Evaluation of anthropogenic secondary organic aerosol tracersfrom aromatic hydrocarbons

Abstract

Products of secondary organic aerosol (SOA) from aromatic volatile organic compounds (VOCs) - 2,3-dihydroxy-4-oxopentanoic acid, dicarboxylic acids, nitromonoaromatics, and furandiones - were evaluated for their potential to serve as anthropogenic SOA tracer5with respect to their (1) ambient concentrations and detectability in PM2:5 in Iowa City, IA, USA; (2) gas-particle partitioning behaviour; and (3) source specificity by way of correlations with primary and secondary source tracersand literature review. A widely used tracer for toluene-derived SOA, 2,3-dihydroxy-4-oxopentanoic acid was only detected in the particle phase (Fp D 1) at low but consistently measurable ambient concentrations (averaging 0.3 ngm-3/. Four aromatic dicarboxylic acids were detected at relatively higher concentrations (9.1-34.5 ngm-3/, of which phthalic acid was the most abundant. Phthalic acid had a low particlephase fraction (Fp D0.26) likely due to quantitation interferences from phthalic anhydride, while 4-methylphthalic acid was predominantly in the particle phase (Fp D 0.82). Phthalic acid and 4-methylphthalic acid were both highly correlated with 2,3-dihydroxy-4-oxopentanoic acid (rsD 0.73, p D 0.003; r5D 0.80, p > 0.001, respectively), suggesting that they were derived from aromatic VOCs. Isophthalic and terephthalic acids, however, were detected only in the particle phase (Fp D 1), and correlations suggested association with primary emission sources. Nitromonoaromatics were dominated by particle-phase concentrations of 4-nitrocatechol (1.6 ngm-3/ and 4-methyl-5-nitrocatechol (1.6 ngm-3/ that were associated with biomass burning. Meanwhile, 4-hydroxy-3-nitrobenzyl alcohol was detected in a lower concentration (0.06 ngm-3/ in the particle phase only (Fp D 1) and is known as a product of toluene photooxidation. Furandiones in the atmosphere have only been attributed to the hotooxidation of aromatic hydrocarbons; however the substantial partitioning toward the gas phase (Fp =0.16) and their water sensitivity limit their application as tracers. The outcome of this study is the demonstration that 2,3-dihydroxy-4-oxopentanoic acid, phthalic acid, 4-methylphthalic acid, and 4-hydroxy-3-nitrobenzyl alcohol are good candidates for tracing SOA from aromatic VOCs.