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

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