Determination of Urban VOC Emissions Ratios and Comparison with Inventories C. Warneke, J. de Gouw, P. Goldan, W. Kuster, J. Holloway, E. Williams, B. Lerner, S. McKeen D.D. Parrish, M. Trainer, F. Fehsenfeld NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO, 80305 carsten.warneke@noaa.gov S. Kato University of Colorado, Boulder, CO E.L. Atlas University of Miami, Miami, FL A. Baker, D. Blake University of California, Irvine, CA ABSTRACT 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 emission ratios thus determined 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 C 2 -C 4 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. INTRODUCTION Volatile organic compounds (VOCs) are emitted into the atmosphere in large quantities from a variety of different natural and anthropogenic sources. VOCs are key ingredients in the formation of ozone and aerosols in polluted air, and play a significant role in determining regional air quality, in the chemistry of the global troposphere, and possibly in the global carbon cycle. On a global scale the biogenic VOC emissions, mainly isoprene, α- and β-pinene and methanol [1, 2], dominate over the anthropogenic sources. In and around urban areas, anthropogenic emissions of VOCs, which are in large part caused by production, storage and use of fossil fuels, usually are more important. In this paper, we will focus on the determination of primary anthropogenic VOC emission ratios from mainly Boston and New York City during NEAQS-ITCT 2k4 (New England Air Quality Study – Intercontinental Transport and Chemical Transformation), which was a large-scale atmospheric chemistry and transport study conducted in July and August of 2004 in New England. Emission ratios of a large number of VOCs versus acetylene and CO were determined using the ship-based measurements