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Background Ozone

A Very Important Component in the Setting of Ozone Standards

During the 2019-2020 rulemaking activity to review the current ozone human health and vegetation standards, the EPA appeared to be mainly focused on estimating how much of current O3 levels can be attributed to sources other than U.S. anthropogenic sources on days when ambient levels exceeded the O3 national standards. While this consideration may be important, background O3 also plays an important role in influencing human health effects risk assessments. The human health risk and exposure assessments play an important role in the margin of safety determinations intended to address uncertainties associated with inconclusive scientific and technical information available at the time of the standard-setting determinations. The margin of safety is also intended to provide a reasonable degree of protection against hazards that research has not yet identified. Background O3 concentrations in the low- and mid-level parts of the distribution of ambient concentrations make up a large fraction of the total O3 levels and potentially can influence those human health risk assessments associated with the margin of safety determinations for the setting of the primary (human health) O3 NAAQS.

Background ozone in the US, which includes both natural, as well as long-range transport from sources in Asia, is a major issue. However, it is not just a consideration in the US but also a consideration worldwide when considering attaining air pollution standards. At times (1) background ozone is greatly associated with the high concentrations experienced in the US Intermountain West (Lefohn et al., 2001; Langford et al., 2009; McDonald-Buller et al., 2011; Jaffe et al., 2018; Mathur et al., 2022) that affect attainability of ozone air quality standards and at other times (2) background ozone contributes on a continuous basis to observed concentrations that influence human health and vegetation risk estimates. In both cases, background ozone influences the recommended levels for ozone standards. With the appropriate understanding of the relative importance of background ozone and how it contributes to observed ozone levels, its place in the decisionmaking process for assessing (1) human health and welfare risks, (2) attainability of ozone standards, and (3) benefits accrued from emission reductions can better be placed into perspective. In December 2015, the EPA issued a white paper to establish a common understanding and foundation for additional conversations on background ozone and to inform any further action taken by the Agency. The paper described several modeling studies that have attempted to estimate background ozone levels by assessing the remaining ozone in a model simulation in which certain emissions were removed. This basic approach, which is often referred to as “zero-out” modeling (i.e., U.S. manmade emissions are removed) or “emissions perturbation” modeling, has been used to estimate background ozone levels. Another modeling technique, referred to as “source apportionment” modeling, can also be used to estimate the sources that contribute to modeled ozone concentrations (please see Lefohn et al., 2014 and Dolwick et al., 2015 as examples). This approach estimates the contribution of certain source categories (e.g., natural sources, non-U.S. manmade sources) to modeled ozone at each model grid cell on an hourly basis. The "source apportionment" modeling approach is the one used in Lefohn et al. (2014) because the authors believed it was more appropriate to use the approach to answer the question "what percentage of the actually observed ambient ozone hourly average (or 8-h average) concentration was associated with background?" Besides calculating background 8-h average concentrations for the human health ozone standard, 1-h average background concentrations are important for determining the relative importance of background to anthropogic contributions for determining W126 exposure values for protecting vegetation.

In the mid-1800s, surface ozone was the focus of many scientific studies to prove its existence, to discover its functions in the atmosphere, and to define its role in affecting the spread of epidemics. Ozone was commonly measured using the Schoenbein ozonoscope method. Schoenbein papers were coated with iodide; the reaction with ozone formed iodine. Ozone concentration was expressed as Schoenbein numbers based on coloration of Schoenbein's test paper. Gases other than ozone influenced the test paper. Observers were cautioned to expose the paper away from possible sources of sulfuric acid. In addition, the coloration tests were affected by atmospheric humidity, air flow, other oxidants, and accidental exposure to direct sunlight.

Despite the method's limitations, starting in the mid-1800s, more than 300 stations recorded ozone exposures in countries such as Austria, Australia, Belgium, England, France, Germany, Russia, and the United States. Only a few stations observed ozone continuously for more than a few years and only data summaries exist. Based on data evaluated, some scientists have concluded that (1) the annual average daily maximum of the surface ozone partial pressure in the Great Lakes area of North America was approximately 0.019 ppm, and (2) the annual average of the European measurements between the 1850s and 1900 were mostly in the range of approximately 0.017 ppm to 0.023 ppm. The authors concluded that these values were approximately half of the mean of the daily maximum of the observations observed during the last 10-15 years in the same geographical regions.

