| The composition of aerosol generated in the photooxidation of a variety of hydrocarbon precursors was investigated in a series of smog chamber experiments. Relative molar loadings of organic acids, alcohols, alkylnitrates, ketone and aldehyde groups, aliphatic C-H groups, and aromatic C-H groups were determined using Fourier Transform Infrared (FTIR) microscopic spectroscopy. Average aerosol molecular structures were postulated based on the known organic chemistry of urban atmospheres. The smog chamber aerosol data were then used in conjunction with several emission inventories (Grosjean and Seinfeld, 1989; Pandis et al., 1992) to estimate the composition of the organic fraction of ambient Los Angeles aerosol. These predictions where compared to ambient measurements made by Palen et al. (1992) during the 1987 Southern California Air Quality Survey (SCAQS). Aerosol compositions were generally consistent with the SCAQS, however, the data revealed two major discrepancies. Ambient loadings of carbonyl bonds were substantially higher than predicted, and aliphatic C-H bonds observed in both the smog chamber and fresh primary sources were absent from the ambient spectra. These observations can be interpreted as evidence for organic oxidation reactions taking place in the aerosol phase in the atmosphere possibly via heterogeneous photocatalysis. |
