We measured the mixing ratios and fluxes of isoprene and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK) from a tall flux tower in metropolitan Houston, Texas, during summertime using a relaxed eddy accumulation (REA) system combined with a dual-channel GC-FID instrument. We show that isoprene was affected dominantly by biogenic emission sources during daytime, but also that tail-pipe emission sources (alongside 2-pentenes) are contributing during the rush hours and at night. The observed daytime mixing ratios of isoprene were much lower than over forested areas due to a comparatively low density of isoprene emitting trees in the tower’s footprint area. Daytime isoprene fluxes displayed the expected light and temperature driven emission characteristics, and a detailed isoprene emissions model explained average fluxes fairly well. Our investigation of isoprene’s oxidation products MACR and MVK showed that both anthropogenic and isoprene oxidation sources exist for MACR, while MVK was strongly dominated by isoprene oxidation between its emission and sampling points due to presumably high local OH radical concentrations. While biogenic emission modeling appears to work well for this urban environment when supplied with accurate input data, emission inventories used for ozone modeling may need to consider tailpipe isoprene (and MACR) emissions to properly account for urban concentrations, particularly higher morning abundances.
