Ozone production in the Phoenix urban plume

In the summer of 1998, the Department of Energy's Atmospheric Chemistry Program conducted an aircraft and surface based field campaign in Phoenix, Arizona, with the overall goal of obtaining a mechanistic understanding of O₃ formation in the metropolitan area. Participants in the study included scientists from the Arizona DEQ, Argonne National Laboratory, Brookhaven National Laboratory, and Pacific Northwest National Laboratory. The DOE G-1 aircraft conducted flights in the morning and afternoon, sampling upwind, over, and downwind of the center city. Measurements included, O₃, NO, NO₂, NOy, VOC's, CO, H₂O₂, organic peroxides, HCHO, aerosol parameters, actinic flux, and meteorological variables. On most days, afternoon O₃ levels were within 20 ppb of morning levels, indicating a relatively inactive photochemistry, despite ample sunshine and high NOx levels. VOC reactivity was relatively low due in part to a near absence of isoprene. Photochemical box model calculations confirm the low production rates for O₃; averages are 4-5 ppb h^-1 downtown at mid-morning, decreasing to 2 ppb h^-1 at downwind locations late in the afternoon. Photochemical calculations are consistent with the following picture: NOx levels over the source region average about 10 ppb over a 2 km boundary layer in the mid-morning. High production rates for O₃ cannot occur until this NOx is removed by reaction or dilution. Removal by reaction is stoichiometrically limited by the rate at which radicals are formed. This rate turns out to be low because production of OH from O¹D is limited by the very dry atmosphere, and production of HO₂ from HCHO photolysis is limited because of the absence of a major precursor, isoprene. An analysis of the concentrations of quasi-conservative species indicates that dispersion only reduces NOx levels by about a factor of 2, between the source region and downwind. High NOx levels therefore persist though much of the peak hours for O₃ production, inhibiting the formation of O₃.