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Observations of peroxynitric acid (HO2NO2) obtained simultaneouslywith those of NO and NO2 provide a sensitive measure of theozone photochemical production rate. We illustrate this technique forconstraining the ozone production rate with observations obtained from theNCAR C-130 aircraft platform during the Megacity Initiative: Local and GlobalResearch Observations (MILAGRO) intensive in Mexico during the spring of2006. Sensitive and selective measurements of HO2NO2 were made insitu using chemical ionization mass spectrometry (CIMS). Observations werecompared to modeled HO2NO2 concentrations obtained from the NASALangley highly-constrained photochemical time-dependent box model. The medianobserved-to-calculated ratio of HO2NO2 is 1.18. At NOxlevels greater than 15 ppbv, the photochemical box model underpredictsobservations with an observed-to-calculated ratio of HO2NO2 of 1.57.As a result, we find that at high NOx, the ozone production ratecalculated using measured HO2NO2 is faster than predicted usingaccepted photochemistry. Inclusion of an additional HOx source fromthe reaction of excited state NO2 with H2O or reduction inthe rate constant of the reaction of OH with NO2 improves theagreement

Inferring ozone production in an urban atmosphere using measurements of peroxynitric acid