Urban & Rural Ozone in Southern Arizona Westar Ozone Conference Salt Lake City, Utah March 2004 Arizona Department of Environmental Quality Peter Hyde

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 Importance of rural ozone concentrations Conclusions

8-hr Phoenix ozone concentrations in Phoenix  2001 – 2003 averages of the annual 4 th 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 ( )

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 sites PHX max ordered high to low 24 max days in

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

Hillside, AZ Ozone Site

Diurnal patterns in 1-hr ozone July 1998 averages

Diurnal patterns in 8-hr ozone July 1998 averages

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

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

Oxy compounds have variable concentrations & are above zero

Background NO & NO2 Levels are Low, but not zero

Rural vs Urban NOx

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: 1.The measured maximum (or design value) concentration 2.The value of the standard 3.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)

Meeting the standard, continued [O3]bk in ppb %R: % reduction to meet the 8-hr O3 standard with [O3]max = 100 ppb 016% 5032% 8080%

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.