PNNL

Abstract

Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NOx environ-ments. Production and loss of IN have a signi?cant in?u-ence on the NOx cycle and tropospheric O3 chemistry. To better understand IN chemistry, a series of photochemical re-action chamber experiments was conducted to determine the IN yield from isoprene photooxidation at high NO concentra-tions (> 100 ppt). By combining experimental data and cal-culated 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 ob-served in a temperate forest atmosphere, during the Southern Oxidant and Aerosol Study (SOAS) ?eld 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 ?eld observations with model simulations, we identi?ed NO as the limiting factor for ambient IN produc-tion 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 after-noon. A signi?cant 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.