A product study of the isoprene+NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; reaction

A E Perring,A L Lockwood,M Graus,A Wisthaler,P B Shepson,L H Mielke,R C Cohen,A Hansel,P J Wooldridge

doi:10.5194/acp-9-4945-2009

Abstract

Abstract. Oxidation of isoprene through reaction with NO3 radicals is a significant sink for isoprene that persists after dark. The main products of the reaction are multifunctional nitrates. These nitrates constitute a significant NOx sink in the nocturnal boundary layer and they likely play an important role in formation of secondary organic aerosol. Products of the isoprene+NO3 reaction will, in many locations, be abundant enough to affect nighttime radical chemistry and to persist into daytime where they may represent a source of NOx. Product formation in the isoprene + NO3 reaction was studied in a smog chamber at Purdue University. Isoprene nitrates and other hydrocarbon products were observed using Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and reactive nitrogen products were observed using Thermal Dissociation–Laser Induced Fluorescence (TD-LIF). The organic nitrate yield is found to be 65±12% of which the majority was nitrooxy carbonyls and the combined yield of methacrolein and methyl vinyl ketone (MACR+MVK) is found to be ∼10%. PTR-MS measurements of nitrooxy carbonyls and TD-LIF measurements of total organic nitrates agreed well. The PTR-MS also observed a series of minor oxidation products which were tentatively identified and their yields quantified These other oxidation products are used as additional constraints on the reaction mechanism.

Highlights

Global isoprene emissions are estimated to be 440–660 Tg/yr (Guenther et al, 2006) and are larger than emissions for any other single organic molecule (Guenther et al, 1995; Bey et al, 2001)
The Thermal Dissociation–Laser Induced Fluorescence (TD-LIF) instrument sampled from the chamber approximately once per hour and observed increases in both PNs
NOy was lost to the walls at an average rate of ∼4% per hour for the 4 h prior to injection, corresponding to a wall loss rate of 5·10−6 s−1 for N2O5 or 2.5×10−6s−1 for HNO3 it should be noted that the overall loss rate of N2O5 may be faster than the wall loss rate alone

Summary

Introduction

Global isoprene emissions are estimated to be 440–660 Tg/yr (Guenther et al, 2006) and are larger than emissions for any other single organic molecule (Guenther et al, 1995; Bey et al, 2001). Even minor changes in the oxidation of isoprene have a major influence on the chemistry of the atmosphere. Isoprene nitrate (IN) production, the focus of this manuscript, is calculated to be a major sink of NOx. The removal of NOx and possible subsequent release (Perring et al, 2009) affects regional and global ozone and the response of ozone to climate change (Horowitz et al, 2007; Wu et al, 2007). The aerosol effects are due both to the role of INs as a NOx sink and to the indirect role of INs as a regulator of available NOx and more directly to the subsequent oxidation of INs leading to low vapor pressure and/or highly water soluble molecules. INs are produced during the daytime as a minor product of OH-initiated isoprene oxidation in the presence of NO (Tuazon and Atkinson, 1990; Werner et al, 1999) and at night as a major product of oxidation by NO3 (Barnes et al, 1990; Skov et al, 1992; Berndt and Boge, 1997)

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Conclusion