Seasonally Resolved Excess Urban Methane Emissions from the Baltimore/Washington, DC Metropolitan Region

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Climate Change Research

Background:

• Urban areas are significant, yet under-quantified, sources of methane (CH4).
• Limited seasonal data hinders accurate assessment of urban CH4 emissions and their climate impact.

Purpose of the Study:

• To quantify seasonal and annual urban CH4 emissions in the Baltimore and Washington, DC metropolitan areas.
• To compare modeled CH4 concentrations with atmospheric observations for emission refinement.
• To identify discrepancies between bottom-up emission inventories and top-down observational data.

Main Methods:

• Utilized CH4 atmospheric observations from four tall tower stations.
• Employed a Lagrangian particle dispersion model to simulate CH4 concentrations.
• Applied a geostatistical inversion method with an ensemble approach for robust emission estimation.
• Incorporated multiple meteorological datasets, emission inventories, and upwind CH4 values.

Main Results:

• Optimized annual CH4 emissions in DC/Baltimore were 39 ± 9 Gg/month, approximately 2.0 times higher than bottom-up inventory estimates.
• Forward simulations showed discrepancies, underestimating CH4 in cooler months and overestimating in summer due to modeled wetland emissions.
• Identified modest seasonal variability in urban CH4 emissions, with summer emissions ~41% lower than winter.

Conclusions:

• Urban CH4 emissions in the studied regions significantly exceed current inventory estimates.
• Seasonal variations in CH4 emissions suggest a dominant contribution from fugitive natural gas sources.
• Findings underscore the need for improved urban emission inventories and seasonal monitoring for effective climate mitigation.