Observations of peroxynitric acid (HO₂ NO₂ ) obtained simultaneously with those of NO and NO₂ provide a sensitive measure of the ozone photochemical production rate. We illustrate this technique for constraining the ozone production rate with observations obtained from the NCAR C-130 aircraft platform during the Megacity Initiative: Local and Global Research Observations (MILAGRO) intensive in Mexico during the spring of 2006. Sensitive and selective measurements of HO₂NO₂ were made in situ using chemical ionization mass spectrometry (CIMS). Observations were compared to modeled HO₂NO₂ concentrations obtained from the NASA Langley highly-constrained photochemical time-dependent box model. The median observed-to-calculated ratio of HO₂NO₂ is 1.18. At NOx levels greater than 15 ppbv, the photochemical box model underpredicts observations with an observed-to-calculated ratio of HO₂NO₂ of 1.57. As a result, we find that at high NOx, the ozone production rate calculated using measured HO₂NO₂ is faster than predicted using accepted photochemistry. Inclusion of an additional HOx source from the reaction of excited state NO₂ with H₂O or reduction in the rate constant of the reaction of OH with NO₂ improves the agreement.
