Natural and anthropogenic methane sources in New England

We have recently completed a methane emissions inventory for the New England region. Methane emissions were calculated to be 0.91 Tg yr^-1, with wetlands and landfills dominating all other sources. Wetlands are estimated to produce 0.33 Tg CH₄ yr^-1, of which 74% come from Maine. Active landfills emit an estimated 0.28 Tg CH₄ yr^-1, 60% of which are generated from twelve landfills. Although uncertainty in the estimate is greater, emissions from closed landfills are on the same order of magnitude as active landfills and wetlands; 0.25 Tg CH₄ yr^-1. Sources of moderate magnitude include ruminant animals (0.05 Tg CH₄ yr^-1) and residential wood combustion (0.03 Tg CH₄ yr^-1). Motor vehicles, natural gas, and wastewater treatment make only minor contributions. New England is heavily forested and the soil uptake of atmospheric methane in upland forests, 0.06 Tg CH₄ yr^-1, decreases emissions from soils by about 18%. Although uncertainties remain, our estimates indicate that even in a highly urbanized region such as New England, natural sources of methane make the single greatest contribution to total emissions, with state totals varying between 8% (Massachusetts) and 92% (Maine). Because emissions from only a few large landfills dominate anthropogenic sources, mitigation strategies focused on these discrete point sources should result in significant improvements in regional air quality. Current federal regulations mandate landfill gas collection at only the largest sites. Expanding recovery efforts to moderately sized landfills through either voluntary compliance or further regulations offers the best opportunity to substantially reduce atmospheric methane in New England. In the short term, however, the large contribution from closed, poorly regulated landfills may make the attribution of air quality improvements difficult. Mitigation efforts toward these landfills should also be a priority.