A Very Important
Part of the Setting of the Ozone Standards
During the 2019-2020 ozone rulemaking
activity to review the current 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 exceed
the O3 NAAQS. However, 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. Background O3 concentrations
in the low- and mid-level part of the distribution of concentrations
make up a large fraction of the total ambient O3 levels and potentially
can influence those human health risk assessments associated
with the margin of safety determinations for the setting of the
primary 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 an issue in the
US but also an issue worldwide when considering attaining air
pollution standards. At times (1) background ozone 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; Jaffe et al., 2018) 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 is 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 that was 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 on 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; 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, 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, 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, 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 with its international research team 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
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, has 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 have 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. Our most recent published article on the
subject (Lefohn et al., 2014) uses adjusted GEOS-Chem model to
estimate background ozone. We are finding 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). 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 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.
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 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 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.
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 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 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
ozone 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 ozone 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 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 (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 have been 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 international
research team is continuing to investigate this important research
area. Please see a list of our publications
in this area.
Clearly, back in 2017 in the US, background
was an issue. Today background has become a major issue, not
just for ozone attainment purposes, but also for assessing the
adequacy of effects models that predict human health and vegetation
effects below 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 governments and tribes
in better understanding natural processes and the role that background
ozone plays in the attainment process. 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
the daily ozone concentrations that are 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
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.
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: https://doi.org/10.1525/elementa.309.
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
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