Oxalic acid is often the single most abundant watersoluble organic compound identified in ambient aerosols, but its precursors have not been identified, and its formation mechanism is not well understood. On the contrary, sulfate as a major aerosol component, its formation pathways have been established, and in-cloud processing is recognized as its major production pathway. Our measurements of aerosol sulfate and oxalate collected across a wide geographical span in the East Asia region, up to Beijing in the north and down to Hong Kong in the south, indicate that the two species are highly correlated among samples collected at the same location and among samples collected at different locations in the Pearl River Delta region. This good correlation is also found in measurements made elsewhere by other researchers. We argue that a common dominant formation pathway, likely in-cloud processing, explains the close tracking of the two chemically distinct species. This also highlights the potential importance of in-cloud processing as a pathway leading to formation of secondary organic aerosols.

The charring of organic materials during carbon analysis bythermal methods makes it difficult to differentiate elemental carbon (EC) from organic carbon (OC). Failure to correct for charring results in the overestimation of EC and the underestimation of OC. The charring characteristics andthermal behaviors of aerosol OC are studied by subjecting hexane and water extracts of ambient aerosols to various analysis conditions. The complete evolution of water-soluble organic carbon (WSOC) aerosol materials is found to require a temperature as high as 850 degrees C and the presence of oxygen. EC would be oxidized under these thermal conditions as well. As a result, thermal methods relying only on temperature for the differentiation of EC and OC would give unreliable OC and EC concentrations. Our investigation also reveals that WSOC accounts for a large fraction (13-66%) of charring, while hexane extractable organic compounds produce little charring. The extent of charring from WSOC, defined as the ratio between pyrolytically generated EC to the total WSOC, is found to increase with the WSOC loading in each analysis when the loadings are below a certain value. This ratio remains constant when the loadings are above this value. This may account for the high variability in the extent of charring among aerosol samples from different locations as well as among samples from a single location collected at different times. Charring is reduced if the residence time at each temperature step in a helium atmosphere is sufficiently long to allow for maximum C evolution at each step. Charring is also influenced by the presence of inorganic constituents such as ammonium bisulfate. For the few tested organic materials, it is observed that ammonium bisulfate enhances the charring of starch and cellulose but reduces the charring of levoglucosan.