | Table of Contents | Multimedia Center

Copyright © 1995-2017 A.S.L. & Associates. All rights reserved.
Privacy Notice