Sources and chemistry of secondary organic aerosols formed from carbonyl compounds

Atmospheric aerosols serve an important role in climate and air quality. However, there are still significant gaps in our scientific understanding of their impacts on climate. One of the greatest factors contributing to uncertainties in our estimations of these impacts can be attributed to the gap in the sources and formation pathways of secondary organic aerosols (SOAs). Carbonyl compounds, in particular, glyoxal and methylglyoxal, are two oxidation products of both anthropogenic and biogenic volatile organic compounds (VOCs) in the atmosphere. Field and modeling studies have indicated that these two compounds can serve as potentially important precursors to SOAs, and alter the physical and chemical properties of the aerosols. The mechanisms and atmospheric significance of these processes pose important questions which need to be addressed. Here, we report experiments targeted to study the following topics: 1) the chemical kinetics of methylglyoxal uptake to aqueous aerosols, and the subsequent formation of SOA material; 2) the oxidative aging of SOA material formed by methylglyoxal; 3) the impact of methylglyoxal on the cloud condensation nuclei (CCN) activity of the aerosol. These studies were conducted using either aerosols generated from bulk solutions of the organic and ammonium sulfate or by exposing the gas-phase organic to pure ammonium sulfate seed aerosols. A number of techniques were utilized including: a custom-built Aerosol Chemical Ionization Mass Spectrometer (Aerosol-CIMS), UV-Vis spectrophotometer, pendant drop tensiometry (PDT), continuous flow stream-wise thermal gradient CCN counter (CFSTGC), aerosol flow tube reactors, and an aerosol chamber. We found that the uptake of methylglyoxal to aerosols is a potentially significant source of light-absorbing SOA in the atmosphere. Additionally, the presence of methylglyoxal leads to surface tension depression with important implications for aerosol CCN activity. The aqueous-phase reaction products of glyoxal and methylglyoxal when NH4^+ is present include species featuring unsaturated C=C bonds such as aldol condensation products and imidazoles. Upon oxidation by O3 and OH, these particles show an increase in light absorption, accompanied by the formation of smaller, more volatile organic acids. Aerosol chamber studies conducted where pure ammonium sulfate particles were exposed to gas-phase methylglyoxal and/or acetaldehyde show significant enhancements in CCN activity, which can increase cloud droplet number concentrations by up to 20%. The results of this work will provide for a more accurate representation of gas-aerosol interactions and cloud formation in climate and atmospheric chemistry models.