A Comparison of Inventoried and Measured U.S. Urban/Industrial Hg Emission Factors during the NOMADSS Experiment
Wednesday, 17 December 2014
We performed an airborne survey of some large anthropogenic mercury (Hg) emission sources in the Southeast U.S. during the 2013 Nitrogen, Oxidants, Mercury and Aerosol Distribution, Sources, and Sinks (NOMADSS) experiment. The observations included speciated atmospheric Hg, and tracers of urban/industrial emissions and associated photochemistry (e.g., carbon monoxide, CO; carbon dioxide, CO2; sulfur dioxide, SO2; nitrogen oxides (NOx); volatile organic compounds, VOCs; ozone, O3; hydroxyl radical, HO·; sulfuric acid, H2SO4) and were made from the National Science Foundation’s/National Center for Atmospheric Research’s C-130 research aircraft. Mercury was measured using the University of Washington’s Detector for Oxidized Hg Species. We derived Hg emission factors (EF) for several U.S. urban areas and large industrial point sources, including coal-fired power plants (CFPPs) in Louisiana, Pennsylvania, Texas, and West Virginia. We compared our measured Hg EFs with inventory-based values from two separate Hg emission inventories provided by the U.S. Environmental Protection Agency – the National Emissions Inventory (NEI) and the Toxics Release Inventory (TRI). We also performed an inter-comparison of the inventory-based Hg EFs. For the CFPPs sampled, we find that actual Hg emissions differed from inventoried values by more than a factor of two in some cases. Measured Hg EFs were weakly correlated with values reported in the NEI: m = 0.71; r2 = 0.47 (p = 0.06; n = 8), whereas EFs derived from the TRI were not meaningfully predictive of the measured values: m = −3.3; r2 = 0.61 (p < 0.05; n = 8). Median absolute differences between measured and inventory-based EFs were ≥50%, relative to the inventory values. The median absolute average difference between the Hg EFs reported in the two inventories was approximately 40%. Our results place quantitative constraints on uncertainties associated with the inventoried Hg emissions. Additionally, our results suggest that the current formulation of the Hg emission inventories critically limits our ability to accurately predict the transport and fate of U.S. urban/industrial emissions of Hg to the atmosphere. These findings are broadly relevant to the design and use of emission inventories for industrial hazardous air pollutants.