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Observations of peroxynitric acid (HO<sub>2</sub>NO<sub>2</sub>) obtained simultaneously with those of NO and NO<sub>2</sub> 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 HO<sub>2</sub>NO<sub>2</sub> were made in situ using chemical ionization mass spectrometry (CIMS). Observations were compared to modeled HO<sub>2</sub>NO<sub>2</sub> concentrations obtained from the NASA Langley highly-constrained photochemical time-dependent box model. The median observed-to-calculated ratio of HO<sub>2</sub>NO<sub>2</sub> is 1.18. At NO<sub>x</sub> levels greater than 15 ppbv, the photochemical box model underpredicts observations with an observed-to-calculated ratio of HO<sub>2</sub>NO<sub>2</sub> of 1.57. As a result, we find that at high NO<sub>x</sub>, the ozone production rate calculated using measured HO<sub>2</sub>NO<sub>2</sub> is faster than predicted using accepted photochemistry. Inclusion of an additional HO<sub>x</sub> source from the reaction of excited state NO<sub>2</sub> with H<sub>2</sub>O or reduction in the rate constant of the reaction of OH with NO<sub>2</sub> improves the agreement.