Budgets for nocturnal VOC oxidation by nitrate radicals aloft during the 2006 Texas Air Quality Study

Industrial emissions in Houston, Texas, and along the U.S. Gulf Coast are a large source of highly reactive anthropogenic volatile organic compounds (VOCs), principally alkenes, that affect air quality in that region. Nighttime oxidation by either O₃ or NO₃ removes these VOCs. This paper presents a regional analysis of nighttime P-3 flights during the 2006 Texas Air Quality Study (TexAQS) to quantify the loss rates and budgets for both NO₃ and highly reactive VOC. Mixing ratios and production rates of NO₃ were large, up to 400 parts per trillion by volume (pptv) and 1-2 parts per billion by volume (ppbv) per hour, respectively. Budgets for NO₃ show that it was lost primarily to reaction with VOCs, with the sum of anthropogenic VOCs (30-54%) and isoprene (10-50%) being the largest contributors. Indirect loss of NO₃ to N₂O₅ hydrolysis was of lesser importance (14-28%) but was the least certain due to uncertainty in the aerosol uptake coefficient for N₂O₅. Reaction of NO₃ with peroxy radicals was a small but nonzero contribution to NO₃ loss but was also uncertain because there were no direct measurements of peroxy radicals. Net VOC oxidation rates were rapid (up to 2 ppbv VOC h⁻¹ in industrial plumes) and were dominated by NO₃, which was 3-5 times more important as an oxidant than O₃. Plumes of high NO₃ reactivity (i.e., short steady state lifetimes, on the order of 1 min) identified the presence of concentrated emissions of highly reactive VOCs from the Houston Ship Channel (HSC), which, depending on the particular VOC, may be efficiently oxidized during overnight transport.