Some scientists have stressed that the estimated ozone concentrations, using the Schoenbein method, should be regarded as approximate rather than absolute. Some have also cautioned that many uncertainties exist when attempting to relate data collected by the Schoenbein method with absolute ozone concentrations. They pointed out that because of relative humidity variation among different monitoring sites, a comparison of Schoenbein values may be invalid.

During the second half of the nineteenth century, precise methods for measuring ozone were not easily available. During this period, one of the only laboratories that made quantitative measurements of surface ozone was the Paris Municipal Observatory, located in Park Montsouris. Beginning in 1876 and continuing for 31 years, daily measurements were carried out. Ozone was related to the amount of arsenite converted to arsenate, which was measured by titration with an iodine solution. Details of the method and data were published in the monthly and annual bulletins of the Observatory. The method has a positive interference when H2O2 and NO2 are present and a negative interference when SO2 is present.

Based on a review of the data obtained using this method, it was reported that the annual maximum at Montsouris occurred in May-June and the minimum in November. It was reported that the average concentration for 31 years, starting in 1876, was approximately 0.014 ppm and showed a tendency to increase. Further, it was also reported, using the ozone data collected at Montsouris between 1876 and 1910, that the annual average ranged from 0.005 to 0.016 ppm, with the average over the entire period being 0.011 ppm.

The quality of the ozone data collected at Montsouris, as well as other locations in the late 1800s and early 1900s, is unclear. Therefore, any comparison of concentrations, inferred from measurements during this period, with current concentrations at "clean" sites should be done with great caution. While the uncertainty of the absolute concentrations is an important consideration, it is important to point out that the observation that annual maximum concentrations occurred during the springtime is an important piece of information. The springtime maximum concentrations may be associated with natural sources of stratospheric ozone that is affecting surface ozone concentrations during this period of the year (Lefohn et al., 2001; Langford et al., 2009; Lefohn et al., 2011; Lefohn et al., 2012; Lefohn et al., 2014; Škerlak et al., 2014, 2019). In addition, it is unknown to what extent the Montsouris data represent ozone concentrations in Europe or the Northern Hemisphere in the last century. It is clear that the monthly average surface ozone concentrations in the last half of the nineteenth century appear to be lower than those currently measured at many rural locations in the eastern United States and Europe. For example, the annual average concentrations estimated for Montsouris were much lower than those calculated for 1980-1987 for the South Pole and Point Barrow (Alaska). However, when reviewing the data, the evidence is not conclusive that the surface ozone concentrations measured in the last half of the nineteenth century at certain locations in either Europe or North America are approximately 50% of those currently monitored at "clean" rural locations. An article on the limitations of using the Schoenbein method to estimate absolute historic ozone concentrations was published in early 1999 in the peer-reviewed journal, Atmospheric Environment.

One alternative approach that has been used is to identify a range of ozone exposures that occur at "clean" sites in the world. Some of the percentile distributions of the hourly average concentrations for some of these clean sites can be viewed. In addition, the percentile of hourly average concentrations for sites that experience low maximum hourly average concentrations over the April - October period can be observed by clicking here.

Note that the maximum hourly average concentrations at some of the most pristine sites in the world today are higher than the low levels observed 100 years ago. Some scientists have suggested that ozone is now increasing everywhere by at least 1% per year. Does that mean that every place in the world today is affected by human-induced activities or are the numbers estimated from the old measurements not reliable? Papers by Oltmans et al. (1998, 2006, 2013), which were published in peer-reviewed journals, report that surface ozone is not increasing in the world at 1% per year. At many monitoring sites, surface ozone is not increasing at all (Lefohn et al., 2017). You can find the full citation to these papers in the Publications section of this website.

