Recent trends in anthropogenic mercury emission in the northeast United States

We combine wintertime measurements of total gaseous mercury (TGM) with a combustion tracer (carbon dioxide) at a background site in Connecticut to investigate changes in regional Hg emission from 1999/2000 through 2003/2004. This method allows timelier flux estimates than inventory approaches. We find a 20% decrease in emission between 1999/2000 and 2003/2004, although the rate has increased since 2001/2002. Air trajectory analysis reveals a source region of Hg influencing the site that encompasses most of the mid-Atlantic and New England states, and Maryland and Delaware, with small interannual variation. The significant interannual changes in Hg emission therefore cannot be fully explained by climatological changes. Inventory calculations of Hg flux from the regional electric power sector show significant correlation with the emission trend derived from the atmospheric measurements, suggesting that the power sector strongly influences the regional Hg flux to the atmosphere. However, averaged over the five winters, the Hg flux from the power sector was 47-75% of the atmospheric flux, which is assumed to be >95% elemental Hg. If 50% or more of Hg emitted by power sources is speciated and deposits locally, the power sector can account for only 23-40% of the observed elemental Hg flux to the atmosphere. Possible explanations include conversion of some reactive Hg emitted by power plants to elemental Hg, underestimation of inventory emissions from sources outside the power sector, the possibility that emissions from municipal and medical waste combustion have not been curbed to the EPA targeted level, or that a combination of these factors exists.