Urban & Rural Ozone in Southern Arizona

1. Urban & Rural Ozone in Southern Arizona Westar Ozone Conference Salt Lake City, Utah March 2004 Arizona Department of Environmental Quality Peter Hyde 602 771-7642

2. OUTLINE 8-hr Phoenix ozone concentrations in Phoenix Relationship between urban (Phoenix) and rural elevated ozone concentrations Diurnal patterns of ozone differ between urban and rural settings Rural ozone: sources and chemistry Background concentrations of VOC and NOx

3. OUTLINE, Continued Importanceof rural ozone concentrations Conclusions

4. 8-hr Phoenix ozone concentrations in Phoenix • 2001 – 2003 averages of the annual 4th high values show two sites in violation of the standard • Concentrations have declined through the years • Proposed nonattainment area will include nearly all of metropolitan area and environs

7. Relationship between urban and rural ozone • Rural 8-hr ozone concentrations, on days with high urban values, are on average 50 to 65% of the urban values • On a day-by-day basis, this rural % varies widely, from 40 to 90%. • Why isn’t this rural % more constant?

8. Average 8-hr ozone: 24 max days in 96-02: metro PHX vs rural

9. Urban & rural components of 24 high 8-hr ozone days (1996- 2002)

10. Rural fraction of elevated urban 8-hr ozone concentrations (24 days)

11. Different rural sites differ in their fraction of the maximum urban value • Background sites do not have equal concentrations • They tend to cluster together, but differences among sites average 20 ppb and are as high as 45 ppb

12. 8-hr max ozone: PHX vs 5 rural sitesPHX max ordered high to low24 max days in 96 - 02

13. Different diurnal patterns between urban and rural • Afternoon maxima are similar throughout, but nocturnal minima differ • Those sites at a high enough elevation to be unaffected by surface temperature inversions have equal ozone concentrations day and night • Rural sites with extremely low emission densities, as long as they’re subject to a surface inversion, have low nocturnal O3

14. Characteristics of sites • Supersite: 1100 ft, midtown Phoenix • Salt R: 1150 ft, east fringe of metro area • Florence: 1505 ft, desert small town, 8,000 • Rye: hamlet, 3200 ft, narrow valley • Palo Verde: 880 ft, desert, isolated • Hillside: 4,200 ft, mountain, isolated • Mt. Ord: 7128 ft, mountain, isolated • Yuma: 138 ft, valley, town of 50,000

21. Hillside, AZ Ozone Site

22. Diurnal patterns in 1-hr ozoneJuly 1998 averages

23. Diurnal patterns in 8-hr ozoneJuly 1998 averages

24. The Origins of Rural Ozone • Without transport from urban areas • ¼ is from the stratosphere • ¾ is from natural tropospheric chemistry • Chemistry involves methane & biogenic VOC emissions NO & NO2 formaldehyde carbon monoxide hydroxy radical acetic and formic acids

25. Background concentrations of ozone precursors • VOC: about 25 ppbC, compared with 175 – 250 in Phoenix • Carbonyls: large sample-to-sample variation at remote background site (Hillside) • HC (2 to 12 C): much lower than urban, but not zero

26. Rural VOCs are 1/10 of urban, but still 25 ppbC

27. Oxy compounds have variable concentrations & are above zero

28. Background NO & NO2 Levels are Low, but not zero

29. Rural vs Urban NOx

30. Importance of rural ozone • Can serve as background for urban O3, but caution is necessary • Measurements of rural ozone necessary, perhaps at more than one site. • Understanding of geographical setting is important • Background value determines degree of control necessary to meet standards

31. Meeting the standard when considering background values The percentage reduction to meet an air quality standard depends on three variables: • The measured maximum (or design value) concentration • The value of the standard • The value of the background that will be unaffected by any emission reductions

32. Meeting the standard, continued • %R = ([O3]max – [O3]std) * 100% • ([O3]max – [O3]bk) • Where • [O3]max = maximum or design value • [O3]std = 8-hr ozone standard = 84 ppb • [O3]bk = background ozone value (40 – 80 ppb)

33. Meeting the standard, continued

34. Conclusions • Elevated concentrations of rural ozone are not well synchronized with elevated urban ozone. • Rural ozone is strongly influenced by mountain-top geography. • Rural ozone appears nearly independent of the degree of nearby population (emissions).

35. Conclusions, continued • At the remotest background locations, VOC and NOx are non-zero and measurable. • The best background location for forecasting, modeling, or understanding urban ozone would be upwind of the city and would have a similar diurnal profile as the urban.