Mercury Emission Ratios from Coal-Fired Power Plants in the Southeastern United States during NOMADSS.

We use measurements made onboard the National Science Foundation's C-130 research aircraft during the 2013 Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) experiment to examine total Hg (THg) emission ratios (EmRs) for six coal-fired power plants (CFPPs) in the southeastern U.S. We compare observed enhancement ratios (ERs) with EmRs calculated using Hg emissions data from two inventories: the National Emissions Inventory (NEI) and the Toxics Release Inventory (TRI). For four CFPPs, our measured ERs are strongly correlated with EmRs based on the 2011 NEI (r(2) = 0.97), although the inventory data exhibit a -39% low bias. Our measurements agree best (to within ±32%) with the NEI Hg data when the latter were derived from on-site emissions measurements. Conversely, the NEI underestimates by approximately 1 order of magnitude the ERs we measured for one previously untested CFPP. Measured ERs are uncorrelated with values based on the 2013 TRI, which also tends to be biased low. Our results suggest that the Hg inventories can be improved by targeting CFPPs for which the NEI- and TRI-based EmRs have significant disagreements. We recommend that future versions of the Hg inventories should provide greater traceability and uncertainty estimates.

Chemical ionization mass spectrometer instrument for the measurement of tropospheric HO2 and RO2.

Laboratory characterizations of the peroxy radical chemical ionization mass spectrometer (PerCIMS) instrument have been performed. The instrument functions by drawing ambient air through a 50-microm-diameter orifice into an inlet held at low pressure. Peroxy radicals (HO2 and RO2) within this air are detected by amplified chemical conversion into a unique ion (HSO4-) via the chemistry initiated by the addition of NO and SO2 to the inlet. HSO4- ions are then quantified by a quadrupole filter mass spectrometer. PerCIMS provides measurements of the sum of peroxy radicals, HO2 + RO2 (HOxROx mode), or the HO2 component only (HO2 mode), achieved through the control of concentration of NO and SO2 added to the instrument. The characterization and response of this instrument have been evaluated through modeling of inlet chemistry and laboratory experiments and have also been demonstrated through successful deployment during field campaigns. The performance of PerCIMS with respect to calibration pressure and relative humidity is reported, as are the sensitivities of the instrument to organic peroxy radicals with different hydrocarbon groups. These data show PerCIMS to be a practical field instrument for the fast and accurate evaluation of the concentration of peroxy radicals over a variety of atmospheric conditions. The estimated accuracy of the derived [HOxROx] concentrations is +/- 35% (at the 95% confidence interval), while [HO2] measurements have accuracies of +/- 41% (at the 95% confidence interval). Typical precision of measurements well above the detection limit is 10%, and typical detection limits are 1 x 10(7) radicals cm(-3) for 15-s averaging times.