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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. OUTLINE. 8-hr Phoenix ozone concentrations in Phoenix
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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
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
OUTLINE, Continued Importanceof rural ozone concentrations Conclusions
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
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?
Average 8-hr ozone: 24 max days in 96-02: metro PHX vs rural
Urban & rural components of 24 high 8-hr ozone days (1996- 2002)
Rural fraction of elevated urban 8-hr ozone concentrations (24 days)
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
8-hr max ozone: PHX vs 5 rural sitesPHX max ordered high to low24 max days in 96 - 02
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
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
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
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
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
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
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)
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).
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.