NATURAL BACKGROUND

AN IMPORTANT ISSUE

If researchers use lower 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 natural background conditions are against which the vegetation effects of higher ozone levels in polluted rural areas of North America can be determined. 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. 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 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 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. 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 controls 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, recently EPA attempted to use chemical transport models to estimate Policy Relevant Background levels.

EPA defines 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 this subjective definition, the U.S. EPA is questioning the use of remote monitoring sites in the world as a reasonable way to establish limits on natural ozone exposures in today's world. Initially EPA believed that PRB could only be estimated using chemical transport models (CTMs). However, based on evidence presented to the Agency that PRB can be determined using empirical data (Oltmans et al., 2008), EPA's Clean Air Scientific Advisory Committee (CASAC) acknowledged that an ozone monitoring site at Trinidad Head (CA) provided helpful information about PRB. Our research is continuing in this important area. 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.

Papers by Oltmans et al. (1998, 2006) report that surface ozone is not increasing at all locations in the world at 1% per year (please see publications list). In many cases, ozone is not increasing. The picture of long-term tropospheric ozone changes is a varied one in terms of both the sign and magnitude of trends and in the possible causes for the changes. In May 2001, Lefohn et al. (2001) published an important paper in the Journal of Geophysical Research (Lefohn, A.S., 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 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.

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). 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.

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.

The importance of properly defining the range of hourly average ozone concentrations that are associated with background is because "Policy Relevant Background" (PRB) concentration or range of concentrations represent what EPA believes would be experienced if the United States and other countries in North America were to initiate a zero anthropogenic emissions strategy. The PRB concentrations define the level below which ozone standards cannot be practicably set. In the 1996 ozone review, the EPA used 0.04 ppm in its health risk assessment evaluations as the level it expected 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 policy-relevant background that was much lower than the 0.04 ppm level. At a monitoring site at Trinidad Head, California, which experiences numerous conditions that meet the definition of Policy Relevant Background (EPA, 2007), frequent occurrences of hourly average concentrations greater than or equal to 0.05 ppm are measured. The EPA's Clean Air Scientific Advisory Committee (CASAC) in August 2006 and March 2007 concluded that there was a large degree of uncertainty associated with the estimates of PRB. In August 2006, the EPA Advisory Committee had concerns with the EPA's human health risk assessment estimates that were based on the Agency's PRB estimates. For further information, please click here.

References

Cooper O.R.; Stohl A.; Hübler G.; Hsie E.Y.; Parrish D.D.; Tuck A.F.; Kiladis G.N.; Oltmans S.J.; Johnson B.J.; Shapiro M.; Moody J.L.; Lefohn A.S. (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.

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.