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.

Glyoxal is a significant atmospheric aldehyde formed from both anthropogenic aromatic compounds and biogenic isoprene emissions. The chemical behavior of glyoxal relevant to secondary organic aerosol (SOA) formation and analysis is examined in GC-MS, electrospray ionization (ESI)-MS, and particle chamber experiments. Glyoxal oligomers are shown to rapidly decompose to glyoxal in GC injection ports at temperatures > or = 120 degrees C. Glyoxal dihydrate monomer is dehydrated at temperatures > or = 140 degrees C during GC analysis but shows only oligomers (n < or = 7) upon ESI-MS analysis. Thus both of these analytical techniques will cause artifacts in speciation of glyoxal in SOA. In particle chamber experiments, glyoxal (at -0.1 Torr) condensed via particle-phase reactions when relative humidity levels exceeded a threshold of -26%. Both the threshold humidity and particle growth rates (-0.1 nm/min) are consistent with a recent study performed at glyoxal concentrations 4 orders of magnitude below those used here. This consistency suggests a mechanism where the surface water layer of solid-phase aerosol becomes saturated with glyoxal dihydrate monomer, triggering polymerization and the establishment of an organic phase.

Volatile furandiones and aldehydes are significant atmospheric oxidation products of aromatic compounds. The mechanism of secondary organic aerosol formation by these compounds was probed using particle chamber observations and macroscale simulations of condensed phases. Growth of inorganic seed aerosol was monitored in the presence of humidity and high concentrations of 2,5-furandione (maleic anhydride), 3-methyl-2,5-furandione (citraconic anhydride), benzaldehyde, and trans-cinnamaldehyde. Particle growth commenced when the gas-phase saturation level of each organic compound and water vapor (relative to its pure liquid), when summed together, reached a threshold near one, implying the formation of a nearly ideal mixed organic/aqueous phase. However, these organics are immiscible with water at the high mole fractions that would be expected in such a phase. Highly acidic dicarboxylic acids produced by the reactions between furandiones and water were shown to rapidly acidify an aqueous phase, resulting in greatly increased benzaldehyde solubility. Thus, the uptake of these organics onto particles in the presence of humidity appears to be reaction-dependent. Finally, it is shown that dicarboxylic acids produced in these reactions recyclize back to furandiones when subjected to normal GC injector temperatures, which could cause large artifacts in gas/particle phase distribution measurements.

The conditions under which semivolatile organic gases condense were studied in a Teflon particle chamber by scanning mobility particle sizing (SMPS) of the resultant particles. Benzaldehyde, maleic and citraconic anhydrides, n-decane, trans-cinnamaldehyde, and citral were introduced in various combinations into a particle chamber containing either particle-free nitrogen or nitrogen with dry seed particles made out of sodium chloride, D-tartartic acid, ammonium sulfate, or 1,10-decanediol. No organic gas was allowed to reach its saturation point relative to the vapor pressure of its pure liquid in any experiment. In the absence of seed particles, organic aerosol particles formed by ternary nucleation when the sum of the individual organic saturation levels reached a threshold between 1.17 and 1.86. With seed particles present, particle sizes began to increase when the sum of organic saturation levels reached 1.0. This size increase corresponds to the establishment and activation of ternary organic layers on the "clean" seed particles, as predicted by absorption partitioning theory. The observed increases in particle volume depended on initial seed particle volume, indicating that either gas diffusion rates or chemical reactions were controlling the rate of uptake.