Prior to 2006, ozone measurements from remote monitoring sites were used to estimate background. EPA (1996) estimated hourly average summer background concentrations of 30-50 ppb and applied a background of 40 ppb in its risk analyses. EPA (2006) cited the work of Fiore et al. (2002, 2003), who applied the GEOS-Chem global model to estimate a mean background concentration range of 15-35 ppb. At that time, EPA (2006) defined North American background (NAB) ozone to include contributions from global anthropogenic and natural sources in the absence of North American (i.e., U.S., Canada, and Mexico) anthropogenic emissions. The level of NAB defines that concentration or range of concentrations that EPA believes would be experienced if the United States and other countries in North America were to initiate a zero emissions strategy. In other words, the NAB concentrations define the level below which ozone standards cannot be set. More recently, EPA (2013) has defined US background (USB) ozone concentrations to include anthropogenic contributions from Canada and Mexico. In 2014, Lefohn et al. (2014), as well as the EPA (2014), introduced the concept of source-apportionment based background ozone. Emissions-influenced-background (EIB) ozone, as introduced by Lefohn et al. (2014), which is almost identical to source-apportionment US Background conditions as defined by the EPA, reflects background concentrations under current emissions-influenced conditions. In urban areas, EIB is chemically decayed but it converges upward toward the higher NAB metric as anthropogenic emissions are reduced in North America. EIB ozone provides to policymakers an indication of current background levels and how much improvement might occur if anthropogenic emissions were reduced at a specific location. While some scientists argue that long-range transport from Asia dominates background ozone concentrations, other scientists believe that natural processes, such as stratospheric intrusions dominate background ozone concentrations across the U.S. This is an important area of science that requires further investigation because answers resulting from this area of science provide to policymakers how much reduction in background ozone concentrations might occur if emissions were to be reduced in Asia. If natural stratospheric intrusions are responsible for replenishing background ozone levels more than long-range transport effects on background ozone, then reductions in background ozone levels might not be as significant as some think if Asian emissions were reduced. A.S.L. & Associates continues its efforts to investigate the importance of the contribution to background ozone from natural stratospheric sources.

As a result of its subjective definition of background, the U.S. EPA has questioned the use of remote monitoring sites in the world as a reasonable way to establish limits on natural ozone exposures in today's world. Based on its definition, EPA concluded initially that NAB could only be estimated using chemical transport models (CTMs). However, scientists (e.g., McDonald-Buller et al., 2011) believed that empirical data at a monitoring site at Trinidad Head, CA allowed for the characterization of NAB.

Although acknowledging EPA's desire to use a model to estimate NAB, the EPA's Clean Air Scientific Advisory Committee (CASAC) in August 2006 concluded that there is a large degree of uncertainty associated with the estimates of NAB using the model. EPA (2007) acknowledged that the monitoring site at Trinidad Head, CA does provide information about NAB concentrations of ozone. Oltmans et al. (2008) described the ozone exposures occurring at the Trinidad Head (CA) monitoring site. It appears based on the results published by Oltmans et al. (2008) that the chemical transport model that EPA used for its NAB estimates for risk assessments for ozone was unable to account for the numerous occurrences of hourly average NAB concentrations greater than or equal to 0.05 ppm measured. The percentile distribution of the hourly average concentrations and the top 10 8-hour average daily maximum concentrations for Trinidad Head are available for review.

A.S.L. & Associates has performed research on identifying background ozone levels since 1985. We were asked to "identify natural background ozone" in the early 1980s for the U.S. EPA, as well as for the National Acid Precipitation Assessment Program (NAPAP). State of Science Report Number 7 for NAPAP summarized our results and we published our findings in the peer-review literature (please see publication list).

