Over the past several years, Dr. Lefohn, A.S.L. & Associates, has collaborated with other researchers from around the world to quantify ozone trends for world-wide "signature" monitoring sites as well as anthropogenically influenced monitoring sites in the United States. The “signature" ozone monitoring stations provide good data for the purpose of assessing possible changes in background levels of ozone. Some of the "signature" sites offer the opportunity to study records that are representative of broad geographic regions where local effects are minimized. We are continually updating our ozone trending analyses.

Introduction

Recently, Lefohn et al. (2008) summarized their trends analyses for surface ozone monitoring sites across the United States. Using statistical trending on a site-by-site basis of the (1) health-based annual 2nd highest 1-hour average concentration and annual 4th highest daily maximum 8-hour average concentration and (2) vegetation-based annual seasonally corrected 24-hour W126 cumulative exposure index, they investigated temporal and spatial statistically significant changes that occurred in surface ozone in the United States for the periods 1980-2005 and 1990-2005 and explored whether differences in trending occur depending upon the selection of the exposure metric. Using the trending results, the analyses quantitatively explore the evidence for the higher hourly average ozone concentrations decreasing faster than the mid- and lower-values.

Results

Figure 1 below from Lefohn et al. (2008) summarizes the findings for the trending of the 4th highest 8-hour ozone metric for the 1980-2005 and 1990-2005 periods.

Figure 1. Trend of 4th highest 8-hour average ozone metric for the (a) 1980-2005 and (b) 1990-2005 periods. This figure was published in Lefohn et. al. (2008). Copyright Elsevier. Please see reference below. Permission granted by Elsevier to reproduce the above figure only on this web page.

Most of the surface ozone monitoring sites analyzed in the study experienced decreasing or no trends. Few monitoring sites experienced increasing trends. For those monitoring sites with declining ozone levels, an initial pattern of rapid decrease in the higher hourly average concentrations, followed by a much slower decrease in mid-level concentrations was observed. In some cases, they observed shifts from the lower hourly average ozone concentrations to the mid-level values. On a site-by-site basis, the majority of monitoring sites (1) changed from negative trend to no trend, (2) continued a negative trend, or (3) remained in the no trend status, when comparing trends for the 1980-2005 to the 1990-2005 time periods. For the three exposure metrics (i.e., annual 2nd highest 1-hour average concentration, annual 4th highest daily maximum 8-hour average concentration, and vegetation-based annual seasonally corrected 24-hour W126 cumulative exposure index, approximately 60% of the monitoring sites shifted from negative trending to no trending status. All regions of the United States were equally affected by the shift in status.

Lefohn et al. (2008) in their paper provide several figures that illustrate the spatial patterns of trends across the United States. The greatest statistically significant decreases in the 2nd highest 1-hour average concentrations and the annual 4th highest daily maximum 8-hour average concentration for the two temporal periods occurred in southern California. Monitoring sites in other portions of the United States experienced lesser decreases than this geographic area. In contrast to the two exposure indices, the vegetation-based 24-hour W126 ozone cumulative index for 1980-2005 experienced significant declines in the midwestern states and the northeastern United States as well as in southern California. For the 1990-2005 period, monitoring sites in southern California and the northeastern United States experienced the greatest decreases in the W126 exposure metric.

When trending was observed, not all months experienced trending. Lefohn et al. (2008) tested for statistically significant changes in the number of hourly average concentrations within specified concentration intervals and identified specific months that experienced shifts in the distribution of the hourly average concentrations. As an example, Figures 2 and 3 below illustrate the changes in the distribution of the hourly average ozone concentrations for a monitoring site located in Reseda in Los Angeles County as reductions occurred over the 1980-2005 and 1990-2005 periods.

Figure 2. Distribution of changes by month for a monitoring site located in Los Angeles County, California (AQS 060371201) for 1980-2005 for the months with significant changes.

Figure 3. Distribution of changes by month for a monitoring site located in Los Angeles County, California (AQS 060371201) for 1990-2005 for the months with significant changes.

