Aqueous-phase Organic Chemistry in the Atmosphere

Abstract

Atmospheric aqueous phases (i.e. cloud, fog and aerosol liquid water) are important reaction media for the processing of organic compounds. Quantitative data on aqueous-phase organic chemistry under the atmospheric context are sparse compared to the data from comparable gas-phase processes. A series of studies was conducted to provide fundamental information on this topic. An online analytical technique, Aerosol Chemical Ionization Mass Spectrometry (Aerosol CIMS), was employed to quantitatively monitor aqueous-phase OH oxidation of glyoxal and methylglyoxal. Quantification of all the major reaction products was achieved, permitting complete reaction mechanisms to be presented. An unexpected class of compounds, α-hydroxyhydroperoxide (α-HHP), was observed during the experiments, and the formation of this class of compounds was further quantified. Formation of α-HHP can be potentially important in aerosol liquid water, affecting aerosol toxicity and the gas-particle partitioning of small aldehydes. Aerosol CIMS was further applied to the OH oxidation of levoglucosan, using a high mass resolution CIMS. Unique reaction trends were observed, from which novel mechanism analysis frameworks were introduced. Aqueous-phase photochemical processing of a variety of light-absorbing organic compounds (Brown Carbon (BrC)) was investigated in the laboratory. The light absorptivity of BrC was significantly altered via these processes, indicating that the chemical processing of BrC species needs to be considered for a sound assessment of their atmospheric implications. Finally, cloud-partitioning of a toxic compound, isocyanic acid (HNCO), was investigated in the field, representing the first online measurement of gas-phase compounds dissolved in cloudwater. A secondary source of HNCO in the ambient air was also observed. Overall, aqueous-phase chemistry leads to reaction products which likely contribute to secondary organic aerosol (SOA) formation. At the same time, aqueous-phase chemistry also facilitates the transformation and removal of specific species with atmospheric significance (e.g. tracer compounds, pollutants, BrC).