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.

Methylglyoxal Uptake Coefficients on Aqueous Aerosol Surfaces.

In order to predict the amount of secondary organic aerosol formed by heterogeneous processing of methylglyoxal, uptake coefficients (γ) and estimates of uptake reversibility are needed. Here, uptake coefficients are extracted from chamber studies involving ammonium sulfate and glycine seed aerosol at high relative humidity (RH ≥ 72%). Methylglyoxal uptake coefficients on prereacted glycine aerosol particles had a strong dependence on RH, increasing from γ = 0.4 × 10-3 to 5.7 × 10-3 between 72 and 99% RH. Continuous methylglyoxal losses were also observed in the presence of aqueous ammonium sulfate at 95% RH (γAS,wet = 3.7 ± 0.8 × 10-3). Methylglyoxal uptake coefficients measured at ≥95% RH are larger than those reported for glyoxal on nonacidified, aqueous aerosol surfaces at 90% RH. Slight curvature in first-order uptake plots suggests that methylglyoxal uptake onto aqueous aerosol surfaces is not entirely irreversible after 20 min. Methylglyoxal uptake by cloud droplets was rapid and largely reversible, approaching equilibrium within the 1 min mixing time of the chamber. PTR-MS measurements showed that each cloud event extracted 3 to 8% of aerosol-phase methylglyoxal and returned it to the gas phase, likely by an oligomer hydrolysis mechanism.

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.