The two figures show the reductions in the number of hourly average concentrations in the higher hourly average concentrations and the increases in the mid-level concentrations as the peak values were reduced. The Thiel estimate was used to estimate trending. The Thiel estimate is determined as the median of slope estimates calculated as the slope of the line passing through two points for all pairs of points in the data set of interest. To test for statistical significance, Kendall's tau test was used to determine significance at the 10% level. Note that the months of March and April exhibited statistically significant trending in the 1980-2005 period, but did not exhibit statistically significant trending over the 1990-2005 period. Over the 1990-2005 period, the month of September exhibited statistically significant trending but did not over the 1980-2005 period.

Conclusions and Recommendations

Most of the surface ozone monitoring sites analyzed in the study experienced decreasing or no trends. Few monitoring sites experienced increasing trends. Lefohn et al. (2008) observed that a statistically significant trend at a specific monitoring site, using one exposure index, did not necessarily result in a similar trend using the other two indices. The authors recommended that because different trending patterns were observed when applying the various exposure indices, a careful selection of ozone exposure metrics is required when assessing trends for specific purposes, such as human health, vegetation, and climate change effects.

 

Part 2 - Ozone Trends of “Signature” Monitoring Stations for Assessing Background Ozone Trending

Introduction

Over the past several years, there have been several articles quoting other sources indicating that surface ozone concentrations are increasing everywhere. Our most current research results do not support this claim. Our research efforts monitor the status of worldwide ozone levels by performing sophisticated analyses using surface ozone and ozonesonde data (e.g., Oltmans et al., 1998; Oltmans et al., 2006). Our research focuses on the results from the available data from four decades (~1967-2007) of observations for the longest records (ozonesondes). Several “signature” stations have 20-30 years of observations for both surface and ozonesondes. The “signature" ozone monitoring stations provide good data for the purpose of assessing possible changes in background levels of ozone. Some of the "Signature" sites offer the opportunity to study records that are representative of broad geographic regions where local effects are minimized.

In December 2008, we presented at the AGU Fall Conference in San Francisco a summary of one of our several research efforts dealing with changes in tropospheric ozone levels. The talk was presented at the Tropospheric Gaseous Composition in the Regional and Global Perspective 1 - A11F-03 session. The title of our presentation was: "Tropospheric Ozone Changes from Surface and Ozonesonde Observations." The research was presented by the lead author, Samuel Oltmans, NOAA Earth System Research Laboratory Global Monitoring Division, Boulder, Colorado. The talk explored the following:

• What are the implications from this record of observations?
• Are the records consistent in regions with multiple records?
• At what geographic scale can conclusions about trends be drawn (global, hemispheric, regional)? and
• Are changes related to precursor emissions, transport variability, stratospheric input?

Results and Conclusions

Based on our analyses, our results and conclusions are

• In the Northern Hemisphere, there is a different pattern of long-term changes between North America, Europe, and Japan;
• Over North America there does not appear to be a significant increase over the 30 years of measurements, although there have been shorter term fluctuations;
• Western Europe saw the largest increases prior to 1990 but a significant decrease in growth rates (and in some cases declines) over the past 15 years over continental Europe;
• In Japan, increases were primarily prior to the mid 1980s, but with recent increases in Okinawa;
• In the Southern Hemisphere mid latitudes, at Cape Point, Cape Grim, and Lauder, ozone has increased significantly with the increases coming primarily in the austral spring;
• In Hawaii (North Pacific tropics), increases appear to be associated with decadal transport shifts;
• The tropical south Pacific (Samoa) has not shown significant changes in tropospheric ozone; and
• At the South Pole, earlier declines have reversed so that overall there has been almost no change.

Some possible implications of recent (i.e., 30 year) observed changes are

• The relationship between emission changes and longer term hemispheric and global tropospheric ozone changes is not adequately understood;
• The climate forcing of tropospheric ozone changes over the recent past is still very uncertain; and
• It will be difficult to assess the climate forcing reductions that can be gained by reducing tropospheric ozone with the current network of observations.

Co-authors of the research study are A. Lefohn, J. Harris, H.-E. Scheel, E. Brunke, H. Claude, D. Tarasick, I. Galbally, G. Bodeker, J. Davies, T. Koide, B. Johnson, C. Meyer, F. Schmidlin, E. Cuevas, A. Redondas, P. Simmonds, and B. Buchman.

References

Lefohn A. S., Shadwick D., and Oltmans S. J. (2008). Characterizing long-term changes in surface ozone levels in the United States (1980-2005). Atmospheric Environment. 42:8252-8262.

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.

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