Abstract. We show that methylglyoxal forms light-absorbing secondary organic material in aqueous ammonium sulfate and ammonium nitrate solutions mimicking tropospheric aerosol particles. The kinetics were characterized using UV-Vis spectrophotometry. The results suggest that the bimolecular reaction of methylglyoxal with an ammonium or hydronium ion is the rate-limiting step for the formation of light-absorbing species, with kNH4+II=5×10−6 M−1 min−1 and kH3O+II≤10−3 M−1 min−1. Evidence of aldol condensation products and oligomeric species up to 759 amu was found using chemical ionization mass spectrometry with a volatilization flow tube inlet (Aerosol-CIMS). Tentative identifications of carbon-nitrogen species and a sulfur-containing compound were also made using Aerosol-CIMS. Aqueous solutions of methylglyoxal, with and without inorganic salts, exhibit significant surface tension depression. These observations add to the growing body of evidence that dicarbonyl compounds may form secondary organic material in the aerosol aqueous phase, and that secondary organic aerosol formation via heterogeneous processes may affect seed aerosol properties.

Abstract. Light-absorbing and high-molecular-weight secondary organic products were observed to result from the reaction of glyoxal in mildly acidic (pH=4) aqueous inorganic salt solutions mimicking aqueous tropospheric aerosol particles. High-molecular-weight (500–600 amu) products were observed when ammonium sulfate ((NH4)2SO4) or sodium chloride (NaCl) was present in the aqueous phase. The products formed in (NH4)2SO4 or ammonium nitrate (NH4NO3) solutions absorb light at UV and visible wavelengths. Substantial absorption at 300–400 nm develops within two hours, and absorption between 400–600 nm develops within days. Pendant drop tensiometry measurements show that the products are not surface-active. The experimental results along with ab initio predictions of the UV/Vis absorption of potential products suggest a mechanism involving the participation of the ammonium ion. If similar products are formed in atmospheric aerosol particles, they could change the optical properties of the seed aerosol over its lifetime.

Glyoxal (G) and methylglyoxal (MG) are potentially important secondary organic aerosol (SOA) precursors. Previous studies of SOA formation by G and MG have focused on either species separately; however, G and MG typically coexist in the atmosphere. We studied the formation of secondary organic material in aqueous aerosol mimic mixtures containing G and MG with ammonium sulfate. We characterized the formation of light-absorbing products using UV-vis spectrophotometry. We found that absorption at 280 nm can be described well using models for the formation of light-absorbing products by G and MG in parallel. Pendant drop tensiometry measurements showed that surface tension depression by G and MG in these solutions can be modeled as a linear combination of the effects of G and MG alone. Product species were identified using chemical ionization mass spectrometry with a volatilization flow tube inlet (Aerosol CIMS). Peaks consistent with G-MG cross-reaction products were observed, accounting for a significant fraction of detected product mass, but most peaks could be attributed to self-reaction. We conclude that cross-reactions contribute to SOA mass from uptake of G and MG, but they are not required to accurately model the effects of this process on aerosol surface tension or light absorption.

Clouds, a key component of the climate system, form when water vapor condenses upon atmospheric particulates termed cloud condensation nuclei (CCN). Variations in CCN concentrations can profoundly impact cloud properties, with important effects on local and global climate. Organic matter constitutes a significant fraction of tropospheric aerosol mass, and can influence CCN activity by depressing surface tension, contributing solute, and influencing droplet activation kinetics by forming a barrier to water uptake. We present direct evidence that two ubiquitous atmospheric trace gases, methylglyoxal (MG) and acetaldehyde, known to be surface-active, can enhance aerosol CCN activity upon uptake. This effect is demonstrated by exposing acidified ammonium sulfate particles to 250 parts per billion (ppb) or 8 ppb gas-phase MG and/or acetaldehyde in an aerosol reaction chamber for up to 5 h. For the more atmospherically relevant experiments, i.e., the 8-ppb organic precursor concentrations, significant enhancements in CCN activity, up to 7.5% reduction in critical dry diameter for activation, are observed over a timescale of hours, without any detectable limitation in activation kinetics. This reduction in critical diameter enhances the apparent particle hygroscopicity up to 26%, which for ambient aerosol would lead to cloud droplet number concentration increases of 8-10% on average. The observed enhancements exceed what would be expected based on Köhler theory and bulk properties. Therefore, the effect may be attributed to the adsorption of MG and acetaldehyde to the gas-aerosol interface, leading to surface tension depression of the aerosol. We conclude that gas-phase surfactants may enhance CCN activity in the atmosphere.atmospheric chemistry | VOCs | indirect effect T he reactive uptake of volatile organic compounds (VOCs) by wet aerosols is a potentially important source of organic matter (OM) (1-3). The α-dicarbonyl species glyoxal and methylglyoxal (MG), along with acetaldehyde and other carbonylcontaining species, belong to this class; they are absorbed by wet aerosol particles (or cloud droplets) and undergo aqueous phase reactions to form low-volatility secondary organic aerosol (SOA) (2, 4-6). The impacts of these processes on aerosol cloud condensation nuclei (CCN) activity and cloud droplet formation are highly uncertain at this time. Few studies have focused on the impact of SOA generated in the aqueous phase on aerosol CCN activity (7-9). It was recently shown that the formation of SOA via the condensation of low-volatility VOC oxidation products, which are generally less hygroscopic than deliquescent inorganic salts, can affect the CCN activity of the seed aerosol (10-15). SOA generated through aqueous-phase chemistry is likely to be highly oxygenated and surface-active, hence making it strongly CCN-active (6,16,17). Some of the VOC precursors themselves, including MG and acetaldehyde, are also surface-active (6,16,17).We studied the changes in the CCN activity of acidified ammonium sulfate seed aerosol...

Surface-active organic material is a key component of atmospheric aerosols. The presence of surfactants can influence aerosol heterogeneous chemistry, cloud formation, and ice nucleation. We review the current state of the science on the sources, properties, and impacts of surfactants in atmospheric aerosols.