On alkyl nitrates, O3, and the “missing NOy”

We describe measurements of NO 2 , total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), and HNO 3 using thermal dissociation followed by laser‐induced fluorescence detection of NO 2 at three continental locations. The ΣAN observations are unique and provide novel constraints on atmospheric photochemistry. At a rural site in California, measurements over a full annual cycle show that ΣANs are routinely 10–20% of NO y . At this rural site, at a suburban site in California and an urban site in Houston, Texas, both the absolute concentration of ΣANs and the fraction of the higher oxides of nitrogen (NO z ) represented by ΣANs 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 ΣANs are consistent with simple chemical models of tropospheric ozone production and with the few prior comprehensive model studies. We also show that ΣANs are a large part, if not all, of the “missing NO y ” reported in many prior experiments and emphasize that the ratios of ΣANs/NO z and of O 3 /ΣANs are especially useful for evaluating chemical models and comparing observations at different sites.