In July 1999, a Harvard University research group published a peer-reviewed paper (Geophysical Research Letters 26:2175-2178) that predicted that the long-range transport of ozone from Asia would increase background ozone levels in the western and eastern U.S. The papers by Oltmans et al. (1998, 2006, 2013) did not indicate that ozone was increasing at the cleanest sites in the world for previous years. In addition, using a moving 15-year trends analysis, Lefohn et al. (2010) and Oltmans et al. (2013) indicated that ozone trends at sites in the western U.S. did not appear to illustrate current increases in surface ozone levels and that in some cases, earlier trend patterns that showed increases were no longer showing such patterns. However, other researchers (e.g., please see papers cited in Cooper et al., 2012) indicate that long-range transport from Asia is enhancing ozone concentrations in the western US and these enhancements may be responsible for observed increasing trends at some western US monitoring sites, as well as possibly other locations across the US. Lin et al. (2012), using the AM3 model, estimated that western US spring and early summer background ozone is routinely elevated by stratospheric ozone with STT-S contributing more than ozone generated from Asian emissions. Similar findings were reported by Ambrose et al. (2011) for the Mount Bachelor area in Oregon. The modeling results reported in Lefohn et al. (2014) support the Lin et al. (2012) findings. For areas east of the Intermountain West, Lin et al. (2012) reported that Asian emissions had minimal impact. This is an important observation that will be addressed in future discussions concerning background ozone.

For over 30 years, we have had an on-going research effort to better understand the range and frequency of occurrence of background ozone levels that may not be affected by emission reduction strategies. In 2001, we published a peer-reviewed paper authored by the research team of Allen Lefohn, Samuel Oltmans, Tom Dann, and Hanwant Singh. Our 2001 paper was possibly one of the first to point out that the stratosphere was contributing at times to natural violations of the 8-h ozone standard. Since 2001, others (e.g., Langford et al., 2009) have also reported on the importance of the stratosphere in resulting in violations of the 8-h ozone standard. In our 2001 paper, we analyzed hourly average ozone concentrations greater than or equal to 0.05 ppm 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 ppm and 0.06 ppm occurred 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. The ozone monitoring sites investigated in the US were Denali National Park (Alaska), Yellowstone National Park (Wyoming), Glacier National Park (Montana), and Voyageurs National Park (Minnesota). In the paper, we noted that the relative contribution of the stratosphere to tropospheric ozone is important because policymakers have promulgated surface ozone standards in the United States and Canada at such levels that exceedances might occur as a result of episodic, naturally occurring events that cannot be significantly altered by implementing emission reduction strategies. Although modeling results have been published questioning our conclusions (e.g., Fiore et al., 2003) about the importance of stratospheric ozone in affecting surface-level ozone concentrations, we believe that there are limitations to the models to adequately quantify the importance of stratospheric-tropospheric exchange (STE) processes that result in enhanced ozone concentrations occurring during the spring months across the US. Lefohn et al. (2014) used adjusted GEOS-Chem model to estimate background ozone. We reported that one has to adjust background estimates from the GEOS-Chem model to provide a more realistic estimate of background ozone. Our research results (Lefohn et al., 2011, 2012, 2014) continue to support our previous conclusions (Lefohn et al., 2001) about the importance of natural stratospheric-tropospheric exchange processes in affecting surface ozone concentrations at both high- and low-elevation monitoring sites across the US.

At western high-elevation sites, the contributions of daily background to total ozone are usually greater than 70% over the entire year (Lefohn et al., 2014). Estimates for high-elevation sites by Dolwick et al. (2015) for the April-October 2007 period agree with estimates by Lefohn et al. (2014). For many of the low-elevation sites across the US, the contributions of background are 50% and higher during non-summer months. Dolwick et al. (2015) report that for low-elevation sites in the western US, background ozone consists of 40-60% for the top 10% of observed 8-hour daily maximum concentration values. In some cases, the contribution is greater than this range for the low-elevation sites.

An Internet-based slide presentation is available for purposes of previewing our original 2001 paper. Also please be sure to check out the answer to our quiz that identifies the month in which the highest 8-hour daily maximum concentration occurred for the 4 remote ozone monitoring sites. Additional information on background ozone can be found in the Air Quality Analyses section of the Table of Contents.

