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

Abstract

[1] 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 NO 3 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 NO 3 and highly reactive VOC. Mixing ratios and production rates of NO 3 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 NO 3 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 NO 3 to N 2 O 5 hydrolysis was of lesser importance (14-28%) but was the least certain due to uncertainty in the aerosol uptake coefficient for N 2 O 5 . Reaction of NO 3 with peroxy radicals was a small but nonzero contribution to NO 3 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 NO 3 , which was 3-5 times more important as an oxidant than O 3 . Plumes of high NO 3 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.

Radical chemistry at night: comparisons between observed and modelled HO<sub>x</sub>, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> during the RONOCO project

“…DMS is a potential sink for NO 3 and source of RO x radicals, with previous measurements at ground level in marine and coastal regions showing DMS to be a significant sink for NO 3 (Carslaw et al, 1997;Allan et al, 1999). DMS concentrations reported over the North Sea and at the Weybourne Atmospheric Observatory on the North Sea coast have indicated high variability at ground level, varying from < 10 to over 300 ppt (Allan et al, 1999;Burgermeister and Georgii, 1991;Carslaw et al, 1997) and typically decreasing rapidly with altitude (Blake et al, 1999;Lunden et al, 2010;Spicer et al, 1996;Andreae et al, 1985). However, DMS was not observed above its 3 ppt limit of detection during the RONOCO campaign.…”

“…Both laboratory and field studies of the value of γ N 2 O 5 are highly variable Riedel et al, 2012;Brown et al, 2011;Escoreia et al, 2010;Tang et al, 2010;Macintyre and Evans, 2010;Brown et al, 2006Brown et al, , 2009Badger et al, 2006;Thornton and Abbatt, 2005;Hallquist et al, 2003;Thornton et al, 2003;Kane et al, 2001;Hu and Abbatt, 1997;Fried et al, 1994;Van Doren et al, 1991;Hanson and Ravishankara, 1991;Mozurkewich and Calvert, 1998) with values varying over an order of magnitude (10 −4 -1). The base model uses a fixed value of γ N 2 O 5 = 0.02, based on a global mean value (Evans and Jacob, 2005).…”

“…Measurements of NO 3 have been overestimated by model calculations in several studies (Mihelcic et al, 1993;Sommariva et al, , 2007, with those of nighttime OH and HO 2 radicals typically underestimated, indicating poor understanding of nighttime tropospheric oxidation processes (Kanaya et al, 1999(Kanaya et al, , 2002(Kanaya et al, , 2007aEmmerson and Carslaw, 2009;Geyer et al, 2003;Faloona et al, 2001;Martinez et al, 2003;Ren et al, 2006). While a number of nighttime studies at ground level close to local sources of NO have observed a limited role of NO 3 in nighttime radical production owing to surface losses of NO 3 and the rapid reaction between NO 3 and NO (Salisbury et al, 2001;Fleming et al, 2006;Sommariva et al, 2007;Kanaya et al, 1999Kanaya et al, , 2002Kanaya et al, , 2007aEmmerson and Carslaw, 2009;Faloona et al, 2001;Martinez et al, 2003;Ren et al, 2003Ren et al, , 2005Ren et al, , 2006Volkamer et al, 2010), several studies of NO 3 and N 2 O 5 above ground level and in more remote regions have indicated a more significant role for NO 3 in nighttime radical production and tropospheric oxidation (Platt et al, 1980;Povey et al, 1998;South et al, 1998;Aliwell et al, 1998;Allan et al, 2002;Stutz et al, 2004;Brown et al, 2003Brown et al, , 2004Brown et al, , 2006Brown et al, , 2007Brown et al, , 2009...…”

“…When primary production of OH by solar photolysis cannot occur, other oxidants dominate, notably O 3 and NO 3 (Mihelcic et al, 1993;Carslaw et al, 1997;Salisbury et al, 2001;Fleming et al, 2006;Warneke et al, 2004;Brown et al, 2009Brown et al, , 2011.…”

New insights into nocturnal nucleation

“…Figure 1 shows the time evolution of the concentrations of the total particle number, ozone and monoterpenes during the experiments. Formation of new particles started earlier for monoterpenes having higher reaction rates with ozone (for the reaction rates, see Calogirou et al, 1998), the minimum ozone level for nucleation to take place being 10 ppb for limonene, 15 ppb for α-pinene and 19 ppb for 3 -carene. Figures 2, 3 and 4 show the number size distributions of particles and ions in the experiments.…”