Analysis of the processing of Nashville urban emissions on July 3 and July18, 1995

This paper analyzes data obtained on July 3 and 18, 1995, during the summer 1995 Southern Oxidant Study (SOS) field campaign. In a previous paper [Nun nermacker et al., 1998] we analyzed measurements of key species that contri bute to formation of O-3 in the Nashville urban plume and presented a semiq uantitative picture of O-3 production in the plume from the point of emissi on to locations where no net O-3 was being formed. In this paper we use a b ox model constrained by observed concentrations of stable species to obtain a detailed mechanistic description of the instantaneous processing of urba n emissions at various times in the chemical evolution of the urban plume. Instantaneous ozone production rates and efficiencies with respect to NOx a nd to primary radical production are examined. At high NOx concentrations i n the fresh urban plume the O-3 production rate was found to be directly pr oportional to the hydrocarbon to NOx reactivity ratio. At lower NO, concent rations, corresponding to the mature urban plume and the background atmosph ere, the O-3 production rate was found to be directly proportional to the N Ox concentration and independent of the hydrocarbon reactivity. NOx was fou nd to be most efficiently used for ozone production at low NOx concentratio ns. In contrast, the efficiency with which the system uses primary radicals was found to be very low at low NOx concentrations and to peak at a NO, co ncentration of approximately 4 ppbv. A sensitivity study of the instantaneo us O-3 production rates to changes in NOx or hydrocarbon concentrations sho wed that the instantaneous O-3 production rate at the center of the urban p lume, when half of the urban NOx emissions had been processed, is hydrocarb on sensitive. However, O-3 production becomes NOx sensitive as the plume ma tures.