Lefohn et al. (2014) have characterized the percent contribution from background ozone to the total ozone observed at the Yellowstone National Park site in Wyoming, as well as 22 other locations across the US. The authors report that the contribution of background ozone at the site in Wyoming was very large (i.e., generally greater than 80-90% of the total surface ozone). The highest ozone concentrations at the site appeared to be associated with stratospheric intrusions.

There are several physical processes at work that are helping to define the distribution of naturally occurring ozone concentrations. All of these processes, plus some important chemical processes, are affecting the ability of the US and Canada to attain their 8-hour ozone standards.

At ozone monitoring sites where the maximum hourly average concentration experienced is low in the United States (i.e., relatively remote, clean monitoring sites), the 8-h daily maximum ozone concentration is near the 0.070 ppm level. These clean, rural sites are discussed in Chapter 3 of the Ozone Criteria Document. In the previous version of the Criteria Document, the distribution of the hourly average concentrations for the clean sites was presented in Tables 4-6 and 4-7. An Adobe PDF file can be downloaded to review some of these values. Note the hourly maximum concentrations at these sites are well above the EPA's defined 0.040 ppm level which was previously assumed for natural background.

Review the top 10 8-hr average daily maximum concentrations, which are derived from data measured at these clean sites. In 1999, the 8-hr daily maximum concentration at Yellowstone National Park in Wyoming was 0.078 ppm. This "episode" occurred on March 25th. This is a period when photochemically produced ozone is much less important than during the summer months. Other processes are at work, such as natural stratospheric contributions. The figure to the right summarizes the 4th highest 8-hour average daily maximum concentration averaged over 3 years for 2008-2010. A larger view of the figure is available. The 3-year averages of the fourth highest 8-hr daily maximum concentrations at these sites are much higher than the EPA's assumption of ozone background values in the range of 0.015 to 0.035 ppm. The 8-hour daily maximum values above 0.040 ppm are not rare, but are very common and in many cases represent background ozone. For many of the clean sites, more than 50% of the 8-hour daily maximum concentrations are above 0.040 ppm. The EPA estimate of background of 0.015 ppm to 0.035 ppm is too low and the simulation models used to predict background levels underestimate the importance of natural processes.

In an interesting attempt to identify background sites, the OTAG Air Quality Analysis Work group estimated background levels by selecting sites in rural areas in the corners of the OTAG region. The arithmetic mean of the daily maximum 1-hour concentrations for the "background" sites can be seen by clicking here. Note that the long-term arithmetic means of the daily maximum 1-hour concentrations are in the 30-50 ppb range. However, it is important to explore the range of values for the 8-hour daily maximum concentrations by clicking here. Note that the top 8-hour daily maximums for the OTAG "background" sites are in the 59 to 90 ppb range. This range is much higher than EPA estimates for natural background levels. If the OTAG sites are actually "background" sites and the 8-hour daily maximum concentrations, in some cases, are so close to the 8-hour ozone standard, how can the standard be attained? On the other hand, perhaps most of the "background" sites identified by OTAG are not background.

There is a substantial background of ozone present in the lower troposphere in the Northern Hemisphere that has a stratospheric origin. As indicated above, there has been considerable discussions over the past several years on the importance of stratospheric ozone in contributing to surface ozone concentrations. Models (e.g., GEOS-CHEM) have been exercised and appear to illustrate that stratospheric ozone is not important for enhancing background ozone monitoring sites except for infrequent exceptional high concentrations. However, Lin et al. (2012) indicate that the AM3 model is able to illustrate the importance of stratospheric ozone affecting surface level ozone concentrations. Hu et al. (2017) have conducted a comprehensive evaluation of the standard version of GEOS-Chem (v10-01) with ozone observations from ozonesondes, the OMI satellite instrument, and MOZAIC-IAGOS commercial aircraft for 2012-2013. The authors reported that the most pronounced model bias was at high northern latitudes in winter-spring, where the model was 10-20 ppb too low. The authors attributed the bias in the model to insufficient stratosphere-troposphere exchange (STE). Empirical evidence shows that stratospheric contributions to surface ozone is important (Lefohn et al., 2001; Cooper et al., 2005; see Lefohn et al., 2014 for additional references) at both high- and low-elevation sites. Chemical transport models, such as GEOS-CHEM, have great uncertainty associated with their predictions and are not able to successfully reproduce the temporal changes in hour-by-hour concentrations (Goldstein et al., 2004). The EPA's 2006 criteria document on ozone (EPA, 2006) and Integrated Science Assessment (EPA, 2013) summarize some of the concerns in using chemical transport models, such as the GEOS-CHEM model, to estimate ozone background levels.

