Abstract
Abstract. The reactive uptake of carbonyl-containing volatile organic compounds (cVOCs) by aqueous atmospheric aerosols is a likely source of particulate organic material. The aqueous-phase secondary organic products of some cVOCs are surface-active. Therefore, cVOC uptake can lead to organic film formation at the gas-aerosol interface and changes in aerosol surface tension. We examined the chemical reactions of two abundant cVOCs, formaldehyde and acetaldehyde, in water and aqueous ammonium sulfate (AS) solutions mimicking tropospheric aerosols. Secondary organic products were identified using Aerosol Chemical Ionization Mass Spectrometry (Aerosol-CIMS), and changes in surface tension were monitored using pendant drop tensiometry. Hemiacetal oligomers and aldol condensation products were identified using Aerosol-CIMS. Acetaldehyde depresses surface tension to 65(±2) dyn cm−1 in pure water (a 10% surface tension reduction from that of pure water) and 62(±1) dyn cm−1 in AS solutions (a 20.6% reduction from that of a 3.1 M AS solution). Surface tension depression by formaldehyde in pure water is negligible; in AS solutions, a 9% reduction in surface tension is observed. Mixtures of these species were also studied in combination with methylglyoxal in order to evaluate the influence of cross-reactions on surface tension depression and product formation in these systems. We find that surface tension depression in the solutions containing mixed cVOCs exceeds that predicted by an additive model based on the single-species isotherms.
Highlights
Organic material is a ubiquitous component of atmospheric aerosols, making up a major fraction of fine aerosol mass, but its sources and influence on aerosol properties are still poorly constrained (Jimenez et al, 2009; Kanakidou et al, 2005)
We investigated the chemical reactions of formaldehyde and acetaldehyde in pure water and concentrated ammonium sulfate (AS) solutions mimicking aerosol water
Aqueous solutions containing varying concentrations of organic compounds with near-saturation concentrations (3.1 M) of AS were prepared in 100 ml Pyrex vessels using Millipore water
Summary
Organic material is a ubiquitous component of atmospheric aerosols, making up a major fraction of fine aerosol mass, but its sources and influence on aerosol properties are still poorly constrained (Jimenez et al, 2009; Kanakidou et al, 2005). Many common organic aerosol species are surfaceactive (Facchini et al, 1999; Shulman et al, 1996). Depressed aerosol surface tension due to film formation may lead to a decrease in the critical supersaturation required for the particle to activate and grow into a cloud droplet as described by Kohler Theory (Kohler, 1936). The surface tension of atmospheric aerosol samples tends to be lower than that predicted based on the combined effects of the individual surfactants identified in the aerosol (Facchini et al, 1999). This is in part because some surfaceactive aerosol organics remain unidentified. The effects of interactions among these species under typical aerosol conditions (i.e. supersaturated salt concentrations, acidic, multiple organic species) are generally unknown
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