Heather Simon, Adam Reff, Benjamin Wells, Neil Frank Office 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 2

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 3

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 4 Chicago Mean May-Sept MDA8: 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 5

6 Satellite-Derived NO 2 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 (95 th percentile) –Low ozone (5 th 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 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

9 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)

10 Trends in High Ozone Concentrations 95 th 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

11 Trends in High Ozone Concentrations 95 th 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 TrendsRural May-Sep Trends

12 Trends in Low Ozone Concentrations 5 th 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

13 Suburban May-Sep TrendsUrban May-Sep TrendsRural May-Sep Trends 5 th 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

14 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

15 Suburban May-Sep TrendsUrban May-Sep TrendsRural 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 16

Appendix 17

Urbanization Classifications based on NLCD land use categories 18 Image reproduced from NLCD 2006 product data downloads: