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 O-3 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 O-3. 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.