UC Irvine

Alkyl and multifunctional nitrates (RONO2, Sigma ANs) have been observed to be a significant fraction of NOy in a number of different chemical regimes. Their formation is an important free radical chain termination step ending production of ozone and possibly affecting formation of secondary organic aerosol. Sigma ANs also represent a potentially large, unmeasured contribution to OH reactivity and are a major pathway for the removal of nitrogen oxides from the atmosphere. Numerous studies have investigated the role of nitrate formation from biogenic compounds and in the remote atmosphere. Less attention has been paid to the role Sigma ANs may play in the complex mixtures of hydrocarbons typical of urban settings. Measurements of total alkyl and multifunctional nitrates, NO2, total peroxy nitrates (Sigma PNs), HNO3 and a representative suite of hydrocarbons were obtained from the NASA DC-8 aircraft during spring of 2006 in and around Mexico City and the Gulf of Mexico. Sigma ANs were observed to be 10-20% of NOy in the Mexico City plume and to increase in importance with increased photochemical age. We describe three conclusions: (1) Correlations of Sigma ANs with odd-oxygen (O-x) indicate a stronger role for Sigma ANs in the photochemistry of Mexico City than is expected based on currently accepted photochemical mechanisms, (2) Sigma AN formation suppresses peak ozone production rates by as much as 40% in the near-field of Mexico City and (3) Sigma ANs play a significant role in the export of NOy from Mexico City to the Gulf Region.