Mark Green and Dave DuBois Desert Research Institute

1. Estimating local versus regional contributions to tropospheric ozone: An example case study for Las Vegas Mark Green and Dave DuBois Desert Research Institute

2. The Problem • With new 8-hour ozone standard of 75 ppb and tendency of polluted air to affect downwind areas, local versus transported pollutant contributions to ozone is important • Monitoring concentrated in urban areas and large geographic areas of the southwestern US may exceed the new standards but sufficient data does not exist to define the areas exceeding standards • How do we even know where to monitor with limited resources to define the areas of high concentrations? • Use existing monitoring data, EI’s, and Chemical Transport Models to help design expanded monitoring network needed

3. This presentation • We present some results of analysis of local versus regional contributions to ground-level ozone in Las Vegas done in support of the Clark County Regional Ozone and Precursors Study (CCROPS) • We demonstrate some methodologies that might be useful elsewhere • Disclaimer/Excuse- we were paid by Clark County, Nevada only to collect upper air data (SODARS, a radar wind profiler, and radiosondes). They paid someone else for analysis of ozone patterns, but we did some analysis anyway because we were interested. So we were not as thorough as we would have been had we gotten paid to do the data analysis.

4. 4th highest O3 2005 Annual number of exceedances Annual number of exceedances Danger if you are downwind of California! (or Vegas or Phoenix)

5. Typical summer California wind flow pattern- combination of sea-breeze and mountain-valley wind circulations

6. Frequency of Tehachapi Pass tracer above background (July 12- Aug 31 1992) High frequency of flow from O3 rich San Joaquin Valley to the SE, then NE

7. Frequency of El Centro tracer above background (July 12 – Aug 31 1992) Convergence zone transports emissions from Mexicali through LA Basin and San Joaquin Valley to the north and east

8. 8 days (all in June and July) with one or more long-term Clark County sites greater than 8 hour standard of 85 ppb O3 (May and June had high frequency of troughs passing through) Nearly every day South Coast maximum > San Joaquin Valley maximum > Clark County maximum

9. High regional ozone 500 mb chart

10. Low regional ozone 500 mb chart

11. Summer 2005daily 8-hour maximums- Clark County, Jean, California Mohave Desert California Mojave Desert maximum typically considerably higher than Clark County maximum- provides high background

12. Summer 2005daily 8-hour maximums- Jean and Clark County maximums Clark County maximum and Jean track well- bigger offset in July to early August than May-June; more local impact?

13. Summer 2005daily 8-hour maximums- San Joaquin Valley, Mojave Desert, Palm Springs Climatology of California ozone patterns- August SJV max> Mohave max>Palm Springs, max shifts northward?

14. Used cluster analysis of wind field patterns to form groups of days with similar surface winds For each cluster of days, used cluster analysis to group hours with similar wind patterns Generated resultant winds and average ozone concentrations for each site for each daily cluster for each group of hours Used difference in ozone concentration from upwind to downwind of Las Vegas to estimate local versus regional transport contributions Generated and plotted HYSPLIT backtrajectories for each cluster Monitoring sites used in cluster analysis

15. Climatology of transport patterns helps build conceptual model- Local terrain forced flow common all summer; strong SW early summer, SJV+LV increment July peak

16. Nighttime downslope flow for terrain forced cluster

17. By 7-10 am heating of east facing Spring Mtns causes upslope flows

18. Late morning to afternoon, valley flow develops, transporting urban precursors to the northwest and O3 is high there. About 9 ppb local enhancement

19. Synoptic flows weak, HYSPLIT backtrajectories from all over, not real helpful

20. San Joaquin Valley + Las Vegas has light SW transport overnight, elevated O3 at high elevation sites

21. SJV + LV high O3 most of monitoring area, high background at Jean + local enhancement of about 9 ppb

22. Backtrajectories for SJV + LV show flow from SF Bay area through San Joaquin Valley, over Tehachapi pass and then into Las Vegas in convergence zone

23. Strong SW flow shows high background O3

24. Afternoon shows highest ozone at background site – 1 exceedance day at Jean

25. Backtrajectories for strong SW flow show about equal frequency from SJV and South Coast

26. Pattern summary table

27. 9-11 am 3-6 pm 7-10 pm 178-544 m AGL Much vertical change in wind direction in morning, becomes SW all levels afternoon 590-1047 m AGL 1183-3711 m AGL

28. Summary • Consideration of regional and local (and global?) transport important in understanding causes of high ozone • Spatial pattern analysis, Cluster analysis and backtrajectory analysis among the methods helpful in developing conceptual models • Far too few rural monitoring sites of O3 and precursors in the western US to understand extent and cause of high ozone in the WRAP region • Need coordinated monitoring, modeling, and data analysis effort over a large region • Use modeling, EI’s, and analysis of existing data to design monitoring networks