Evaluation of discrepancy between measured and modelled oxidized mercury species
L. Zhang et al. (2012), in a recent report, compared model estimates with new observations of oxidized and particulate mercury species (Hg 2+ and Hg p) in the Great Lakes region and found that the sum of Hg 2+ and Hg p varied between a factor of 2 to 10 between measurements and model. They suggested too high emission inputs as Hg 2+ and too fast oxidative conversion of Hg 0 to Hg 2+ and Hg p as possible causes. This study quantitatively explores measurement uncertainties in detail. These include sampling efficiency, composition of sample, interfering species and calibration errors. Model (Global/Regional Atmospheric Heavy Metals Model – GRAHM) sensitivity experiments are used to examine the consistency between various Hg measurements and speciation of Hg near emission sources to better understand the discrepancies between modelled and measured concentrations of Hg 2+ and Hg p. We find that the ratio of Hg 0, Hg 2+ and Hg p in the emission inventories, measurements of surface air concentrations of oxidized Hg and measurements of wet deposition are currently inconsistent with each other in the vicinity of emission sources. Current speciation of Hg emissions suggests higher concentrations of Hg 2+ in air and in precipitation near emission sources; however, measured air concentrations of Hg 2+ and measured concentrations of Hg in precipitation are not found to be significantly elevated near emission sources compared to the remote regions. The averaged unbiased root mean square error (RMSE) between simulated and observed concentrations of Hg 2+ is found to be reduced by 42% and for Hg p reduced by 40% for 21 North American sites investigated, when a ratio for Hg 0 : Hg 2+ : Hg p in the emissions is changed from 50 : 40 : 10 (as specified in the original inventories) to 90 : 8 : 2. Unbiased RMSE reductions near emissions sources in the eastern United States and Canada are found to be reduced by up to 58% for Hg 2+. Significant improvement in the model simulated spatial distribution of wet deposition of mercury in North America is noticed with the modified Hg emission speciation. Measurement-related uncertainties leading to lower estimation of Hg 2+ concentrations are 86%. Uncertainties yielding either to higher or lower Hg 2+ concentrations are found to be 36%. Finally, anthropogenic emission uncertainties are 106% for Hg 2+. Thus it appears that the identified uncertainties for model estimates related to mercury speciation near sources, uncertainties in measurement methodology and uncertainties in emissions can close the gap between modelled and observed estimates of oxidized mercury found in L. Zhang et al. (2012). Model sensitivity simulations show that the measured concentrations of oxidized mercury, in general, are too low to be consistent with measured wet deposition fluxes in North America. Better emission inventories (with respect to speciation), better techniques for measurements of oxidized species and knowledge of mercury reduction reactions in different environments (including in-plume) in all phases are needed for improving the mercury models.