1. 1 Ozone Trends at Cleanest 10% Sites March 2010 Till Stoeckenius, Lan Ma, Lynsey Parker ENVIRON International Corp. & Gregory Stella Alpine Geophysics, LLC

2. 2 Scope  Map locations of cleanest ozone sites and evaluate relationships with precursor emissions and population  Compute, summarize and display trends in ozone design values at sites with lowest ozone levels during the period 1999 – 2009

3. 3 Data Handling Procedures  Ozone design value (DV) is the annual 4 th highest daily maximum 8-hour average averaged over three consecutive years Computed ozone DV for each overlapping three year period starting with 1999 – 2001 and ending with 2007-2009 Missing annual 4 th highest daily max 8-hr average is filled in via EPA trend approach: interior values using linear interpolation, end points set equal to adjacent value Data associated with exceptional events that have received EPA concurrence are omitted  DV calculations based on EPA data handling conventions (except 2009) Observation rate: at least 75% valid daily 8-hr max values for each year; at least 90% valid values for three-year average Count as DV if three-year average is greater than the standard of 0.075ppm, even if it fails observation rate requirement above 2009 ozone DV valid if >50% valid daily 8-hour maximum values due to incomplete data set for 2009

4. 4 Monitoring Site Selection  Changes to monitoring network (new or discontinued sites) can introduce spurious trends  Identify trends sites with nearly complete data 1999 - 2009 Monitoring site with at least 7 out of 9 valid annual DV (between 1999-2001 and 2007- 2009) are used in trend study

5. 5 Procedure  For each overlapping 3-year attainment period Rank DVs over all monitoring sites Calculate the 10 th percentile Identify all sites falling at or below the 10 th percentile value  Count number of attainment periods each site falls in the bottom 10%

6. 6 Clean Ozone Site Locations Cleanest sites: fall in bottom 10% nationally at least 76% of the time 9

7. 7 Characteristics of Clean Sites  Coastal with prevailing on-shore winds: CA coast, FL Atlantic coast, extreme southeast TX coast  Remote rural locations: MT, ND, WY, northern MN  High elevation – above polluted boundary layer: Lake Tahoe, CO Rockies, Mt. Washington NH  Unfavorable weather (cloudy, windy): Puget Sound WA, Willamette Valley OR  More investigation needed: Rio Grande, Iowa  High NOx levels: Boston, Chicago

8. 8 Ozone vs. Emissions by County  County annual NOx + VOC emissions  Bottom axis shows avg. number of years DVs at sites in county appear in bottom 10% of all sites nationally over 1999 – 2009  No least squares trend but cleanest sites are only located in counties with low-end emissions  Relationships masked by county locations relative to neighboring source regions and prevailing wind directions

9. 9 Ozone vs. Population by County  Results similar to previous slide Population figures from 2000 Census

10. 10 Trends at Clean Ozone Sites: National  1 st, 5 th, and 10 th percentile DV of all sites in each three-year period

11. 11 Trends at Clean Ozone Sites: National Design Value RankSlope (ppm/yr) 10th Percentile-0.001 5th Percentile< ± 0.0005 1st Percentile< ± 0.0005  Negative (downward) trend in 10 th percentile DVs  No trends in 1 st and 5 th percentile DVs

12. 12 Trends at Clean Ozone Sites: Eastern & Midwestern RPOs

13. 13 Trends at Clean Ozone Sites: Western States

14. 14 Trends at Clean Ozone Sites: Regional Slope (ppm/yr) Design Value RankEast & MidwestWest 10th Percentile-0.001< ± 0.0005 5th Percentile-0.001< ± 0.0005 1st Percentile< ± 0.0005  Downward (negative) trend in East/Midwest at 10 th and 5 th percentiles  Flat trends in West  10 th percentile values much lower in West than in East/Midwest

15. 15 No. of Sites Exceeding Threshold  20 percentage point decline in fraction of sites exceeding 0.070 ppm  Over 95% of all sites have DVs greater than 0.060 ppm and this value has declined by just 1 percentage point in 9 years

16. 16 Eastern State Emission Trends (VOC+NOx)  NOx reductions in eastern states have resulted in reductions in total NOx plus VOC emissions

17. 17 Western State Emission Trends (VOC+NOx)  NOx emissions have declined steadily since 1999  VOC emissions impacted by large-scale wildfire events  Ozone data impacted by exceptional events such as wildfires were excluded from DV trend calculations

18. 18 Conclusions  Most of the cleanest sites are located on windward coasts and in low population areas  Virtually every monitored location in the 48 states has DV exceeding 0.060 ppm  Emission reductions since 1999 have had virtually no impact on DVs at or below 0.060 ppm