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.
It 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), which were published in peer-reviewed journals
report that surface ozone is not increasing in the world
at 1% per year. In many cases, surface ozone is not increasing
at all. You can find the full citation to these papers in the
Publications section of this web page.
EPA has defined Policy-Relevant Background
(PRB) as those concentrations that would occur in the United
States in the absence of anthropogenic emissions in continental
North America (i.e., the United States, Canada, and Mexico).
PRB concentrations include contributions from (1) natural sources
everywhere in the world and (2) anthropogenic sources outside
the United States, Canada, and Mexico. Contributions to PRB O3
include photochemical actions involving natural emissions of
VOCs, NOx, and CO as well as the long-range transport of O3 and
its precursors from outside North America and the stratospheric-tropospheric
exchange (STE) of O3. Natural sources of O3 precursors include
biogenic emissions, wildfires, and lightning. Biogenic emissions
from agricultural activities are not considered in the formation
of PRB O3. The level of PRB 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 PRB concentrations
define the level below which O3 standards cannot be set.
As a result of the 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 PRB could only be
estimated using chemical transport models (CTMs). However, based
on scientific testimony in 2005, the EPA and its Clean Air Scientific
Advisory Committee (CASAC) became aware that empirical data at
a monitoring site at Trinidad Head, CA allowed for the characterization
of PRB without the use of highly uncertain modeling results.
Although acknowledging EPA's desire to
use a model to estimate PRB, 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 PRB using
the model. EPA (2007) acknowledged that the monitoring site at
Trinidad Head, CA does provide information about policy-relevant
background 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 PRB estimates for risk assessments for ozone was unable to
account for the numerous occurrences of hourly average PRB 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 1989, when the firm
was asked to "identify natural background ozone" 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) did not indicate that ozone
was increasing at the cleanest sites in the world for previous
years. Additional research is continuing in this area.
For several years,
we have had an on-going 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. In that 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 about the importance
of stratospheric ozone in affecting surface-level ozone concentrations,
we believe the models are unable to adequately quantify the importance
of stratospheric-tropospheric exchange (STE) that appears to
be affecting some of the enhanced ozone concentrations occurring
during the spring months across the US. Some of the shortcomings
of the models are documented in EPA's 2006 Ozone Criteria Document
(EPA, 2006). In the Ozone Staff Paper (EPA, 2007), the Agency
concluded that PRB O3 concentrations at the surface were generally
predicted to be in the range of 15 to 35 ppb in the afternoon,
and they tended to decline under conditions conducive to high
O3 episodes. The EPA believed that PRB was highest during spring
and declined into summer. Higher values tended to occur at higher
elevations during spring due to contributions from hemispheric
pollution and stratospheric intrusions. The EPA stated that stratospheric
contribution to surface O3 was typically well below 20 ppb and
only rarely elevated O3 concentrations at low-altitude sites
and only slightly more often elevated them at high-altitude sites.
Our most current research results continue to support our previous
conclusions (Lefohn et al., 2001) about the importance of stratospheric-tropospheric
exchange processes in affecting surface ozone concentrations
at both high- and low-elevation monitoring sites across the US.
An Internet-based
slide presentation is available for purposes
of previewing our 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.
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
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 policy-relevant 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 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 influencing background ozone monitoring sites. Empirical
evidence shows that stratospheric contributions to surface O3
is important (Lefohn et al., 2001; Cooper et al.,
2005) 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) summarizes 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 PRB
is 20-40 ppb for the United States. The report notes that the
discussion by Lefohn, Oltmans, Dann, and Singh (2001) that occurrences
of hourly average concentrations associated with PRB are higher
than this level are associated either with high-elevation sites
or with more distant North American pollution. Since 2001, when
we published the Lefohn et al. (2001) paper, evidence that has
been published in the peer-review literature has continued to
build that the stratosphere likely plays a more important role
in the observed ozone surface concentrations at both high- and
low-elevation monitoring sites than the models have predicted.
In addition, our research on PRB, using empirical data, indicates
that levels are higher than 20-40 ppb in the United States. 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.
Our most recent work on background ozone (Oltmans et al., 2010)
discusses the importance of Eurasian biomass burning and how
it influences policy-relevant background ozone concentrations
in the US.
It is important that the range of PRB hourly
average concentrations is correctly characterized. The inadequate
characterization of PRB 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 research in this important area is
continuing.
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.
Reference
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.
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
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observations. Atmospheric Environment. 44:4497-4509.
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Agency (2006) Air Quality Criteria for Ozone and Related Photochemical
Oxidants. Research Triangle Park, NC: Office of Research and
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