There is a growing understanding that secondary organic aerosol (SOA) can form through reactions in atmospheric waters (i.e., clouds, fogs, and aerosol water). In clouds and wet aerosols, water-soluble organic products of gas-phase photochemistry dissolve into the aqueous phase where they can react further (e.g., with OH radicals) to form low volatility products that are largely retained in the particle phase. Organic acids, oligomers and other products form via radical and non-radical reactions, including hemiacetal formation during droplet evaporation, acid/base catalysis, and reaction of organics with other constituents (e.g., NH₄⁺). <br><br> This paper provides an overview of SOA formation through aqueous chemistry, including atmospheric evidence for this process and a review of radical and non-radical chemistry, using glyoxal as a model precursor. Previously unreported analyses and new kinetic modeling are reported herein to support the discussion of radical chemistry. Results suggest that reactions with OH radicals tend to be faster and form more SOA than non-radical reactions. In clouds these reactions yield organic acids, whereas in wet aerosols they yield large multifunctional humic-like substances formed via radical-radical reactions and their O/C ratios are near 1

Previous experiments demonstrated that aqueous OH radical oxidation of glyoxal yields low-volatility compounds. When this chemistry takes place in clouds and fogs, followed by droplet evaporation (or if it occurs in aerosol water), the products are expected to remain partially in the particle phase, forming secondary organic aerosol (SOA). Acidic sulfate exists ubiquitously in atmospheric water and has been shown to enhance SOA formation through aerosol phase reactions. In this work, we investigate how starting concentrations of glyoxal (30−3000 μM) and the presence of acidic sulfate (0−840 μM) affect product formation in the aqueous reaction between glyoxal and OH radical. The oxalic acid yield decreased with increasing precursor concentrations, and the presence of sulfuric acid did not alter oxalic acid concentrations significantly. A dilute aqueous chemistry model successfully reproduced oxalic acid concentrations, when the experiment was performed at cloud-relevant concentrations (glyoxal <300 μM), but predictions deviated from measurements at increasing concentrations. Results elucidate similarities and differences in aqueous glyoxal chemistry in clouds and in wet aerosols. They validate for the first time the accuracy of model predictions at cloud-relevant concentrations. These results suggest that cloud processing of glyoxal could be an important source of SOA.

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Aqueous hydroxyl radical (∼10 −12 M) oxidation of glycolaldehyde (1 mM), followed by droplet evaporation, forms secondary organic aerosol (SOA) that exhibits an effective liquid vapor pressure and enthalpy of vaporization of ∼10 −7 atm and ∼70 kJ/mol, respectively, similar to the mix of organic acids identified in reaction samples. Salts of these acids have vapor pressures about three orders of magnitude lower (e.g., ammonium succinate ∼10 −11 atm), suggesting that the gas-particle partitioning behavior of glycolaldehyde SOA depends strongly on whether products are present in the atmosphere as acids or salts. Several reaction samples were used to simulate cloud droplet evaporation using a vibrating orifice aerosol generator. Samples were also analyzed by ion chromatography (IC), electrospray ionization mass spectrometry (ESI-MS), IC-ESI-MS, and for total carbon. Glycolaldehyde SOA mass yields were 50-120%, somewhat higher than yields reported previously (40-60%). Possible reasons are discussed: (1) formation of oligomers from droplet evaporation, (2) inclusion of unquantified products formed by aqueous photooxidation, (3) differences in gas-particle partitioning, and (4) water retention in dried particles. These and similar results help to explain the enrichment of organic acids in particulate organic matter above clouds compared with those found below clouds, as observed previously in aircraft campaigns.

The dynamics of the 18 O( 1 D) + 44 CO 2 oxygen isotope exchange reaction has been studied using a crossed molecular beam apparatus at collision energies of 4.2 and 7.7 kcal/mol. At both collision energies, two reaction channels are observed: isotope exchange in which quenching to O( 3 P) occurs and isotope exchange in which the product oxygen atom remains on the singlet surface. Electronic quenching of O( 1 D) is the major channel at both collision energies, accounting for 84% of isotope exchange at 4.2 kcal/mol and 67% at 7.7 kcal/mol. Both channels proceed via a CO 3 * complex that is long-lived with respect to its rotational period. Combined with recent ab initio and statistical calculations by Mebel et al., the long complex lifetimes suggest that statistical isotope exchange occurs in the CO 3 * complex (apart from zero-point energy isotope effects), although the existence of a small, dynamically driven unconventional isotope effect in this reaction cannot yet be ruled out. These new molecular-level details may help provide a more quantitative understanding of the heavy isotope enrichment in CO 2 observed in the stratosphere. † Part of the special issue "Richard Bersohn Memorial Issue".

There is a growing understanding that secondary organic aerosol (SOA) can form through reactions in atmospheric waters (i.e., clouds, fogs, and aerosol water). In clouds and wet aerosols, water-soluble organic products of gas-phase photochemistry dissolve into the aqueous phase where they can react further (e.g. with OH radicals) to form low volatility products that are largely retained in the particle phase. Organic acids, oligomers and other products form via radical- and non-radical reactions, including hemiacetal formation during droplet evaporation, acid/base catalyzation, and reaction of organics with other constituents (e.g. NH₄⁺). <br><br> This paper uses kinetic modeling, experiments conducted with aqueous carbonyl solutions in the presence and absence of OH radicals, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, and the literature to describe aqueous chemistry at cloud- and aerosol-relevant concentrations and during droplet evaporation. At least for aqueous reactions of glyoxal with OH radicals, chemical modeling can reproduce experiments conducted at cloud-relevant concentrations without including radical–radical reactions, whereas radical–radical reactions become dramatically more important at higher concentrations. We demonstrate that reactions with OH radicals tend to be faster and form more SOA than "non-radical" reactions (e.g., acid catalyzation)