The photochemical formation and fate of nitric acid in the metropolitan detroit area: Ambient, captive-air irradiation and modeling results

Abstract

Outdoor smog chamber experiments, a photochemical model and ambient air measurements were used to evaluate the formation rate and fate of HNO 3 on photochemically active days in the Detroit metropolitan area during the summer of 1981. The primary tool used to ascertain the rate of HNO 3 formation and its dependence on the O 3 precursors, nonmethane hydrocarbons (NMHC) and nitrogen oxides ( NO x = NO + NO 2) was outdoor smog chambers filled with ambient air. Based on the NO x decay observed in the chambers, a first-order (in NO x ) HNO 3 formation coefficient, k{ HNO 3}, of 10.6 ± 1.9 % h −1 was derived from 14 air samples irradiated on 10 sunny days. Additions to the chambers at the start of the irradiations of NMHC, NO x or clean air did not strongly affect k{ HNO 3}. In contrast, O 3 formation was markedly affected by the additions; NO x additions inhibited O 3 formation, whereas NMHC additions enhanced ozone formation. A photochemical model, referred to as the carbon-bond model, version 3 (CB-3), was used to predict kHNO 3, the sensitivity of kHNO 3 to the additions, and the nitrogen-containing product distribution in the chambers. While the model satisfactorily predicted the mean kHNO 3, it failed to predict the observed insensitivity of kHNO 3 to the additions. The model also predicted that HNO 3 was by far the major nitrogen-containing product of NO x oxidation by a ratio of 3:1 over all other species; peroxyacetyl nitrate (PAN) accounted for most of the rest of the oxidized NO x . The atmospheric concentrations of HNO 3 expected on photochemically active days (in the absence of HNO 3 removal) were predicted using the rate of formation of HNO 3 relative to O 3 formation in the chambers together with the net amount of O 3 formed photochemically in the atmosphere. The PAN concentration was also predicted, using the yield of PAN relative to HNO 3 obtained from the CB-3 model. The predictions were compared to maximum atmospheric concentrations of these species derived from a set of measurements at sites downwind of the city. The comparison suggests that the photochemical HNO 3 source was strongly attenuated by HNO 3 removal processes, the most likely of which was its reaction with basic, soil-related aerosols.