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? A paper by Oltmans et
al. (1998), which was published in early 1998 in the peer-reviewed
journal, Geophysical Research Letters, reports that surface
ozone is not increasing in the world at 1% per year. In many
cases, ozone is not increasing at all. You can find the full
citation to this paper 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 that PRB could only be estimated
using chemical transport models (CTMs). Based on scientific testimony,
the EPA was advised in December 2005 that empirical data at a
monitoring site at Trinidad Head, CA allow 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 represent policy-relevant background concentrations
of ozone. Oltmans et al. (2008) described the O3 exposures occurring
at this PRB 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 paper
by Oltmans et al. (1998) did not indicate that ozone was
increasing at the cleanest sites in the world for previous years.
Additional research is continuing in this area. Oltmans et
al. (2006) updated their original 1998 findings and reached
similar conclusions as stated in their 1998 paper. Please see the publication
list for the reference.
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, confirming that background ozone
levels are higher and that the natural short-term variability
is more frequent and greater than previously believed. Although
modeling results have been published questioning our conclusions
about the importance of stratospheric ozone in affecting surface-level
ozone concentrations, we believe there are serious shortcomings
associated with the modeling efforts, some of which have been
documented in EPA's 2006 Ozone Criteria Document (EPA, 2006).
Our most current research results continue to support our previous
conclusions about the importance of stratospheric-tropospheric
exchange processes in affecting surface ozone concentrations
at both high- and low-elevation monitoring sites.
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 2006-2008. A larger view of the figure
is available. Note that 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. Stratospheric processes
play a significant role in defining these background ozone concentrations.
In May 2001, we published an important paper in the Journal
of Geophysical Research (A.S. Lefohn, S.J. Oltmans, T. Dann,
and H.B. Singh. 2001. Present-day variability of background ozone
in the lower troposphere. Jour. Geophys. Res. 106 (D9): 9945
- 9958). In order to better understand the frequency, spatial,
and temporal characteristics of this background ozone burden,
we analyzed hourly average ozone concentrations greater than
or equal to 0.05 and 0.06 ppm that were experienced during the
photochemically quiescent months in the winter and spring at
several rural sites across southern Canada, the northern United
States, and northern Europe. Our results were mostly consistent
and indicated that hourly average ozone concentrations greater
than or equal to 0.05 and 0.06 ppm occur frequently during the
winter and spring months. Most occurrences were during April
and May but sometimes as late as June. In some, but not all,
of the cases that were studied, a plausible explanation for the
higher ozone values was the presence of upper tropospheric and
stratospheric air that was transported down to the surface.
There has been considerable
debate over the past several years on the importance of stratospheric
ozone in contributing to surface ozone concentrations. Low spatial
resolution models (e.g., GEOS-CHEM), which are 2 degrees by 2.5
degrees in resolution, have been exercised to illustrate that
stratospheric ozone is not important. 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 background.
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 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 an important role in
the observed ozone surface concentrations. 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.
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
at Trinidad Head, California, during ITCT 2K2. J. Geophys. Res.
109, D23S17, doi:10.1029/2003JD004406.
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.
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.
Oltmans S. J., Lefohn A. S., Harris
J. M., Galbally I., Scheel H. E., Bodeker G., Brunke E., Claude
H., Tarasick D., Johnson B.J., Simmonds P., Shadwick D., Anlauf
K., Hayden K., Schmidlin F., Fujimoto T., Akagi K., Meyer C.,
Nichol S., Davies J., Redondas A., and Cuevas E. (2006) Long-term
changes in tropospheric ozone. Atmospheric Environment. 40:3156-3173.
Oltmans S. J., Lefohn A. S., Harris J. M. and Shadwick
D. (2008) Background ozone levels of air entering the west coast
of the U.S. and assessment of longer-term changes. Atmospheric
Environment. 42:6020-6038.
U.S. Environmental Protection
Agency (2006) Air Quality Criteria for Ozone and Related Photochemical
Oxidants. Research Triangle Park, NC: Office of Research and
Development; EPA/600/R-05/004af. February.
U.S. Environmental Protection
Agency (2007) Review of the National Ambient Air Quality Standards
for Ozone: Policy Assessment of Scientific and Technical Information
OAQPS Staff Paper. Research Triangle Park, NC: Office of Air
Quality and Planning and Standards, EPA-452/R-07-003. January.
