Airborne Observations of Mercury Emissions from the Chicago/Gary Urban/Industrial Area during the 2013 NOMADSS Campaign

Atmospheric emissions from the Chicago/Gary urban/industrial area significantly enhance ambient mercury (Hg) concentrations and lead to increased levels of atmospheric mercury deposition within the Lake Michigan Basin (Gratz et al., 2013a; Gratz et al., 2013b; Landis and Keeler, 2002; Landis et al., 2002; Vette et al., 2002). In this study we use airborne observations of total atmospheric Hg (THg) collected over Lake Michigan during summer 2013 as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) field campaign to quantify the outflow of total atmospheric Hg from the Chicago/Gary urban/industrial area. We use concurrent airborne measurements of THg, carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2) to calculate measured enhancement ratios (ER) and thus characterize Chicago/Gary emissions. We determine the observed THg/CO ER in outflow from Chicago/Gary to be 2.11x10-7 mol mol-1, which is comparable to values reported in the literature for other major U.S. urban/industrial areas (Radke et al., 2007; Talbot et al., 2008; Weiss-Penzias et al., 2013). We also employ the FLEXPART Lagrangian transport and dispersion model to simulate air mass transport during plume encounters. We convolve the emissions of each species from the 2011 U.S. EPA National Emissions Inventory (NEI) with the FLEXPART-modeled air mass transport to compare our observations to inventoried emission ratios (EmR). We find that the inventoried THg/CO EmRs are biased low by -63% to -67% compared to the observed ERs for the Chicago/Gary area. This suggests that there are many small emission sources that are not fully accounted for within the inventory, and/or that the re-emission of legacy Hg is a significant source of THg to the atmosphere in this region.