Molecular characterization of urban organic aerosol in tropical India: contributions of primary emissions and secondary photooxidation
1Institute of Low Temperature Science, Hokkaido University, Sapporo, 060-0819, Japan
2Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, 600036, India
3State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, China
4Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China
Abstract. Organic molecular composition of PM10 samples, collected at Chennai in tropical India, was studied using capillary gas chromatography/mass spectrometry. Fourteen organic compound classes were detected in the aerosols, including aliphatic lipids, sugar compounds, lignin products, terpenoid biomarkers, sterols, aromatic acids, hydroxy-/polyacids, phthalate esters, hopanes, Polycyclic Aromatic Hydrocarbons (PAHs), and photooxidation products from biogenic Volatile Organic Compounds (VOCs). At daytime, phthalate esters were found to be the most abundant compound class; however, at nighttime, fatty acids were the dominant one. Di-(2-ethylhexyl) phthalate, C16 fatty acid, and levoglucosan were identified as the most abundant single compounds. The nighttime maxima of most organics in the aerosols indicate a land/sea breeze effect in tropical India, although some other factors such as local emissions and long-range transport may also influence the composition of organic aerosols. However, biogenic VOC oxidation products (e.g., 2-methyltetrols, pinic acid, 3-hydroxyglutaric acid and β-caryophyllinic acid) showed diurnal patterns with daytime maxima. Interestingly, terephthalic acid was maximized at nighttime, which is different from those of phthalic and isophthalic acids. A positive relation was found between 1,3,5-triphenylbenzene (a tracer for plastic burning) and terephthalic acid, suggesting that the field burning of municipal solid wastes including plastics is a significant source of terephthalic acid. Organic compounds were further categorized into several groups to clarify their sources. Fossil fuel combustion (24–43%) was recognized as the most significant source for the total identified compounds, followed by plastic emission (16–33%), secondary oxidation (8.6–23%), and microbial/marine sources (7.2–17%). In contrast, the contributions of terrestrial plant waxes (5.9–11%) and biomass burning (4.2–6.4%) were relatively small. This study demonstrates that, in addition to fossil fuel combustion and biomass burning, the open-burning of plastics in urban area also contributes to the organic aerosols in South Asia.