Abstract
Abstract. Previous experiments have demonstrated that the aqueous OH radical oxidation of methylglyoxal produces low volatility products including pyruvate, oxalate and oligomers. These products are found predominantly in the particle phase in the atmosphere, suggesting that methylglyoxal is a precursor of secondary organic aerosol (SOA). Acetic acid plays a central role in the aqueous oxidation of methylglyoxal and it is a ubiquitous product of gas phase photochemistry, making it a potential "aqueous" SOA precursor in its own right. However, the fate of acetic acid upon aqueous-phase oxidation is not well understood. In this research, acetic acid (20 μM–10 mM) was oxidized by OH radicals, and pyruvic acid and methylglyoxal experimental samples were analyzed using new analytical methods, in order to better understand the formation of SOA from acetic acid and methylglyoxal. Glyoxylic, glycolic, and oxalic acids formed from acetic acid and OH radicals. In contrast to the aqueous OH radical oxidation of methylglyoxal, the aqueous OH radical oxidation of acetic acid did not produce succinic acid and oligomers. This suggests that the methylgloxal-derived oligomers do not form through the acid catalyzed esterification pathway proposed previously. Using results from these experiments, radical mechanisms responsible for oligomer formation from methylglyoxal oxidation in clouds and wet aerosols are proposed. The importance of acetic acid/acetate as an SOA precursor is also discussed. We hypothesize that this and similar chemistry is central to the daytime formation of oligomers in wet aerosols.
Highlights
Large uncertainties remain in the predicted impact of secondary organic aerosol (SOA) on air quality, climate and human health (Kanakidou et al, 2005; Hallquist et al, 2009)
The formation of glyoxylic, glycolic and oxalic acid is supported by the existence of products with m/z− of 73, 75 and 89 in the online electrospray ionization mass spectrometry (ESI-MS) analysis of the aqueous OH radical oxidation of acetic acid (Fig. 1), and was further verified by ion chromatography (IC)-ESI-MS analysis (Fig. 2)
With the exception of glycolic acid, this dilute aqueous acetic acid chemistry is contained in the methylglyoxal plus OH radical chemical model published by Tan et al (2010)
Summary
Large uncertainties remain in the predicted impact of secondary organic aerosol (SOA) on air quality, climate and human health (Kanakidou et al, 2005; Hallquist et al, 2009). While the refined treatment of intermediate volatility organic compounds (IVOCs) has in some cases brought predicted organic aerosol mass into better alignment with measured mass (Pye et al, 2010; Jathar et al, 2011), formation of SOA through gas phase reactions followed by partitioning into particulate organic matter (from traditional or IVOC precursors) does not account for observed atmospheric SOA properties. Products of gas-phase photochemistry have greater access to liquid water than to particulate organic matter for partitioning. Laboratory and modeling experiments demonstrate that aqueous oxidation of small water-soluble organic compounds forms SOA with high O/C ratios
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