Heather Simon, Adam Reff, Benjamin Wells, Neil FrankOffice of Air Quality Planning and Standards, US EPA

Ozone Trends Across the United States over a Period of Decreasing NOx

Emissions

Importance of Ozone Distribution

• Ozone is an important atmospheric oxidant: changes in spatial and temporal ozone patterns affect NOx cycling and secondary aerosol formation

• Changing spatial and temporal patterns will affect overall ozone exposure to individuals and populations

• Ozone damage to plants is most reflective of cumulative rather than peak ozone exposure

• Changing spatial patterns affect how areas will implement controls to meet air quality goals

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Motivation: Chicago Case Study• Past Modeling analysis has predicted future NOx reductions will lead to changes

in spatial and temporal patterns of ozone– Increasing nighttime and decreasing daytime concentrations– Shift in seasonal pattern: peaks occur earlier in the year

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Motivation: Chicago Case Study• Past Modeling analysis has predicted future NOx reductions will lead to changes

in spatial and temporal patterns of ozone– Different response seen in urban core versus suburban and outlying areas

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Chicago Mean May-Sept MDA8: 2006-2008 obs

Chicago Mean May-Sept MDA8:75% NOx cuts

What Can Ambient Data Tell Us About These Trends?

• We have an extensive ground-based ozone monitoring network with data dating back to the 1970’s

• The past 15 years provide a “natural experiment” to look at ambient ozone trends over a period of dramatically decreasing NOx emissions • U.S. NOx emissions dropped from 22.6 million TPY in 2000

to 12.9 million TPY in 2011 (43%)

• Caution: past trends may not reflect behavior which will occur from future emissions reductions

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Satellite-Derived NO2 Trends

Aura OMI tropospheric NO2 Column Density: 2005

Aura OMI tropospheric NO2 Column Density: 2011

Ozone Trends at Monitoring Sites across the U.S. Segregated by:

• Percentile – High ozone (95th percentile)– Low ozone (5th percentile)– Mid-range ozone (mean/median)

• Region• Season

– Warm season (May – September)– Cool season (April – October)

• Degree of Urbanization 7

Percent of Monitors with Statistically Significant Ozone

Trends from 1998-2011

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• Looked at statistically significant trends in seasonal means of 8-hr daily max ozone (p-value < 0.05)– Only ~20% of monitors nationally

have statistically significant trends in mean ozone

• This plot shows trends by degree of urbanization and season– Rural sites on left (sum-win)– Suburban sites in middle (sum-win)– Urban sites on right (sum-win)

Mean Ozone

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• Ozone trends were evaluated by ozone metric (columns) and by region of the US (rows)

• This talk will focus on differences in trends for different portions of the ozone distribution (metric)

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Trends in High Ozone Concentrations

• 95th Percentile ozone concentrations have decreased across seasons, regions, and urban classifications

• Demonstrates large scale success of US control strategies which target peak ozone concentrations• 1-2 ppb/yr 13 – 26 ppb decreases over this time period

75th percentile5th percentile 25th percentile 50th percentile 95th percentile mean

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Trends in High Ozone Concentrations

• 95th Percentile ozone concentrations have decreased across seasons, regions, and urban classifications

• Demonstrates large scale success of US control strategies which target peak ozone concentrations• 1-2 ppb/yr 13 – 26 ppb decreases over this time period

Suburban May-Sep TrendsUrban May-Sep Trends Rural May-Sep Trends

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Trends in Low Ozone Concentrations

• 5th Percentile ozone concentrations have increased across seasons, regions, and urban classifications

• Past studies have characterized the influence of increasing background ozone levels on low ozone concentrations

• Little work has looked at impact of regional NOx reductions75th percentile5th percentile 25th percentile 50th percentile 95th percentile mean

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Suburban May-Sep TrendsUrban May-Sep Trends Rural May-Sep Trends

• 5th Percentile ozone concentrations have increased across seasons, regions, and urban classifications

• Past studies have characterized the influence of increasing background ozone levels on low ozone concentrations

• Little work has looked at impact of regional NOx reductions

Trends in Low Ozone Concentrations

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Trends in Midrange Ozone Concentrations

• Both increasing and decreasing trends seen in median ozone• Decreases most commonly seen in rural sites• Increases most commonly seen in urban sites• Linkage to degree of urbanization suggests that these

trends may be related to NOx disbenefits

75th percentile5th percentile 25th percentile 50th percentile 95th percentile mean

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Suburban May-Sep TrendsUrban May-Sep Trends Rural May-Sep Trends

Trends in Midrange Ozone Concentrations

• Both increasing and decreasing trends seen in mid-range ozone• Decreases most commonly seen in rural sites• Increases most commonly seen in urban sites• Linkage to degree of urbanization suggests that these

trends may be related to NOx disbenefits

Conclusions

• Increase in low ozone concentrations and decrease in high ozone concentrations leads to a compression of the observed range of ozone values

• Modeling suggests that this compression will also lead to less temporal and spatial variability

• Health studies which often derive correlations based on area-wide averages of ozone should consider implications of these changes for the design of their studies

• US control strategies have been largely successful at targeting peak ozone concentrations

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Appendix

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Urbanization Classifications

based on NLCD land use

categories

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Image reproduced from NLCD 2006 product data downloads: http://www.mrlc.gov/nlcd06_data.php