Characterization of secondary atmospheric photooxidation products: Evidence for biogenic and anthropogenic sources
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Measurements of the biogenic hydrocarbons isoprene and 2-methyl-3-buten-2-ol (MBO), their first-, second-, and third-generation photooxidation products methacrolein (MACR), methyl vinyl ketone (MVK), acetone, 2-hydroxy-2-methylpropanal (2-HMPR), glycolaldehyde, hydroxyacetone, methylglyoxal, and glyoxal, and carbon monoxide (CO), were obtained above a ponderosa pine plantation, near the Blodgett Forest Research Station, California on 15-19 August and 11-15 September 2000. Diurnal cycles for all the compounds were similar, with maximum mixing ratios in the afternoon and minimum mixing ratios in the early morning. The diurnal cycles of the second-generation isoprene photooxidation products were similar to their precursors, while changes in 2-HMPR, a compound unique to MBO photooxidation, lagged behind corresponding changes in MBO. These observations are consistent with transport of isoprene and its photooxidation products from a strong source several hours upwind, and a predominantly local source of MBO with in situ photochemical production of 2-HMPR. Glycolaldehyde and hydroxyacetone mixing ratios exceeded the mixing ratios of their biogenic precursors on days with high CO, and were generally correlated more strongly with CO than with their biogenic precursors. The agreement between measured product ratios and ratios predicted by a box model improved when anthropogenic precursors were added to the model. Quantification of the source apportionment demonstrated that photooxidation of anthropogenic precursors contributed significantly to the mixing ratios of glycolaldehyde and hydroxyacetone measured in this rural environment. More research into the photochemistry and yields of multifunctional carbonyl compounds from hydrocarbon photooxidation is needed in order for models to accurately predict their role in tropospheric chemistry.