In September 2009, the National Research Council released the report, Global Sources of Local Pollution. In the report, the Committee stated that modeling and analysis supported the finding that background ozone is 20-40 ppb for the United States. The NRC report noted that the discussion by Lefohn, Oltmans, Dann, and Singh (2001) that occurrences of hourly average concentrations associated with background ozone are higher than the level indicated in the NRC report and that the NRC believed that the levels reported by Lefohn et al. (2001) were associated either with high-elevation sites or with more distant North American pollution. The conclusions in the NRC report were unfortunately inaccurate. Since 2001, when we published the Lefohn et al. (2001) paper, evidence has been published in the peer-review literature indicating the importance of stratospheric ozone in enhancing observed ozone surface concentrations at both high- and low-elevation monitoring sites. As indicated above, we believe the GEOS-Chem model did not adequately handle the stratosphere and that it is possible to adjust the GEOS-Chem model to obtain a much better esstimate of background ozone (please see Lefohn et al., 2014). In addition, our research on background ozone, using empirical data, indicates that levels are higher than 20-40 ppb in the United States. One of our research papers on background ozone (Oltmans et al., 2010) discusses the importance of Eurasian biomass burning and how it influences background ozone concentrations in the US. Our research is continuing on this matter and current results published in the peer-review literature support our previous conclusions that hourly levels greater than or equal to 50 ppb occur more frequently as a result from natural sources than models suggest. The enhanced hourly average concentrations influenced by background ozone also result in enhanced 8-h average concentrations that are used in the ozone standard-setting process. At many high-elevation sites, much of this enhancement is due to naturally occurring stratospheric ozone influencing ground-level ozone. Since our 2001 paper, many more peer-reviewed research papers continue to be published that support our earlier conclusions.

It is important that the range of background hourly average concentrations is correctly characterized. The inadequate characterization of background ozone at both low- and high-elevation sites will lead to 1) inflated human health risk estimates and 2) overly optimistic policy expectations on the levels to which hourly average ozone concentrations can be lowered as a result of emission reduction requirements. Our research is continuing to investigate this important research area. Please see a list of our publications in this area.

Today background continues to be an important issue, not just for ozone attainment purposes, but also for assessing the adequacy of effects models that predict human health and vegetation effects at background levels. Background ozone cannot necessarily be reduced if natural processes, such as stratospheric transport to the surface, play a major role in influencing ambient ozone levels measured at monitors across the country. Researchers, as well as policymakers, need to be very careful in their assessments and understand and communicate the limitations of the models and the analytical methods used in estimating the range of background ozone levels. However, while there are uncertainties in estimating background ozone, one should not draw the conclusion that the models today cannot be used to assist federal/state governments and tribes in better understanding natural processes and the role that background ozone plays in the attainment process, as well as in margin of safety determinations. While it is obvious that additional resources are needed to continue to improve models, attention should be directed towards a serious effort (involving scientists, regulators, policymakers, etc.) to place into better perspective the relative contribution of background ozone to daily ozone concentrations measured routinely across the US. This information is required not just for ozone attainment purposes but also for human health and vegetation risk assessments. By studying background ozone, one can learn much about his or her environment and the natural processes that affect our daily living conditions. By better understanding nature, one learns more about how our actions affect our environment. Additional information can be found at other web pages on this site.


