Articles | Volume 15, issue 19
Research article
09 Oct 2015
Research article |  | 09 Oct 2015

Observation of isoprene hydroxynitrates in the southeastern United States and implications for the fate of NOx

F. Xiong, K. M. McAvey, K. A. Pratt, C. J. Groff, M. A. Hostetler, M. A. Lipton, T. K. Starn, J. V. Seeley, S. B. Bertman, A. P. Teng, J. D. Crounse, T. B. Nguyen, P. O. Wennberg, P. K. Misztal, A. H. Goldstein, A. B. Guenther, A. R. Koss, K. F. Olson, J. A. de Gouw, K. Baumann, E. S. Edgerton, P. A. Feiner, L. Zhang, D. O. Miller, W. H. Brune, and P. B. Shepson

Abstract. Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NOx environments. Production and loss of IN have a significant influence on the NOx cycle and tropospheric O3 chemistry. To better understand IN chemistry, a series of photochemical reaction chamber experiments was conducted to determine the IN yield from isoprene photooxidation at high NO concentrations (> 100 ppt). By combining experimental data and calculated isomer distributions, a total IN yield of 9(+4/−3) % was derived. The result was applied in a zero-dimensional model to simulate production and loss of ambient IN observed in a temperate forest atmosphere, during the Southern Oxidant and Aerosol Study (SOAS) field campaign, from 27 May to 11 July 2013. The 9 % yield was consistent with the observed IN/(MVK+MACR) ratios observed during SOAS. By comparing field observations with model simulations, we identified NO as the limiting factor for ambient IN production during SOAS, but vertical mixing at dawn might also contribute (~ 27 %) to IN dynamics. A close examination of isoprene's oxidation products indicates that its oxidation transitioned from a high-NO dominant chemical regime in the morning into a low-NO dominant regime in the afternoon. A significant amount of IN produced in the morning high NO regime could be oxidized in the low NO regime, and a possible reaction scheme was proposed.

Short summary
Hydroxynitrates from isoprene oxidation were quantified both in the laboratory and through field studies. The yield of hydroxynitrates 9(+4/-3)% derived from chamber experiments was applied in a zero-dimensional model to simulate the production and loss of isoprene hydroxynitrates in an ambient environment during the 2013 Southern Oxidant and Aerosol Study (SOAS). NOx was determined to be the limiting factor for the formation of isoprene hydroxynitrates during SOAS.
Final-revised paper