Methylamine's Effects on Methylglyoxal-Containing Aerosol: Chemical, Physical, and Optical Changes

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 similar 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.