INVESTIGATION OF AMBIENT METHANE CONCENTRATION, SOURCES, AND TRENDS IN THE BALTIMORE-WASHINGTON REGION

Abstract

Methane, an important and not yet fully understood greenhouse gas, has a global warming potential 25 times that of carbon dioxide over 100 years, although with an atmospheric lifetime much shorter than carbon dioxide. Controlling methane emissions is a useful way to avoid some of the adverse effects of climate change at least on short time scales. Natural sources include wetlands, ruminants, and wildfires, while anthropogenic sources include the production, transmission, distribution, and use of natural gas, livestock, and landfills. In the US, natural gas and petroleum systems, anthropogenic sources, are the second-largest source of methane emissions. Urban areas are a significant source of anthropogenic methane emissions, primarily fugitive emissions from natural gas distribution and usage.We studied methane observations from five towers in the Baltimore-Washington (BWR) region – two urban towers ARL (Arlington, VA), NEB (Northeast Baltimore, MD), and one rural tower, BUC (Bucktown, MD). Methane measurements from these three towers displayed distinct seasonal and diurnal cycles with maxima at night and in the early morning, which indicated significant local emissions. We concluded from our analysis that anthropogenic methane emissions dominate at the urban sites whereas wetland emissions dominate at the rural site. We compared observed enhancements (mole fractions above the 5th percentile) to simulated methane enhancements using the WRF-STILT model driven by two EDGAR inventories – EDGAR 4.2 and EDGAR 5.0. We did a similar comparison between model and observations with vertical gradients. We concluded that both versions of EDGAR underestimated the regional anthropogenic emissions of methane, but version 5.0 had a more accurate spatial representation. We ran the model with WETCHARTs to account for wetland emissions which significantly reduced the bias between model and observations especially in summer at the rural site. We investigated winter methane observations from three towers in the BWR including a ten-year record, 2013-2022, from BUC, located ~100 km southeast of these urban areas. We combined the observations with a HYSPLIT clustering analysis for all years to determine the major synoptic patterns influencing methane mixing ratios at BUC. For methane concentrations above global background, the cluster analysis revealed four characteristic pathways of transport into BUC – from the west (W), southwest (SW), northwest (NW), and east (E) and these showed significant differences in methane mixing ratios. We corroborated our conclusions from BUC using 2018-2022 data from towers in Stafford, Virginia (SFD), and Thurmont, Maryland (TMD); results confirmed the influence of synoptic pattern, typically associated with frontal passage, on methane. No significant temporal trend over the global background was detected overall or within any cluster. For BUC, low concentrations were observed for air off the North Atlantic Ocean (E cluster) and flowing rapidly behind cold fronts (NW cluster). High methane mixing ratios were observed, as expected, in the W cluster due to the proximity of the BWR and oil and gas operations in the Marcellus. Less expected were high mixing ratios for the SW cluster – we attribute these to agricultural sources in North Carolina. Swine production, ~500 km to the SW, impacts methane in eastern Maryland as much or more than local urban emissions plus oil and gas operations 100–300 km to the west; this supports the high end of emission estimates for animal husbandry and suggests strategies for future research and mitigation.