Glyoxal and methyl glyoxal are common secondary atmospheric pollutants, formed from aromatic and terpene precursors. Both compounds are extremely water-soluble due to dihydrate formation and partition into cloudwater. In this work, FTIR-ATR and mass measurements indicate that both compounds remain primarily in the condensed phase due to oligomer formation when aqueous solution droplets are evaporated, regardless of concentration (> or = 1 mM) or, for glyoxal, droplet evaporation rate. FTIR spectral analyses suggestthat oligomer formation is triggered by conversion from dihydrate to monohydrate forms, which are still nonvolatile but contain reactive carbonyl groups. Methyl glyoxal hemiacetal formation is observed by changes in the C-0/C=0 stretch peak area ratio. The formation of glyoxal oligomers is detected by a dramatic shift of the C-0 stretching peak toward low frequencies. Glyoxal oligomer peaks at 1070 cm(-1), 950 cm(-1), and 980 cm(-1) are assigned to free C-OH stretch, dioxolane-linked C-OC asymmetric stretch, and tentativelyto non-dioxolane-linked C-OC stretches, respectively. Acids have little effect on glyoxal oligomer formation; however, base interrupts oligomer formation by catalyzing glyoxal hydration and disproportionation to glycolic acid. Since glyoxal and methyl glyoxal are commonly found in cloudwater and are expected to remain largely in the aerosol phase when cloud droplets evaporate, this process may be a source of secondary organic aerosol by cloud processing.

[1] Glyoxal reacts with methylamine in drying cloud droplet/ aerosol surrogates to form high molecular mass oligomers along with smaller amounts of 1,3-dimethylimidazole and light-absorbing compounds. The patterns observed by highresolution time-of-flight aerosol mass spectrometry indicate that oligomers form from repeated imine units. The reactions are 1st order in each reactant: rate-limiting imine formation is followed by rapid dimer and oligomer formation. While excess methylamine evaporates from the droplet, half the glyoxal does not, due to self-oligomerization reactions that occur in the absence of methylamine. Glyoxal irreversibly traps volatile amine compounds in the aerosol phase, converting them into oligomers. This is the first reported mechanism for the formation of stable secondary organic aerosol (SOA) material from methylamine, a substance with only one carbon, and could produce as much as 11 Tg SOA yr À1 globally if glyoxal reacts exclusively by this pathway. Citation: De Haan, D. O., M. A. Tolbert, and

Density functional theory (B3LYP//6-311+G*) calculations, including Poisson-Boltzmann implicit solvent and free energy corrections, are applied to study the hydration of methylglyoxal and the subsequent formation of dimeric species in solution. Our calculations show that, unlike glyoxal, fully hydrated species are not thermodynamically favored over their less hydrated counterparts, nor are dioxolane ring species the thermodynamic sink, which is in agreement with experimental data. Instead, we find that aldol condensations are the most favored oligomerization reactions for methylglyoxal. These results differ from those of glyoxal, which, lacking the methyl group, cannot access the enol structure leading to aldol condensation. For methylglyoxal, the product from nucleophilic attack at the aldehyde rather than the ketone was favored. Our results help explain some of the observed differences between methylglyoxal and glyoxal, in particular the different array of oligomers formed.

Density functional theory (B3LYP//6-311+G) calculations including Poisson-Boltzmann implicit solvent were used to study the hydration of glyoxal and subsequent formation of dimeric species in solution. Our calculations show that the dioxolane ring dimer is the thermodynamic sink among all monomers and dimers with varying degrees of hydration. Although fully hydrated species are thermodynamically favored over their less hydrated counterparts, we find that a preliminary dehydration step precedes dimerization and ring closure. Ring closure of the open dimer monohydrate to the dioxolane ring dimer is kinetically favored over both hydration to the open dimer dihydrate and ring closure to form the dioxane ring dimer. The kinetic barriers for different geometric approaches for dimerization suggest an explanation why oligomerization stops after the formation of a dioxolane ring trimer as observed experimentally.

In this study, we have explored the phase behavior and the ice nucleation properties of secondary organic aerosol made from aqueous processing (aqSOA). AqSOA was made from the dark reactions of methylglyoxal with methylamine in simulated evaporated cloud droplets. The resulting particles were probed from 215 to 250 K using Raman spectroscopy coupled to an environmental cell. We find these particles are in a semisolid or glassy state based upon their behavior when exposed to mechanical pressure as well as their flow behavior. Further, we find that these aqSOA particles are poor depositional ice nuclei, in contrast to previous studies on simple mixtures of glassy organics. Additionally, we have studied the effect of ammonium sulfate on the phase, morphology, and ice nucleation behavior of the aqSOA. We find that the plasticizing effect of ammonium sulfate lowers the viscosity of the aqSOA, allowing the ammonium sulfate to effloresce within the aqSOA matrix. Upon humidification, the aqSOA matrix liquefies before it can depositionally nucleate ice, and the effloresced ammonium sulfate can act as an immersion mode ice nucleus. This change in the mode of nucleation is accompanied by an increase in the overall ice nucleation efficiency of the aqSOA particles.

The uptake of glyoxal by a variety of organic and inorganic aerosol types was examined in a Teflon chamber. Rapid glyoxal uptake was observed for all liquid-phase aerosols at all relative humidity levels tested (< 5 to 50% RH). Even for aerosol with known water content, Henry's Law cannot predict glyoxal uptake: H* > (3 +/- 1.5) x 10(8) mol kg(-1) atm(-1) for l-tartaric acid, H* > (1 +/- 0.5) x 10(8) for dl-malic acid and H* = (2 +/- 1) x 10(7) for malonic acid aerosol. Other liquid-phase aerosol particles containing amine functional groups (arginine, aspartic acid, and glutamic acid) took up even more glyoxal (H* > 3 x 10(8)). The trend of higher glyoxal uptake onto aerosol containing more nucleophilic organic compounds suggests that glyoxal is reacting with organic compounds in the aerosol phase. Solid-phase aerosol showed RH-dependent glyoxal uptake, likely due to the existence of surface water layers. However, particle growth rates were the highestfor sodium sulfate aerosol. For organic aerosol, growth rates correlated with the acidity of the carboxylic acid groups of the aerosol material, suggesting that glyoxal uptake is enhanced by mildly acidic conditions.

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.

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.