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 k 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/m 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 cm molecule s at 294 K and activation energy E = 64 ± 37 kJ/mol.

Aqueous methylglyoxal chemistry has often been implicated as an important source of oligomers in atmospheric aerosol. Here we report on chemical analysis of brown carbon aerosol particles collected from cloud cycling/photolysis chamber experiments, where gaseous methylglyoxal and methylamine interacted with glycine, ammonium, or methylammonium sulfate seed particles. Eighteen N-containing oligomers were identified in the particulate phase by liquid chromatography/diode array detection/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry. Chemical formulas were determined and, for 6 major oligomer products, MS fragmentation spectra were used to propose tentative structures and mechanisms. Electronic absorption spectra were calculated for six tentative product structures by an ab initio second order algebraic-diagrammatic-construction/density functional theory approach. For five structures, matching calculated and measured absorption spectra suggest that they are dominant light-absorbing species at their chromatographic retention times. Detected oligomers incorporated methylglyoxal and amines, as expected, but also pyruvic acid, hydroxyacetone, and significant quantities of acetaldehyde. The finding that ∼80% (by mass) of detected oligomers contained acetaldehyde, a methylglyoxal photolysis product, suggests that daytime methylglyoxal oligomer formation is dominated by radical addition mechanisms involving CHCO*. These mechanisms are evidently responsible for enhanced browning observed during photolytic cloud events.

Formaldehyde and acetaldehyde are commonly found in cloud droplets because of reversible partitioning and hydration reactions. An SOA formation pathway was recently identified in which these common aldehydes are irreversibly incorporated into imidazole derivatives formed by reaction with dicarbonyl species and ammonium salts or amine species. Here we use ultraviolet−visible and nuclear magnetic resonance kinetic measurements to determine the influence of formaldehyde and acetaldehyde on aqueous methylglyoxal chemistry. The presence of formaldehyde increases imidazole product formation rates by factors of 2 and ≥5 in reactions with ammonium sulfate and amines, respectively, and increases imidazole product yields in methylglyoxal + amine reactions by more than an order of magnitude. Acetaldehyde is less likely to be incorporated into imidazole products and increases formation rates and yields only in reactions involving amines. We estimate that aqueous formation of imidazoles could generate as much as 1.05 Tg of C/year SOA from formaldehyde and 3.8 Tg of C/year or 7 Tg/year SOA overall, limited by the availability of aqueousphase glyoxal and methylglyoxal. While this upper limit represents a negligible formaldehyde sink, it is ∼5% of current estimates of global SOA formation. Formaldehyde's channeling of aqueous dicarbonyl chemistry toward production of imidazoles limits the formation of other oligomer products, including brown carbon species.

Methylamine, a common atmospheric amine species, is found in the gas, particle, and aqueous phases. It has been shown to form light-absorbing, oligomeric species in reactions with methylglyoxal and other aldehyde species in bulk aqueous-phase experiments and when mixed into seed aerosol as a sulfate salt. Here, we explore the influence of multiphase methylamine chemistry on aerosol production, properties, and molecular composition. When methylglyoxal aerosol particles were exposed to ∼2 ppm methylamine gas in a humid chamber, rapid browning was observed, but not growth. Aerosol bounce measurements indicated that particles became slightly more viscous and hydrophobic upon methylamine exposure. Subsequent cloud processing increased both viscosity and hygroscopicity but had little effect on browning, consistent with high-resolution mass spectrometry results showing that aerosol oligomer dicarbonyl functional groups were transformed into cationic imidazole rings. Photolytic cloud processing triggered the incorporation of hydroxyacetone and acetal radicals into oligomers. Because dicarbonyl species are a major component of atmospheric aerosol particles, these results suggest that methylamine exposure and cloud processing will slowly increase brown carbon content, viscosity, and hygroscopicity of atmospheric aerosol.

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 to 5.7 × 10 between 72 and 99% RH. Continuous methylglyoxal losses were also observed in the presence of aqueous ammonium sulfate at 95% RH (γ = 3.7 ± 0.8 × 10). 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.