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Ozone Trends

Over the past several years, Dr. Lefohn, A.S.L. & Associates, has collaborated with other researchers around the world to quantify ozone trends for special monitoring sites, as well as anthropogenically influenced monitoring sites in the United States. The special ozone monitoring stations provide good data for the purpose of assessing possible changes in background levels of ozone. Some of the 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.

Part 1 - Ozone Trends of Anthropogenically Influenced Monitoring Sites


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.


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 Theil estimate was used to estimate trending. The Theil 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.

The Mann-Kendall (M-K) nonparametric test is utilized to test for a significant trend. Advantages of the M-K test are

  • No distributional assumption is made;


  • No assumption of any specific functional form for the behavior of the data through time is made. Thus, the M-K test is universally applicable across all sites, seasons, and different continuous summary exposure metrics (e.g., percentiles, means, and cumulative exposure indices, such as the W126 and AOT40 vegetation exposure metrics); and


  • The M-K test is resistant to the effects of outlying observations. The results are not unduly affected by particularly high or low values that occur during time series analyses.

For estimating the magnitude of a trend, the Theil-Sen (also called Sen-Theil, Theil, or Sen) estimator can be used. It possesses the same attributes described above for the M-K test (i.e., there are no distributional or functional form assumptions and the estimator is resistant to outliers). The Theil-Sen (T-S) estimator, similar to the M-K technique, is also universally applicable. In cases where simple linear regression is appropriate (assuming key assumptions are met), the slope of the regression line and the T-S estimator are asymptotically equivalent.

For more information about the Theil estmate and the Kendall's tau test, please see the discussion in Section 3 in Lefohn et al. (2018).

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.

Lefohn et al. (2010a) updated the trending results reported in Lefohn et al. (2008). The updated trending periods were 1980-2008 (29 years) and 1994-2008 (15 years). In addition to updating the trends analysis, the authors focused on 12 urban and 15 rural monitoring sites. The trending results provide examples on why it is important to investigate the change in the trending pattern with time (e.g., moving 15-year trending) in order to assess how year-to-year variability may influence the trend calculation. Several research investigations have explored trending from the beginning of a data series until the latest date for collection of data. However, these types of trends analyses do not take into consideration the possibility that the rate of trending over the period of record has changed and in some cases the trending has ceased. Lefohn et al. (2010a) took a closer look at changes in the rate of trending and drew conclusions about the importance of using 15-year moving trends to quantify trending rates. The paper's abstract is available on our publications web page.

Conclusions and Recommendations

Most of the surface ozone monitoring sites analyzed in the Lefohn et al. (2008, 2010a) studies experienced decreasing or no trends. Few monitoring sites experienced increasing trends. Lefohn et al. (2008, 2010a) 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. Lefohn et al. (2017) performed a detailed analysis using several human health and vegetation exposure metrics and quantified the differences in trends using different metrics for sites in the US, EU, and China. As in previous analyses, Lefohn et al. (2017) noted that using the identical hourly average ozone concentrations, different trend patterns occurred based on the selection of the specific exposure indices (e.g., metrics focused on the higher end of the distribution versus those indices focused on the lower end of the distribution). In some cases, the high end of the distribution moved downward, while the low end of the distribution shifted upward. The lower hourly averaged ozone concentrations moved upward due to less NOx scavenging as NOx emissions were reduced (Simon et al., 2015; Lefohn et al., 2017; Lefohn et al., 2018). For assessing biological effects, the highest hourly average ozone concentrations are more important than the mid- and low-level values (Hazucha and Lefohn, 2007; Lefohn et al., 2010b; Musselman et al., 2006).


Part 2 - Ozone Trends of Special Monitoring Stations for Assessing Background Ozone Trending


Over the past several years, there have been several articles quoting other sources indicating that surface ozone concentrations are increasing everywhere. Our 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; Oltmans et al., 2013). Our research focuses on the results from the available data from four decades of observations for the longest records (ozonesondes). Several key stations have 20-30 years of observations for both surface and ozonesondes. The key ozone monitoring stations provide good data for the purpose of assessing possible changes in background levels of ozone. Some of the sites offer the opportunity to study records that are representative of broad geographic regions where local effects are minimized.

