NOy partitioning and the role of alkyl nitrates in Tecamac, Mexico

The reactive nitrogen oxide (NOy) budget observed at the T1 site in Tecamac, MX during the MILAGRO campaign in March 2006 allows us to probe the role of alkyl nitrates (ANs) and peroxy nitrates (PNs) in a polluted megacity region. Comparing data from the Mexico City region to previous field sites at Granite Bay, California and Houston, Texas demonstrate that each site has a characteristic nitric acid and alkyl nitrate production efficiency depending on the balance of NOx and VOCs. This balance in turn affects the ozone production efficiency. Unlike Granite Bay and Houston, little isoprene was observed at Tecamac: the result is an alkyl nitrate (AN) population with a higher observed branching ratio (~7 %). The observed branching ratio is consistent with predictions based on the VOC distribution and predicted OH reactivity at the site. These predictions suggest that aromatics and multifunctional ANs such as hydroxyalkyl nitrates make up a large fraction of ANs in the Mexico City plume, and have implications to ozone production and air quality in the region.