Water-soluble organic compounds (WSOCs) are abundant in atmospheric aerosols, typically accounting for 20~80% of particulate organic matter mass. Due to their affinity for water, WSOCs play an active role in aerosol-water interaction, and thus influence hygroscopic properties of aerosols, which in turn affect cloud formation processes and earth's radiation balance. Despite their abundance and significance, the sources of WSOCs are not well understood. Some primary sources (e.g., biomass burning) are known to emit WSOCs. It is also known from smog chamber experiments that photochemical oxidation of volatile organic compounds lead to less volatile oxygenated compounds that reside in the aerosol phase and are water-soluble because of the presence of polar functional groups. More recent wo...[ Read more ]

Water-soluble organic compounds (WSOCs) are abundant in atmospheric aerosols, typically accounting for 20~80% of particulate organic matter mass. Due to their affinity for water, WSOCs play an active role in aerosol-water interaction, and thus influence hygroscopic properties of aerosols, which in turn affect cloud formation processes and earth's radiation balance. Despite their abundance and significance, the sources of WSOCs are not well understood. Some primary sources (e.g., biomass burning) are known to emit WSOCs. It is also known from smog chamber experiments that photochemical oxidation of volatile organic compounds lead to less volatile oxygenated compounds that reside in the aerosol phase and are water-soluble because of the presence of polar functional groups. More recent work points to in-cloud/fog processes as a potentially important source for WSOCs. Work in this thesis aims to improve our understanding of the sources and formation mechanisms of WSOCs in atmospheric aerosols. Multiple approaches have been taken, including field measurements and controlled laboratory experiments. The thesis consists of the following four parts:

(1) The formation mechanism of the most abundant WSOC species, oxalate, was investigated by synthesizing field measurement data obtained by our group and those available in the literature. Our measurements of aerosol sulfate and oxalate across a wide geographical span in the East Asia region, up to Beijing in the north and down to Hong Kong in the south, indicated that the two species were highly correlated. This good correlation was also found in measurements made elsewhere in the world by other researchers. Through a detailed analysis of factors influencing ambient oxalate, it can be argued that a common dominant formation pathway, likely in-cloud processing, explains the close tracking of the two chemically distinct species. This result also highlights the potential importance of in-cloud processing as a pathway leading to the formation of secondary organic aerosols.

(2) Size distributions of water-soluble organic carbon and oxalate in ambient aerosols were measured in Shenzhen in the summer and the winter of 2005. Both water-soluble organic carbon and oxalate had a dominant droplet mode, a small condensation mode, and a small coarse mode. The sources and formation mechanisms of oxalate and water-soluble organic carbon were inferred in reference to the well-understood size distribution characteristics for the inorganic species (SO₄^2-, Νa⁺ , K⁺, and Ca²⁺). The droplet mode oxalate was mostly produced from in-cloud aqueous-phase reactions, while the condensation mode oxalate had a photochemical origin, and the coarse mode oxalate might have come from the adsorption of gas-phase precursors. The droplet mode WSOC was found to be correlated well with the biomass tracer, K⁺, suggesting biomass burning as a significant source of WSOC during the chosen sampling periods. The condensation mode of WSOC was a result of complicated mixing of both primary sources and secondary gas-to-particle conversion, and the course mode WSOC was likely attributed to microbial and biochemical degradation of organic debris in soils.

(3) Size distribution characteristics of elemental carbon (EC) emissions in the size range of 0.056-18 μm from Chinese vehicles were measured by collecting size-segregated aerosol samples in a roadway tunnel (Zhujiang Tunnel) in Guangzhou. EC showed a dominant accumulation mode with a mass median aerodynamic diameter (MMAD) of 0.42 μm. This MMAD value was much larger than those reported for EC emissions from vehicles in developed countries (~0.1 μm). EC particles of 0.42 μm are effective cloud condensation nuclei (CCN). An important implication of this finding is that fresh EC particles from Chinese vehicles could readily undergo cloud processing and form internal mixtures with sulfate in the residue droplet mode particles. The potential of in-cloud/wet aerosol oxidation of EC and other insoluble carbonaceous materials leading to the formation of WSOCs was investigated and reported in part 4.

(4) Laboratory experiments were carried out to simulate H₂O₂ oxidation of insoluble carbonaceous aerosol materials in cloud water and wet particles. Water-insoluble aerosol materials and a soot sample were soaked in an acidic H₂O₂ solution for 10-20 hours. WSOCs were produced in this process and the yield of WSOCs from the insoluble aerosol carbon material ranged from 12 to 41% on a carbon mass basis and the yield was 3% from the soot sample. Comparisons of the characteristics of the derived WSOCs with those of the original aerosol WSOCs revealed high chemical similarity between the two, suggesting the atmospheric relevance of this formation pathway.

This thesis work has identified that biomass burning and heterogeneous oxidation processes (e.g., in-cloud processing, oxidation on wet particles) are important sources for WSOCs. Future work should be directed at quantifying the contributions of these sources and elucidating the molecular formation pathways.