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If vegetation researchers apply lower ozone concentrations in their control chambers than those concentrations expected to occur at areas which experience the lowest maximum hourly average concentrations in the world, yield reductions may be overestimated for some vegetation. This would make it difficult to use these data to establish standards to protect vegetation from surface ozone.

The challenge is to identify what the range of natural background ozone concentrations is and then use this range of concentrations to estimate vegetation effects in polluted rural areas of North America. At one end of the spectrum, natural background can be defined as unpolluted conditions in pre-industrial times (i.e., absolutely unpolluted air in which there is no human interference). For a number of reasons, this definition of natural background is not realistic for characterizing ozone exposures to be used as controls in vegetation research. First, we do not know with much confidence what past unpolluted conditions were (see Tarasick et al., 2019). Second, even if all anthropogenic emissions of ozone precursors were eliminated, it is unlikely that ozone concentrations in North America would return to pre-industrial levels. Since pre-industrial times, major land use changes have occurred. It is probable that these changes have modified the emissions of ozone precursors from natural sources and, thus, changed the concentrations of ozone. A third reason is that vegetation is no longer exposed to those ozone levels that may have existed hundreds of years ago; it is possible that vegetation has adapted to these changed levels.

However, some scientists have used data from over 100 years ago to compare to present levels. 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 of the daily maximum of the surface ozone partial pressure in the Great Lakes area of North America was approximately 0.019 ppm, and (2) the European measurements between the 1850s and 1900 experienced annual averages 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 most recent times 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 not be valid.

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. We have published information on the limitations of using the Schoenbein method to estimate absolute historic ozone concentrations. Please see our publications list for more information.

An alternative approach that has been employed is to identify a range of ozone exposures that occur at "clean" sites in the world. Although the sites are not free from human influence, the ozone concentrations at these sites may be appropriate to use as control exposures for vegetational researchers as pragmatic and defensible surrogates for natural background levels. 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. 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? In previous years, the US EPA accepted the approach of using remote monitoring sites in the world as a reasonable way to establish limits on natural ozone exposures in today's world. However, EPA has relied more recently on using chemical transport models to estimate North American Background (NAB) or US Background (USB) levels.

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 NAB level 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 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.

As a result of its subjective definition of modeled 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 background could only be estimated using chemical transport models (CTMs). However, scientists (e.g., McDonald-Buller et al., 2011) concluded that empirical data at a monitoring site at Trinidad Head, CA allowed for the characterization of background ozone without the use of highly uncertain modeling results.

Although acknowledging EPA's desire to use a model to estimate, 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 using the model. EPA (2007) acknowledged that the monitoring site at Trinidad Head, CA does provide information about 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 estimates for risk assessments for ozone was unable to account for the numerous occurrences of hourly average 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 we were requested 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. 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 westen U.S. did not appear to illustrate current increases in surface O3 levels and that in some cases, early 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) believe that long-range transport from Asia is enhancing O3 concentrations in the western US, as well as possibly other locations across the US. For areas east of the Intermountain West, Lin et al. (2012) reported that Asian emissions have minimal impact on surface ozone concentrations. Our most recent trending results indicate that inconsistencies exist in the hypothesis that long-range transport from Asia is causing the increases in ozone concentrations in the western US. At several monitoring sites in the western US, surface O3 is not increasing (Lefohn et al., 2017). In addition to long-range transport from Asia, surface ozone is enhanced from natural stratospheric sources (Mathur et al., 2022). Mathur et al. (2022) noted that background O3 across the continental United States is composed of a sizable and spatially variable fraction that is of stratospheric origin (29%-78%). Wang et al. (2020) reported that the stratospheric influence on summertime high surface O3 events makes a significant contribution to the surface O3 variability where background surface O3 exceeds the 95th percentile, especially over western U.S. Lin et al. (2012), using the AM3 model, 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. Langford et al. (2009) has reported deep STT contributing to high surface O3 using lidar and surface measurements from the Front Range of the Colorado Rocky Mountains during the 1999 O3 season (March-October). Their results showed that the stratospheric source was not only significant but could directly lead to exceedances of the 2008 U.S. NAAQS standards in a major metropolitan area.

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. 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 (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 published article on the subject (Lefohn et al., 2014) used adjusted GEOS-Chem model to estimate background O3. We are found that the adjusted background estimates from GEOS-Chem provided a much 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 stratospheric-tropospheric exchange processes in affecting surface ozone concentrations at both high- and low-elevation monitoring sites across the US.

Lefohn et al. (2014) 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 reported 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 appeared to be associated with stratospheric intrusions.

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; see Lefohn et al., 2014 for list of additional publications) 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). Our research results and the published results of others 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.

In late September 2009, the National Research Council released the report, Global Sources of Local Pollution. In the report, the Committee stated that modeling and analysis supports the finding that background O3 (i.e, policy-relevant background) is 20-40 ppb for the United States. The NRC report noted that the discussion by Lefohn, Oltmans, Dann, and Singh (2001) that occurrences of hourly average concentrations associated with background O3 are higher than the level indicated in the NRC report and that the NRC believed that the levels reported by Lefohn et al. (2001) were associated either with high-elevation sites or with more distant North American pollution. The conclusions in the NRC report were unfortunately inaccurate. Since 2001, when we published the Lefohn et al. (2001) paper, evidence has been published in the peer-review literature indicating the importance of stratospheric 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 at some sites 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. 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.

