Reply to comment by Ted Michaels on “Recent trends in anthropogenic mercury emission in the northeast United States”: Waste‐to‐energy's low mercury emissions

[1] Sigler and Lee [2006] (hereinafter referred to as SL06) combined measurements of mercury (Hg) and a combustion tracer at a background site to estimate regional Hg emission rates in the northeast United States from 1999/2000 to 2003/2004. Significant interannual variation in regional Hg emission was observed. Inventory calculations of Hg flux from the regional electric power sector suggested that the power sector strongly influences annual variation in Hg emission in the northeast but may not account for as large of a percentage of the total atmospheric Hg flux as expected given previous inventory estimates. [2] We thank Michaels [2007] (hereinafter referred to as M07) for comments on SL06. M07 cited substantial reductions in Hg emissions from municipal waste combustors (MWC) nationwide since 1990 and a clear ability of MWC to achieve federal emissions standards and suggested improper speculation by SL06 as to the role of MWC in Hg emissions in the northeast. Here we briefly address several of the important issues raised by M07. [3] We stress that SL06 did not claim that MWC cannot or do not test significantly lower than U.S. emission standards as defined by the Clean Air Act. More importantly, SL06 did not speculate that ‘‘high mercury emission levels in the northeast might be attributable’’ to MWC and failure to achieve standards, as suggested by M07, or to the performance of any other source category. SL06 cited multiple sources which indicate significant reductions in Hg emissions from MWC in several northeastern states since the late 1990s and were led to specifically investigate inventory Hg flux from the electric power sector because the substantial decline in MWC emissions due to legislative impact would leave power plants as by far the most important Hg emission category in the northeast [U.S. Environmental Protection Agency (EPA), 1997]. Rather than qualifying or searching for reasons behind high emission rates, SL06 sought to understand the observed interannual variation in Hg emissions determined from atmospheric measurements and gain insight into the relative contributions of different sources to total emissions in the northeast. [4] SL06 posited several potential contributing factors for the observation that power sector emissions were unable to account for as high a percentage of total Hg emissions in the northeast as expected. Among them (SL06, paragraph 38) was the possibility that ‘‘emissions from municipal and medical waste combustion emissions have not been reduced to the level targeted by EPA in 1997 and still contribute significantly to the regional Hg emission rate.’’ While not intended to suggest that MWC are responsible for ‘‘high mercury emission levels’’ in the northeast or are unable to test below federal standards, this contention could be misleading and deserves clarification. [5] As M07 noted, U.S. Hg emissions from large MWC declined significantly (95%) between 1990 and 2000 because of Maximum Achievable Control Technologies (MACT) compliance [EPA, 2002]. According to EPA [1997], New Source Performance Standards (NSPS) adopted in 1995 sought to reduce U.S. Hg emissions from municipal and medical waste combustion by at least an additional 90% over 1995 levels by 2000. Using inventory estimates presented by EPA [1997] as a baseline, we therefore expected MWC emissions to have declined from roughly one third of total Hg emission (1995 inventory) in the source region considered by SL06 (New England, MidAtlantic, Maryland, and Delaware) to on the order of 5% during the observation period (1999/2000–2003/2004), assuming no changes in emission among other major sources. This is clearly not the case, despite the dramatic reductions in bulk Hg emissions from MWC that have been achieved during the past decade. For example, recent inventory data suggest that MWC may have contributed approximately 21% of total Hg emissions in the northeast (not including Pennsylvania, Maryland, and Delaware) as recently as 2002, roughly the midpoint of the observation period of SL06, despite an 86% reduction in total MWC emissions in the same region since 1998 [Northeast States for Coordinated Air Use Management (NESCAUM), 2005]. We also note that total Hg emissions from MWC in New England, New York, and New Jersey were approximately 8.5 Mg in 1995 [EPA, 1997]. If a 90% reduction were JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D13314, doi:10.1029/2006JD008069, 2007 Click Here for Full Article