Isoprene and its oxidation products, methacrolein and methylvinyl ketone, at an urban forested site during the 1999 Southern Oxidants Study

Isoprene (ISOP) and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK), were measured at an urban forested site in Nashville, T ennessee, as part of the 1999 Southern Oxidants Study (SOS). Hourly observa tions were performed at Cornelia Fort Airpark for a 4 week period between J une 13 and July 14. At the midday photochemical peak (1200 local standard t ime, LST), average mixing ratios of isoprene, MACR, and MVK were 410 parts per trillion by volume (pptv), 240 pptv, and 430 pptv, respectively. Median isoprene, MACR, and MVK mixing ratios were 400 pptv, 200 pptv, and 360 ppt v, respectively, at 1200 LST. An emissions inventory calculation for Davids on County, encompassing Nashville, suggests that MACR and MVK were produced predominately from isoprene oxidation rather than direct combustion emissi ons. The observations are compared with results from two chemical models: a simple sequential reaction scheme and a one-dimensional (1-D) numerical bo x model. The daytime ratios of MVK/ISOP and MACR/ISOP varied in a systemati c manner and can be reproduced by the analytical solution of the sequential reaction scheme. Air masses with more photochemically aged isoprene were o bserved during SOS 1999 at Cornelia Fort (0.3-1.6 hours) compared to the SO S 1990 canopy study at Kinterbish (0.1-0.6 hours). This is consistent with the proximity of the tower inlets to the forest canopies during both campai gns. Isoprene had a chemical lifetime of 20 min at the average observed mid day HO mixing ratio of 8 x 10(6) molecules/cm(3). As a result, significant conversion of isoprene to its oxidation products was observed on the timesc ale of transport from the dense forest canopies surrounding Nashville. The systematic diurnal behavior in the MVK/MACR ratio can also be simulated wit h a 1-D photochemical box model. General agreement between the observations of MACR and MVK during SOS 1999 with the two chemical models suggests we h ave a comprehensive understanding of the first few stages of isoprene oxida tion in this urban forested environment.