Field observations of regional and urban impacts on NO2, ozone, UVB, and nitrate radical production rates in the Phoenix air basin

In the May and June of 1998, field measurements were taken at a site near the Usery Pass Recreation Area, 27 miles from the downtown Phoenix area, overlooking Phoenix and Mesa, Arizona. This site was selected to examine the impacts of the Phoenix urban plume on the Usery Pass Recreation Area and surrounding regions. Data were obtained for ultraviolet-B (UVB) radiation, nitrogen dioxide (NO2), peroxyacetyl nitrate (PAN), ozone (O3), and carbon monoxide (CO). Nocturnal plumes of NO2 (in tens of ppb), observed near midnight, were correlated with CO and anti-correlated with O3. This behavior was consistent with the titration of locally generated NO by boundary layer O3 to form the nighttime NO2 plumes that were subsequently transported into the Usery Pass Recreation area. Nitrate radical (NO3) production rates were calculated to be very high on the edges of these nocturnal plumes. Examination of O3 and PAN data also indicates that Phoenix is being affected by long-range transport of pollutants from the Los Angeles to San Diego areas. A regional smoke episode was observed in May, accompanied by a decrease in UVB of factor of two and a decrease in O3 and an increase in methyl chloride. Low level back trajectories and chemical evidence confirm that the smoke event originated in northern Mexico and that the reduced O3 levels observed at Usery Pass could be partially due to reduced photolysis rates caused by carbonaceous soot aerosols transported in the smoke plume. The results are discussed with regard to potential effects of local pollution transport from the Phoenix air basin as well as an assessment of the contributions from long-range transport of pollutants to the background levels in the Phoenix-Usery Pass area.