Lohman Kristen, Seigneur Christian, Edgerton Eric, Jansen John
Atmospheric & Environmental Research, Inc., 2682 Bishop Drive, Suite 120, San Ramon, California 94583, USA.
Environ Sci Technol. 2006 Jun 15;40(12):3848-54. doi: 10.1021/es051556v.
Measurements of speciated mercury (Hg) downwind of coal-fired power plants suggest that the Hg(II)/(Hg0 + HgII) ratio (where HgII is divalent gaseous Hg and Hg0 is elemental Hg) decreases significantly between the point of emission and the downwind ground-level measurement site, but that the SO2/(Hg0 + HgII) ratio is conserved. We simulated nine power plant plume events with the Reactive & Optics Model of Emissions (ROME), a reactive plume model that includes a comprehensive treatment of plume dispersion, transformation, and deposition. The model simulations fail to reproduce such a depletion in HgII. A sensitivity study of the impact of the HgII dry deposition velocity shows that a difference in dry deposition alone cannot explain the disparity. Similarly, a sensitivity study of the impact of cloud chemistry on results shows that the effect of clouds on Hg chemistry has only minimal impact. Possible explanations include HgII reduction to Hg0 in the plume, rapid reduction of HgII to Hg0 on ground surfaces, and/or an overestimation of the HgII fraction in the power plant emissions. We propose that a chemical reaction not included in current models of atmospheric mercury reduces HgII to Hg0 in coal-fired power plant plumes. The incorporation of two possible reduction pathways for HgII (pseudo-first-order decay and reaction with SO2) shows better agreement between the model simulations and the ambient measurements. These potential HgII to Hg0 reactions need to be studied in the laboratory to investigate this hypothesis. Because the speciation of Hg has a significant effect on Hg deposition, models of the fate and transport of atmospheric Hg may need to be modified to account for the reduction of HgII in coal-fired power plant plumes if such a reaction is confirmed in further experimental investigations.
对燃煤发电厂下风向特定形态汞（Hg）的测量表明，Hg(II)/(Hg0 + HgII) 比率（其中HgII为二价气态汞，Hg0为元素汞）在排放点与下风向地面测量点之间显著降低，但SO2/(Hg0 + HgII) 比率保持不变。我们使用排放反应与光学模型（ROME）模拟了九次发电厂烟羽事件，ROME是一个反应性烟羽模型，全面处理了烟羽扩散、转化和沉降。模型模拟未能重现HgII的这种减少。HgII干沉降速度影响的敏感性研究表明，仅干沉降差异无法解释这种差异。同样，云化学对结果影响的敏感性研究表明，云对汞化学的影响很小。可能的解释包括烟羽中HgII还原为Hg0、地表HgII快速还原为Hg0，和/或发电厂排放中HgII含量的高估。我们提出，当前大气汞模型中未包含的一种化学反应会使燃煤发电厂烟羽中的HgII还原为Hg0。纳入HgII的两种可能还原途径（准一级衰变和与SO2反应）后，模型模拟与环境测量结果的一致性更好。这些潜在的HgII到Hg0反应需要在实验室中进行研究以验证这一假设。由于汞的形态对汞沉降有显著影响，如果在进一步的实验研究中证实了这种反应，大气汞迁移转化模型可能需要进行修改，以考虑燃煤发电厂烟羽中HgII的还原。
