WESTAR Ozone Transport Analysis

1. WESTAR Ozone Transport Analysis Clinton P. MacDonald Dianne S. Miller Sean Raffuse Tim S. Dye Sonoma Technology, Inc. Petaluma, CA WESTAR Fall Business Meeting Boise, ID September 27, 2006 STI-3037

2. Overview • Project goal: To quantify the contribution of transported ozone to peak ozone concentrations in six western cities for several years • Phoenix, AZ • Las Vegas, NV • Denver, CO • Salt Lake City, UT • Farmington, NM • Seattle, WA

3. Source: http://www.al.noaa.gov/WWWHD/Pubdocs/assessment94/common-questions.html Background – Ozone (1 of 2) • Ozone occurs naturally • Concentrations of stratospheric ozone up to 12,000 ppb protect earth’s inhabitants from ultraviolet radiation. • Concentrations of tropospheric ozone are typically 35 to 40 ppb. • Ozone concentrations above natural background levels often occur due to photochemical reactions of NOx and VOCs.

4. Background – Ozone (2 of 2) • Sources of NOx and VOCs • automobile exhaust • solvent fumes • many other anthropogenic emission sources • natural emissions from trees and wildfires

5. Background – Peak Local Ozone • Peak local ozone concentrations • = • Natural ozone • + • Transported ozone(generated from emissions from upwind cities and natural events such as wildfires) • + • Local ozone(generated from local anthropogenic emissions)

6. Questions (1 of 2) • Background ozone • What is the typical natural background ozone concentration? • Transport • How much ozone, above natural background, was transported into each city? • What are the typical source areas for transported ozone?

7. Questions (2 of 2) • Local contribution • What amount of locally generated ozone was produced at each city? • Contribution comparison • On what percentage of high-ozone days was peak local 8-hr ozone dominated by locally generated ozone compared to transported ozone? • How do these percentages change for each city by transport direction? • How do the contribution amounts compare among the cities?

8. Methodology (1 of 3) Peak Local Ozone = Incoming (natural background + transported anthropogenic) + Local Anthropogenic • System developed to calculate incoming ozone • Ran multiple backward trajectories each day for five years (2001–2005, April–October); • Determined the daily peak 8-hr ozone concentration at the last site near which each trajectory passed during daylight hours prior to entering each city; and • Averaged all concentrations to estimate incoming boundary layer concentration each day.

9. Methodology (2 of 3) 72-hr backward trajectory • Site B is the last site passed, but it is in the buffer. • D is the next nearest site, but Site C is chosen because the trajectory passed Site C after Site D. • The trajectory would be assigned the daily peak 8-hr ozone concentration value from Site C. • If Site C had been passed overnight and Site D passed during the day (10 a.m. to 5 p.m. local time for the city), Site D would have been selected instead. City

10. Date: 07/18/2003 Incoming peak 8-hr ozone: 63 ppb Natural background ozone: 35 ppb Transported anthropogenic ozone: 28 ppb Locally generated anthropogenic ozone: 40 ppb Peak local 8-hr ozone: 103 ppb Dominant source direction: SE Salt Lake City Methodology (3 of 3) • Sample plot showing the trajectories and component concentrations for Salt Lake City on July 18, 2003 Peak Local Ozone = Incoming (natural background + transported anthropogenic) + Local Anthropogenic

11. Results – Salt Lake City • Computed averages from the daily results

12. Results – Las Vegas

13. Las Vegas Salt Lake City Results – Locally Generated Ozone

14. Results – Transported Ozone Salt Lake City Las Vegas

15. Results – Daily Contribution Las Vegas Salt Lake City

16. Results – Frequency, Salt Lake City Local days are characterized by local contribution that is at least two-thirds of the ozone beyond natural background. Transport days are characterized by transport contribution that is at least two-thirds of the ozone beyond natural background.

17. Results – Frequency, Las Vegas

18. Results – Highest Days, Salt Lake City Contributions and source regions for the five highest ozone days.

19. Results – Highest Days, Las Vegas Contributions and source regions for the five highest ozone days.

20. Results – Summary (1 of 2) • Summary of average contributions on days when peak local 8-hr ozone concentrations were at least 70 ppb

21. Results – Summary (2 of 2) • Summary of average contributions on days when peak local 8-hr ozone concentrations were at least 85 ppb

22. Recommendations (1 of 4) From Final Report and subsequent WESTAR Committee review • Expand the data set • Nighttime data • Tribal, industrial, and special study data from various organizations • Use transport information to determine placement of new rural monitors to better estimate background ozone • Determine whether certain transport directions are excluded because of lack of data • Las Vegas • Arizona – border transport issues • Use 20-year satellite ozone data set to investigate how background ozone levels are changing in time (new idea)

23. Recommendations (2 of 4) • Explore the regional and seasonal magnitude of natural background ozone • Investigate sources of ozone • Fires • Did ozone generated from fire smoke impact ozone concentrations in cities? • How much ozone was generated by fire smoke? • How does the contribution of smoke to ozone vary by transport direction, city, and time of year? • Anthropogenic emissions • Couple emission density maps from the WRAP modeling with trajectories

24. Recommendations (3 of 4) • Smoke from the Bar Complex and Ralston Fires and certain meteorological conditions led to high ozone in Sacramento. • This was the first two-day Unhealthy ozone episode in September in the past five years. • Estimates reveal that the smoke led to a doubling of NOx emissions for a typical summer day. • Based on historical ozone concentrations on days with similar weather conditions, we estimated that the smoke contributed approximately 10 to 15 ppb to the peak 8-hr average ozone concentrations.

25. Recommendations (4 of 4) • Determine meteorological characteristics often associated with high ozone conditions • Helps organizations determine episodes that may be caused by natural events • Helps direct the modification of natural events policies at the national level • Compare modeled ozone with results from this work (lower priority)

26. Final Report • MacDonald C.P., Miller D.S., and Raffuse S.M. (2006) Regional and local contributions to peak local ozone concentrations in six western cities. Final report prepared for the Western States Air Resources Council, Seattle, WA, by Sonoma Technology, Inc., Petaluma, CA, STI-906004-2970-FR, May.

27. Acknowledgments • The authors extend thanks to • Bob Lebens of WESTAR for managing this project and for his important technical advice • Other members of the WESTAR ozone work group for their valuable input: • Steve Arnold, Colorado Department of Environmental Quality • Mike Sundblom, Arizona Department of Environmental Quality • Phil Allen, Oregon Department of Environmental Quality • Brock LeBaron, Bob Clark, and Dave Strohm, Utah Division of Environment Quality