Oltmans et al. (2013) discusses the longer-term (i.e., 20-40 years) tropospheric ozone time series obtained from surface and ozonesonde observations that we analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time-series model, running 15-year trends, and changes in the distribution by month in the ozone mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (~30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10-15 years. It is uncertain if the flattening of the ozone change over Japan reflects the impact of ozone transported from continental East Asia in light of reported ozone increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and midlatitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and ozone has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on ozone precursors have likely been a factor in the leveling off or decline from earlier ozone increases.

As indicated earlier in the Part 1 discussion, it is important to investigate the rate-of-change in trending and not just analyze for trends using the beginning and ending periods of monitoring. For example, Logan et al. (2012) described the decrease in ozone over Europe since 1998, with the largest decrease during the summertime. Using Zugspitze data for 1978-1989 and the mean time series from three Alpine stations since 1990, Logan et al. (2012) found that the ozone increased substantially in 1978-1989 (i.e., 6.5-10 ppb but began to exhibit a reduced rate of increase in the 1990s (i.e., 2.5-4.5 ppb) with decreases in the 2000s (i.e., 4 ppb) in summer with no significant changes in other seasons. Overall in summer no trend was noted for the 1990-2009 period.

Conclusions and Recommendations

Our results suggest that on a hemispheric scale it is currently difficult to observe the projected increases in tropospheric ozone that models indicate may occur from Asian emissions. This may result from the lack of such ozone increases or that changes resulting from precursor reductions in North America and Europe have made the influence of Asian precursor emissions more difficult to detect. Parrish et al. (2017) noted that over the past decades, a long-term increase in baseline ozone has been observed at the North American west coast, They have concluded based on their analyses that this increase has ended. These results complement our observation that changes in hourly average ozone distributions at the low-, mid and high-level ranges for sites investigated in our most recent study do not indicate that background ozone concentrations continue to increase in the most recent decades. As indicated in our analysis and others, at many of the investigated sites earlier ozone increases have reached a plateau and in some cases begun to decrease. In order to follow future trend patterns, it will be important to use techniques that capture the time evolution of ozone changes such as the 15-year trend periods used by Lefohn et al. (2010a) and Oltmans et al. (2013) or other methods that detect these changes. In particular it will be important to determine if the widespread flattening or declining ozone concentrations reported here reflect longer-term changes in precursor ozone emissions.


Hazucha, M. J.; Lefohn, A. S. (2007) Nonlinearity in Human Health Response to Ozone: Experimental Laboratory Considerations. Atmospheric Environment. 41:4559-4570.

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.

Lefohn, A. S., Shadwick, D., Oltmans, S. J. (2010a). 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., Hazucha, M.J., Shadwick, D., Adams, W.C. (2010b). An Alternative Form and Level of the Human Health Ozone Standard. Inhalation Toxicology. 22 (12):999-1011.

Logan, J.A. et al. (2012). Changes in ozone over Europe: Analysis of ozone measurements from sondes, regular aircraft (MOZAIC) and alpine surface sites, Journal of Geophysical Research, 117, D09031, doi:10.1029/2011JD16952.

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.

Lefohn, A.S., Malley, C.S., Smith, L., Wells, B., Hazucha, M., Simon, H., Naik, V., Mills, G., Schultz, M.G., Paoletti, E., De Marco, A., Xu, X., Zhang, L., Wang, T., Neufeld, H.S., Musselman, R.C., Tarasick, T., Brauer, M., Feng, Z., Tang, T., Kobayashi, K., Sicard, P., Solberg, S., and Gerosa. G. (2018). Tropospheric ozone assessment report: global ozone metrics for climate change, human health, and crop/ecosystem research. Elem Sci Anth. 2018;6(1):28. DOI:

Musselman R. C., Lefohn A. S., Massman W. J., and Heath, R. L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.

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., 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. 67:331-351.

Parish D. D., Petropavlovskikh I., Oltmans S. J. (2017). Reversal of long-term trend in baseline ozone concentrations at the North American west Coast. Geophys Res Lett In Press. DOI: 10.1002/2017GL074960s.

Simon H, Reff A, Wells B, Xing J, Frank N. (2015). Ozone trends across the United States over a period of decreasing NOx and VOC emissions. Environ Sci Technol 49: 186-195.

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