A Very Important
Part of the Setting of the Ozone Standards
On December 22, 2017, the US EPA responded
to state and tribal recommendations for ozone area designations
by indicating the anticipated area designations for the remaining
portions of the US. This follows a November 6, 2017 designation
by the EPA of most of the US attainment/unclassifiable
for the 2015 ozone standards. These actions followed an August
2, 2017 Withdrawal of Extension of Deadline for Promulgating
Designations for the 2015 Ozone National Ambient Air Quality
Standards notice that was issued in June 2017 in the Federal
Register. The June notice indicated that there was insufficient
information to complete area designations for the 2015 ozone
standards. The result of this action would have extended the
deadline by one year, until October 1, 2018. For more information, please
the EPA has decided to continue the ozone area designation process,
the Agency is still concerned about the effect that background
has on attainment of the 2015 ozone standards. Specifically,
the key reasons that the EPA proposed the delay in June 2017
understanding the role of background ozone levels;
accounting for international transport; and
consideration of exceptional events demonstrations.
Background ozone (O3) in the US, which
includes both natural as well as long-range transport from sources
in Asia, is obviously a major issue. However, it is not just
an issue in the US but also an issue worldwide when considering
attaining air pollution standards. At times (1) background O3
is greatly associated with high concentrations experienced in
the US Intermountain West (Lefohn et al., 2001; Langford et al.,
2009; McDonald-Buller et al., 2011) that affect attainability
of O3 air quality standards and at other times (2) background
O3 contributes on a continuous basis to observed concentrations
that influence human health and vegetation risk estimates. In
both cases, background O3 influences the recommended levels for
O3 standards. With the appropriate understanding of the relative
importance of background O3 and how it is contributes to observed
O3 levels, its place in the decisionmaking process for assessing
(1) human health and welfare risks, (2) attainability of O3 standards,
and (3) benefits accrued from emission reductions can better
be placed into perspective. In December 2015, the US EPA issued
a white paper paper to establish a common understanding and foundation
for additional conversations on background ozone and to inform
any further action by the Agency.
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 on that the
observation that annual maximum concentratioins 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; Lefohn et al., 2011; Lefohn et al., 2012; Lefohn et al.,
2014). 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
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 web page.
Prior to 2006, O3 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) O3 to include contributions from global anthropogenic
and natural sources in the absence of North American (i.e., U.S.,
Canada, 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 O3 standards cannot be set. More recently, EPA (2013) has
defined US background (USB) O3 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 O3. Emissions-influenced-background
(EIB) O3, 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 O3 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 O3 concentrations, other scientists believe
that natural processes, such as stratospheric intrusions dominate
background O3 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 O3 concentrations might occur if
emissions were to be reduced in Asia. If natural stratospheric
intrusions are responsible for replenishing background O3 levels
more than long-range transport effects on background O3, then
reductions in background O3 levels might not be as significant
as some think if Asian emissions were reduced. A.S.L. & Associates
continues its efforts with its international research team to
investigate the importance of the contribution to background
O3 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 O3 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
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 O3 trends at sites in the western U.S. did not appear to
illustrate current increases in surface O3 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 O3 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, has estimated that western US spring
and early summer background O3 is routinely elevated by stratospheric
O3 with STT-S contributing more than O3 generated from Asian
emissions. Similar findings were reported by Ambrose et al. (2011)
for the Mount Bachelor area in Oregon. The 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 have minimal impact.
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. Our most recent published article on the
subject (Lefohn et al., 2014) uses adjusted GEOS-Chem model to
estimate background O3. We are finding that one has to adjust
background estimates from the GEOS-Chem model to provide a more
realistic estimate of background O3. 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 O3 are
usually greater than 70% over the entire year (Lefohn et al.,
2014). Recent 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 O3 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.
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
Lefohn et al. (2014)
have characterized the percent contribution from background O3
to the total O3 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 O3 at the
site in Wyoming was very large (i.e., generally greater than
80-90% of the total O3). The highest O3 concentrations at the
site were 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.
The Canadian Country-Wide ozone standard of 0.065 ppm for the
4th highest 8-hour ozone concentration averaged over 3 years
will be almost impossible to attain on a consistent basis. In
other words, at most locations, the sites will more than likely
go in and out of attainment as a function of the meteorology.
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
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
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 debate 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
O3 affecting surface level O3 concentrations. Recently, Hu et
al. (2017) 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 O3 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 late September 2009,
the National Research Council released the report, Global
Sources of Local Pollution. In the report, the Committee
states that modeling and analysis supports the finding that background
O3 is 20-40 ppb for the United States. The NRC report notes that
the discussion by Lefohn, Oltmans, Dann, and Singh (2001) that
occurrences of hourly average concentrations associated with
background O3 are higher than the level indicated in the NRC
report and that the NRC believes that the levels reported by
Lefohn et al. (2001) are 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 O3 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 O3 (see
Lefohn et al., 2014). In addition, our research on background
O3, 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 O3 also result
in enhanced 8-h average concentrations that are used in the O3
standard-setting process. At many high-elevation sites, much
of this enhancement is due to naturally occurring stratospheric
O3 influencing ground-level O3. Since our 2001 paper, many more
peer-reviewed research papers have been published that support
our early findings.
It is important that the range of background
hourly average concentrations is correctly characterized. The
inadequate characterization of background O3 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 O3 concentrations can be lowered
as a result of emission reduction requirements. Our international
research team is continuing to investigate this important research
area. Please see a list of our publications
in this area.
Is background O3 an important issue or
is it just an academic exercise? Clearly, in 2017 in the US,
the background O3 issue is front and center. Researchers and
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
O3 levels. By studying background ozone, one learns 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
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),
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, http://dx.doi.org/10.1016/j.atmosenv.2017.08.036.
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.
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.
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. http://dx.doi.org/10.1016/j.atmosenv.2013.11.033.
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
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