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Published October 9, 2015 | Supplemental Material + Published
Journal Article Open

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

Abstract

Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NO_x environments. Production and loss of IN have a significant influence on the NO_x cycle and tropospheric O_3 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.

Additional Information

© 2015 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. Received: 3 June 2015 – Published in Atmos. Chem. Phys. Discuss.: 2 July 2015; Revised: 19 September 2015 – Accepted: 28 September 2015 – Published: 9 October 2015. We thank the organizers of the SOAS study, especially Ann Marie Carlton. We appreciate help from Jozef Peeters at the University of Leuven in elucidating the uncertainties associated with the current LIM1 mechanism. We acknowledge funding from the National Science Foundation (NSF) grant 1228496 and US Environmental Protection Agency (EPA) STAR grant 83540901.

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Supplemental Material - acp-15-11257-2015-supplement.pdf

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