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, Tennessee, as part of the 1999 Southern Oxidants Study (SOS). Hourly observations were performed at Cornelia Fort Airpark for a 4 week period between June 13 and July 14. At the midday photochemical peak (1200 local standard time, 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 pptv, respectively, at 1200 LST. An emissions inventory calculation for Davidson County, encompassing Nashville, suggests that MACR and MVK were produced predominately from isoprene oxidation rather than direct combustion emissions. The observations are compared with results from two chemical models: a simple sequential reaction scheme and a one-dimensional (1-D) numerical box model. The daytime ratios of MVK/ISOP and MACR/ISOP varied in a systematic manner and can be reproduced by the analytical solution of the sequential reaction scheme. Air masses with more photochemically aged isoprene were observed during SOS 1999 at Cornelia Fort (0.3-1.6 hours) compared to the SOS 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 campaigns. Isoprene had a chemical lifetime of 20 min at the average observed midday 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 timescale of transport from the dense forest canopies surrounding Nashville. The systematic diurnal behavior in the MVK/MACR ratio can also be simulated with 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 have a comprehensive understanding of the first few stages of isoprene oxidation in this urban forested environment.