Correction to Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol.

[This corrects the article DOI: 10.1021/acsearthspacechem.3c00035.].

Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol.

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO2. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark. In more atmospherically relevant chamber experiments where suspended aqueous aerosol particles are exposed to gas-phase glyoxal and SO2, the formation of detectable amounts of BrC requires an OH radical source and occurs most rapidly after a cloud event. From these observations we infer that this photobrowning is caused by radical-initiated reactions as evaporation concentrates aqueous-phase reactants and aerosol viscosity increases. Positive-mode electrospray ionization mass spectrometric analysis of aerosol-phase products reveals a large number of CxHyOz oligomers that are reduced rather than oxidized (relative to glyoxal), with the degree of reduction increasing in the presence of OH radicals. This again suggests a radical-initiated redox mechanism where photolytically produced aqueous radical species trigger S(IV)-O2 auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions especially if aerosol-phase O2 is depleted. This process may contribute to daytime BrC production and aqueous-phase sulfur oxidation in the atmosphere. The BrC produced, however, is about an order of magnitude less light-absorbing than wood smoke BrC at 365 nm.

Kinetics, Products, and Brown Carbon Formation by Aqueous-Phase Reactions of Glycolaldehyde with Atmospheric Amines and Ammonium Sulfate.

Glycolaldehyde (GAld) is a C2 water-soluble aldehyde produced during the atmospheric oxidation of isoprene and many other species and is commonly found in cloudwater. Previous work has established that glycolaldehyde evaporates more readily from drying aerosol droplets containing ammonium sulfate (AS) than does glyoxal, methylglyoxal, or hydroxyacetone, which implies that it does not oligomerize as quickly as these other species. Here, we report NMR measurements of glycolaldehyde's aqueous-phase reactions with AS, methylamine, and glycine. Reaction rate constants are smaller than those of respective glyoxal and methylglyoxal reactions in the pH range of 3-6. In follow-up cloud chamber experiments, deliquesced glycine and AS seed particles were found to take up glycolaldehyde and methylamine and form brown carbon. At very high relative humidity, these changes were more than 2 orders of magnitude faster than predicted by our bulk liquid NMR kinetics measurements, suggesting that reactions involving surface-active species at crowded air-water interfaces may play an important role. The high-resolution liquid chromatography-electrospray ionization-mass spectrometric analysis of filter extracts of unprocessed AS + GAld seed particles identified sugar-like C6 and C12 GAld oligomers, including proposed product 3-deoxyglucosone, with and without modification by reactions with ammonia to diimine and imidazole forms. Chamber exposure to methylamine gas, cloud processing, and simulated sunlight increased the incorporation of both ammonia and methylamine into oligomers. Many C4-C16 imidazole derivatives were detected in an extract of chamber-exposed aerosol along with a predominance of N-derivatized C6 and C12 glycolaldehyde oligomers, suggesting that GAld is capable of forming brown carbon SOA.

Brown Carbon Production in Ammonium- or Amine-Containing Aerosol Particles by Reactive Uptake of Methylglyoxal and Photolytic Cloud Cycling.

The effects of methylglyoxal uptake on the physical and optical properties of aerosol containing amines or ammonium sulfate were determined before and after cloud processing in a temperature- and RH-controlled chamber. The formation of brown carbon was observed upon methylglyoxal addition, detected as an increase in water-soluble organic carbon mass absorption coefficients below 370 nm and as a drop in single-scattering albedo at 450 nm. The imaginary refractive index component k450 reached a maximum value of 0.03 ± 0.009 with aqueous glycine aerosol particles. Browning of solid particles occurred at rates limited by chamber mixing (<1 min), and in liquid particles occurred more gradually, but in all cases occurred much more rapidly than in bulk aqueous studies. Further browning in AS and methylammonium sulfate seeds was triggered by cloud events with chamber lights on, suggesting photosensitized brown carbon formation. Despite these changes in optical aerosol characteristics, increases in dried aerosol mass were rarely observed (<1 µg/m3 in all cases), consistent with previous experiments on methylglyoxal. Under dry, particle-free conditions, methylglyoxal reacted (presumably on chamber walls) with methylamine with a rate constant k = (9 ± 2) × 10-17 cm3 molecule-1 s-1 at 294 K and activation energy Ea = 64 ± 37 kJ/mol.