Secondary Atmospheric Photooxidation Products: Evidence for Biogenic and Anthropogenic Sources

Isoprene, 2-methyl-3-buten-2-ol (MBO), their photooxidation products, and carbon monoxide (CO) were measured above a Ponderosa pine plantation, near Blodgett Forest Research Station, CA, from August 15-19 and September 11-15, 2000. An automated gas chromatograph-flame ionization detector system was used to measure isoprene, its "first-generation" photooxidation products, methyl vinyl ketone and methacrolein, MBO, and the MBO photooxidation product, acetone. A mist chamber was used for collection of the MBO photooxidation product 2-hydroxy-2-methylpropanal (2-HMPR), and the "second-generation" isoprene photooxidation products glycolaldehyde, hydroxyacetone, and methylglyoxal. Carbonyl and hydroxyl groups in these compounds were derivatized with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and bis-(trimethylsilyl)trifluoroacetamide (BSTFA), respectively, and the derivatives were quantified by using gas chromatography with ion trap mass spectrometry. Isoprene, MBO, and their photooxidation products had similar diurnal cycles, with maximum mixing ratios in the afternoon and minimum mixing ratios in the early morning. The highest mean mixing ratios were observed for glycolaldehyde (0.69 ppb), hydroxyacetone (0.42 ppb), and 2-HMPR (0.30 ppb), while methylglyoxal was lower (0.13 ppb), consistent with its shorter lifetime in the atmosphere. Secondary isoprene photooxidation products had diurnal cycles similar to their precursors, while changes in 2-HMPR lagged behind corresponding changes in MBO. These observations are consistent with transport of isoprene and its photochemical products from a source several hours upwind, and a 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. They were also more highly correlated with CO than with their biogenic precursors, suggesting that oxidation of anthropogenic compounds contributed to their production.