Chamber studies of SOA formation from aromatic hydrocarbons: observation of limited glyoxal uptake

S. Nakao,D. R. Cocker III,C.-L. Chen,J. Zhang,P. Tang,Y. Liu

doi:10.5194/acp-12-3927-2012

Abstract

Abstract. This study evaluates the significance of glyoxal acting as an intermediate species leading to secondary organic aerosol (SOA) formation from aromatic hydrocarbon photooxidation under humid conditions. Rapid SOA formation from glyoxal uptake onto aqueous (NH4)2SO4 seed particles is observed in agreement with previous studies; however, glyoxal did not partition significantly to SOA (with or without aqueous seed) during aromatic hydrocarbon photooxidation within an environmental chamber (RH less than 80%). Rather, glyoxal influences SOA formation by raising hydroxyl (OH) radical concentrations. Four experimental approaches supporting this conclusion are presented in this paper: (1) increased SOA formation and decreased SOA volatility in the toluene + NOx photooxidation system with additional glyoxal was reproduced by matching OH radical concentrations through H2O2 addition; (2) glyoxal addition to SOA seed formed from toluene + NOx photooxidation did not increase SOA volume under dark; (3) SOA formation from toluene + NOx photooxidation with and without deliquesced (NH4)2SO4 seed resulted in similar SOA growth, consistent with a minor contribution from glyoxal uptake onto deliquesced seed and organic coatings; and (4) the fraction of a C4H9+ fragment (observed by Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer, HR-ToF-AMS) in SOA from 2-tert-butylphenol (BP) oxidation was unchanged in the presence of additional glyoxal despite enhanced SOA formation. This study suggests that glyoxal uptake onto aerosol during the oxidation of aromatic hydrocarbons is more limited than previously thought.

Highlights

Aerosol contributes to climate change and adversely affects air quality (Seinfeld and Pandis, 2006; Finlayson-Pitts and Pitts, 1999)
Significant Secondary organic aerosol (SOA) formation by glyoxal uptake onto deliquesced (NH4)2SO4 was observed under dark conditions (Fig. 2)
The significance of glyoxal uptake in SOA formation from aromatic hydrocarbon photooxidation was evaluated for the first time

Summary

Introduction

Aerosol contributes to climate change and adversely affects air quality (Seinfeld and Pandis, 2006; Finlayson-Pitts and Pitts, 1999). Secondary organic aerosol (SOA) is formed from oxidative processing of volatile organic compounds in the atmosphere. Previous researchers have estimated approximately 70 % of organic aerosols are secondary in nature (Hallquist et al, 2009 and references therein). SOA formation is described solely by gas-toparticle partitioning of semi-volatile oxidation products of volatile organic compounds (VOCs) (Odum et al, 1996; Pankow, 1994). Glyoxal was previously ignored as a SOA precursor due to its high vapor pressure (6 orders of magnitude too high, Volkamer et al, 2007); the current view is that glyoxal can contribute to SOA formation by uptake into water (cloud, fog, and wet aerosols) followed by radical and nonradical reactions to produce low volatility products (Lim et al, 2010 and references therein). The global emission of glyoxal is estimated to be 45 Tg yr−1 (Fu et al, 2008); globally, the major precursor of glyoxal is isoprene (21 Tg yr−1) (Fu et al, 2008), while aromatic hydrocarbons are the main precursors in urban areas (e.g., 70∼79 % in Mexico City, Volkamer et al, 2007)

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