We present two full years of continuous C6-C8 aromatic compound measurements by PTR-MS at the KCMP tall tower (Minnesota, US) and employ GEOS-Chem nested grid simulations in a Bayesian inversion to interpret the data in terms of new constraints on US aromatic emissions. Based on the tall tower data, we find that the RETRO inventory (year-2000) overestimates US C6-C8 aromatic emissions by factors of 2.0-4.5 during 2010-2011, likely due in part to post-2000 reductions. Likewise, our implementation of the US EPA’s NEI08 overestimates the toluene flux by threefold, reflecting an inventory bias in non-road emissions plus uncertainties associated with species lumping. Our annual top-down emission estimates for benzene and C8 aromatics agree with the NEI08 bottom-up values, as does the inferred contribution from non-road sources. However, the NEI08 appears to underestimate on-road emissions of these compounds by twofold during the warm season. The implied aromatic sources upwind of North America aremore than double the prior estimates, suggesting a substantial underestimate of East Asian emissions, or large increases there since 2000. Long-range transport exerts an important influence on ambient benzene over the US: on average 43% of its wintertime abundance in the US Upper Midwest is due to sources outside North America. Independent aircraft measurements show that the inventory biases found here for C6-C8 aromatics also apply to other parts of the US, with notable exceptions for toluene in California and Houston, Texas. Our best estimates of year-2011 contiguous US emissions are 206 (benzene), 408 (toluene), and 822 (C8 aromatics) GgC.
Bibliographical noteFunding Information:
This study was supported by the National Science Foundation (grants 0937004 and 1148951), by the Minnesota Supercomputing Institute, and by a University of Minnesota Doctoral Dissertation Fellowship. We thank Tom Nelson and Minnesota Public Radio for their logistical support at the KCMP tall tower. PTR-MS measurements during DISCOVER-AQ and DC-3 were supported by the Austrian Space Applications Programme (ASAP, bmvit, FFG-ALR). TM was supported by the NASA Postdoctoral Program. We acknowledge the US Environmental Protection Agency for providing 2006 and 2010 North American emission inventories. These emission inventories are intended for research purposes and were developed for Phase 2 of the Air Quality Model Evaluation International Initiative (AQMEII) using information from the 2008-based modeling platform (http://www.epa.gov/ttn/ chief/emch/index.html#2008) as a starting point. Measurements from the KCMP tall tower described here are available for download at http://www. atmoschem.umn.edu/data.htm.