Properly defining the range of hourly average ozone concentrations associated with background is important because if the United States and other countries were to initiate a zero anthropogenic emissions strategy to achieve low-level ozone standards, unanticipated exceedances would occur. The range of background concentrations define the level below which ozone standards cannot be practicably established. In the 1996 ozone review, the EPA used 0.04 ppm in its health risk assessment evaluations as the level it predicted as background for an 8-hr daily maximum concentration for clean sites. In its review of the ozone standard in 2006 (U.S. EPA, 2006), the EPA used a model with great uncertainty to define ranges of concentrations for background that were much lower than the 0.04 ppm level used in 1996. At a monitoring site at Trinidad Head, California, which experiences numerous conditions that meet the definition of background, occurrences of hourly average concentrations greater than or equal to 0.05 ppm are measured. For further information, please click here.



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

Cooper, O.R.; A. Stohl; G. Hübler; E.Y. Hsie; D.D. Parrish; A.F. Tuck; G.N. Kiladis; S.J. Oltmans; B.J. Johnson; M. Shapiro; J.L. Moody; A.S. Lefohn. (2005) Direct transport of mid-latitude stratospheric ozone into the lower troposphere and marine boundary layer of the tropical Pacific Ocean. J. Geophys. Res., 110, D23310, doi:10.1029/2005JD005783.

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

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

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

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

Langford, A.O., Aikin, K.C., Eubank, C.S., Williams, E.J. (2009). Stratospheric contribution to high surface ozone in Colorado during springtime. Geophysical Research Letters 36, L12801.

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. 84:275-288.

Lefohn, A.S., Malley, C.S., Simon, H., Wells. B., Xu, X., Zhang, L., Wang, T., 2017. Responses of human health and vegetation exposure metrics to changes in ozone concentration distributions in the European Union, United States, and China. Atmospheric Environment 152: 123-145. doi:10.1016/j.atmosenv.2016.12.025.

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

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

McDonald-Buller, E.C., Allen, D.T., Brown, N., Jacob, D.J., Jaffe, D., Kolb, C.E., Lefohn, A.S., Oltmans, S., Parrish, D.D., Yarwood, G., Zhang, L. (2011) Establishing policy relevant background (PRB) ozone concentrations in the United States. Environmental Science & Technology 45, doi:10.1021/es2022918, 9484-9497.

Oltmans S. J., Lefohn A. S., Scheel H. E., Harris J. M., Levy H. II, Galbally I. E. , Brunke E. G., Meyer C. P., Lathrop J. A., Johnson B. J., Shadwick D. S., Cuevas E., Schmidlin F.J ., Tarasick D. W., Claude H., Kerr J. B., Uchino O., and Mohnen V. (1998) Trends of Ozone in the Troposphere. Geophysical Research Letters. 25:139-142.

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.

Oltmans, S.J., Lefohn, A.S., Harris, J.M., Tarasick, DW., Thompson, AM., Wernli, H., Johnson, B.J., Novelli, P.C., Montzka, S.A., Ray, J.D., Patrick, L.C., Sweeney, C., Jefferson, A., Dann, T., Davies, J., Shapiro, M., Holben, B.N. (2010). Enhanced ozone over western North America from biomass burning in Eurasia during April 2008 as seen in surface and profile observations. Atmospheric Environment. 44:4497-4509.

Oltmans, S.J., Lefohn, A.S., Shadwick, D., Harris, J.M., Scheel, H.-E., Galbally, I., Tarasick, D.A., Johnson, B.J., Brunke, E., Claude, H., Zeng, G., Nichol, S., Schmidlin, F., Redondas, A., Cuevas, E., Nakano, T., Kawasato, T. (2013). Recent Tropospheric Ozone Changes - A Pattern Dominated by Slow or No Growth. Atmospheric Environment. doi:10.1016/j.atmosenv.2012.10.057. 67: 331-351.

Tarasick et al. (2019). Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties. Elementa: Science of the Anthropocene. 7:39.

US Environmental Protection Agency (1996) Review of National Ambient Air Quality Standards for Ozone - Assessment of Scientific and Technical Information, OAQPS Staff Paper. EPA-452/R-96-007, Office of Air Quality Planning and Standards, Research Triangle Park, NC. June 1996. US Environmental Protection Agency.

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.

US Environmental Protection Agency (2013) Integrated Science Assessment for Ozone and Related Photochemical Oxidants. EPA/600/R-10/076F. Research Triangle Park, NC: Office of Research and Development. February.

Wang, X., Wu, Y., Randel, W., Tilmes, S. (2020). Stratospheric contribution to the summertime high surface ozone events over the western united states. Environ. Res. Lett. 15 (2020) 1040a6.

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