Determination of Urban VOC Emissions Ratios and Comparison with Inventories

During the NEAQS-ITCT2k4 campaign in New England anthropogenic VOCs and CO were measured downwind from New York City and Boston. The emission ratios of VOCs relative to CO and acetylene were calculated using a method in which the ratio of a VOC with acetylene is plotted versus the photochemical age. The intercept at the photochemical age of zero gives the emission ratio. The so determined emission ratios were compared to other measurement sets, including data from the same location in 2002, canister samples collected inside New York City and Boston, aircraft measurements from Los Angeles in 2002 and the average urban composition of 39 U.S. cities. All the measurements show fairly good agreement. The measured emission ratios also agree well with vehicle exhaust data indicating that a major source of VOCs in urban areas are automobiles. A comparison with an anthropogenic emission inventory shows a rather poor agreement, especially for the C2-C4 alkanes and most oxygenated species. The inventory overestimated toluene for example by almost a factor of three, which caused an air quality forecast model (WRF-CHEM) using this inventory to over-predict the toluene mixing ratio by about a factor of three as well.