On alkyl nitrates, O3, and the ‘‘missing NOy’’

We describe measurements of NO2, total peroxy nitrates (ÆPNs), total alkyl nitrates (ÆANs), and HNO3 using thermal dissociation followed by laser-induced fluorescence detection of NO2 at three continental locations. The ÆAN observations are unique and provide novel constraints on atmospheric photochemistry. At a rural site in California, measurements over a full annual cycle show that ÆANs are routinely 10–20% of NOy. At this rural site, at a suburban site in California and an urban site in Houston, Texas, both the absolute concentration of ÆANs and the fraction of the higher oxides of nitrogen (NOz) represented by ÆANs are greater than or equal to values reported in any prior observations. Although the contrast with prior observations is striking, we show that large abundances of ÆANs are consistent with simple chemical models of tropospheric ozone production and with the few prior comprehensive model studies. We also show that ÆANs are a large part, if not all, of the ‘‘missing NOy’’ reported in many prior experiments and emphasize that the ratios of ÆANs/NOz and of O3/ÆANs are especially useful for evaluating chemical models and comparing observations at different sites.