(644a) Formation of Humic-Like Secondary Organic Material in Aqueous Aerosol Mimics

We have observed that methylglyoxal forms surface-active and light-absorbing oligomeric products in aqueous inorganic salt solutions mimicking tropospheric aerosol particles. The solutions studied were saturated in the salt of interest ((NH4)2SO4, NaCl, NH4NO3) and the concentration of organics in the mixture was maintained at 1-10% of the solute mass, consistent with ambient aerosol composition. The product mixtures were characterized using UV-Vis spectrophotometry, MALDI-MS, pendant drop tensiometry, Aerosol-CIMS, and HPLC with UV detection. The products formed in aqueous solutions containing (NH4)2SO4 or NH4NO3 absorb light at UV and visible wavelengths. The absorption spectrum consists of a nearly featureless broad absorption band that extends to longer wavelengths with increasing reaction time. Significant absorption at 550 nm is exhibited after 12 hours, with absorption at up to 700 nm developing within 2-3 days. The results suggest a mechanism involving the participation of the ammonium ion. Solutions initially containing > 0.05 M methylglyoxal exhibit significant surface tension depression compared to ammonium sulfate solutions without organics. The surface tension depression follows a Langmuir-like dependence on initial methylglyoxal concentration, with a minimum (saturation) surface tension of 39.9(+/-0.6) dynes/cm. Mixtures of methylglyoxal and glyoxal also form surface-active organic products. The formation of surface-active secondary organic material in aqueous tropospheric aerosols, besides affecting CCN activity, could limit SOA formation via heterogeneous chemistry by inhibiting the uptake of glyoxal, methylglyoxal, and other gas-phase SOA precursors.