Ambrose, J.L., Reidmiller, D.R., Jaffe, D.A. (2011). Causes of high O3 in the lower free troposphere over the Pacific Northwest as observed at the Mt. Bachelor Observatory. Atmospheric Environment 45, 5302-5315.

Cooper, O.R.; A. Stohl; G. Hübler; E.Y. Hsie; D.D. Parrish; A.F. Tuck; G.N. Kiladis; S.J. Oltmans; B.J. Johnson; M. Shapiro; J.L. Moody; A.S. Lefohn. (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.

Cooper, O.R., Gao, R.S., Tarasick, D., Leblanc, T., Sweeney, C. (2012) Long-term ozone trends at rural ozone monitoring sites across the United States, 1990-2010. Journal of Geophysical Research 117 D22307, doi:10.1029/2012JD018261.

Dolwick, P., Akhtar, F., Baker, K., Possiel, N., Simon, H., Tonnesen, G., 2015. Comparison of background ozone estimates over the western United States based on two separate model methodologies. Atmospheric Environment 109: 282-296, doi: 10.1016/j.atmosenv.2015.01.005.

Fiore, A. M., Jacob, D.J., Bey, I., Yantosca, R.M., Field, B.D., Fusco, A.C., Wilkinson, J.G. (2002) Background ozone over the United States in summer: Origin, trend, and contribution to pollution episodes. Journal of Geophysical Research 107(D15), 4275, doi:10.1029/2001JD000982.

Fiore, A., Jacob, D.J., Liu, H., Yantosca, R.M., Fairlie, T.D., Li, Q. (2003) Variability in surface ozone background over the United States: Implications for air quality policy. Journal of Geophysical Research 108 (D24), 4787, doi:10.1029/2003JD003855.

Goldstein, A. H.; Millet, D. B.; McKay, M.; Jaegle, L.; Horowitz, L.; Cooper, O.; Hudman, R.; Jacob, D. J.; Oltmans, S.; Clarke, A. (2004) Impact of Asian emissions on observations at Trinidad Head, California, during ITCT 2K2. J. Geophys. Res. 109, D23S17, doi:10.1029/2003JD004406.

Hu, L., Jacob, D.J., Liu, X., Zhang, Y., Zhang, L., Kim, P.K., Sulprizio, M.P., Yantosca, R.M. (2017). Global budget of tropospheric ozone: Evaluating recent model advances with satellite (OMI), aircraft (IAGOS), and ozonesonde observations. Atmospheric Environment 167: 323-334,

Jaffe, D.A., Cooper, O.R., Fiore, A.M., Henderson, B.H., Tonnesen, G.S., Russell, A.G., Henze, K., Langford, A.O., Lin, M., Moore, T. (2018). Scientific assessment of background ozone over the U.S.: Implications for air quality management. Elem Sci Anth, 6: 56. DOI:

Langford, A.O., Aikin, K.C., Eubank, C.S., Williams, E.J. (2009) Stratospheric contribution to high surface ozone in Colorado during springtime. Geophys. Res. Lett.

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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.

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.

Lefohn, A.S., Wernli, H., Shadwick, D., Oltmans, S.J., Shapiro, M. (2012) Quantifying the frequency of stratospheric-tropospheric transport affecting enhanced surface ozone concentrations at high- and low-elevation monitoring sites in the United States. Atmospheric Environment 62, 646-656.

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.

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.

Lin, M., Fiore, A.M., Cooper, O.R., Horowitz, L.W., Langford, A.O., Levy II, H., Johnson, B.J., Naik, V., Oltmans, S.J., Senff, C.J. (2012). Springtime high surface ozone events over the western United States: Quantifying the role of stratospheric intrusions. Journal of Geophysical Research 117, D00V22, doi:10.1029/2012JD018151.

Mathur, R., Kang, D., Napelenok, S.L., Xing, J., Hogrefe, C., Sarwar, G., et al. (2022). How have divergent global emission trends influenced long-range transported ozone to North America? Journal of Geophysical Research: Atmospheres, 127, e2022JD036926.

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, doi:10.1021/es2022918, 9484-9497.

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.

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