Role of methyl group number on SOA formation from monocyclic aromatic hydrocarbons photooxidation under low-NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; conditions

L Li,C.-L Chen,S Nakao,P Tang,D R Cocker Iii

doi:10.5194/acp-16-2255-2016

Abstract

Abstract. Substitution of methyl groups onto the aromatic ring determines the secondary organic aerosol (SOA) formation from the monocyclic aromatic hydrocarbon precursor (SOA yield and chemical composition). This study links the number of methyl groups on the aromatic ring to SOA formation from monocyclic aromatic hydrocarbons photooxidation under low-NOx conditions (HC/NO > 10 ppbC : ppb). Monocyclic aromatic hydrocarbons with increasing numbers of methyl groups are systematically studied. SOA formation from pentamethylbenzene and hexamethylbenzene are reported for the first time. A decreasing SOA yield with increasing number of methyl groups is observed. Linear trends are found in both f44 vs. f43 and O / C vs. H / C for SOA from monocyclic aromatic hydrocarbons with zero to six methyl groups. An SOA oxidation state predictive method based on benzene is used to examine the effect of added methyl groups on aromatic oxidation under low-NOx conditions. Further, the impact of methyl group number on density and volatility of SOA from monocyclic aromatic hydrocarbons is explored. Finally, a mechanism for methyl group impact on SOA formation is suggested. Overall, this work suggests that, as more methyl groups are attached on the aromatic ring, SOA products from these monocyclic aromatic hydrocarbons become less oxidized per mass/carbon on the basis of SOA yield or chemical composition.

Highlights

Aromatic hydrocarbons are major anthropogenic secondary organic aerosol (SOA) precursors (Kanakidou et al, 2005; Henze et al, 2008)
SOA yields from the photooxidation of seven monocyclic aromatic hydrocarbons are calculated as the mass-based ratio of aerosol formed to hydrocarbon reacted (Odum et al, 1996)
SOA yield as a function of particle mass concentration (M0) for all seven monocyclic aromatic precursors (Fig. 1) includes experiments listed in both Table 1 and S2

Summary

Introduction

Aromatic hydrocarbons are major anthropogenic secondary organic aerosol (SOA) precursors (Kanakidou et al, 2005; Henze et al, 2008). Monocyclic aromatic hydrocarbons with more than four methyl groups have scarcely been investigated in previous ambient studies, possibly due to a vapor pressure decrease with carbon number (Pankow and Asher, 2008; Table S1 in the Supplement). A recent study observed that compounds with low vapor pressure are available to evaporate into the atmosphere (Voand Morris, 2014). Hydrocarbon reactivity and OH reaction rate constant (kOH) increase with methyl group number (kOH Table S1; Glasson and Tuesday, 1970; Calvert et al, 2002; Atkinson and Arey, 2003; Aschmann et al, 2013). Photooxidation occurs rapidly once these low-vapor-pressure aromatic hydrocarbons evaporate into atmosphere. An increase in carbon number is associated with a decrease in vapor pressure (Pankow and Asher, 2008). Higher-carbonnumber products with a similar number of functional groups

Methods

Results

Conclusion