Emissions from Gas Distribution and Postmeter Leaks May Drive Seasonal and Diurnal Trends in Methane Emissions in Baltimore

We estimated diurnal and seasonal trends in methane (CH4) emissions and evaluated their relationships with trends in natural gas usage data in Baltimore, MD, USA. We used Monin-Obukhov Similarity Theory for flux estimation at urban, suburban, and rural sites using several years of tower mole fraction measurements and compared flux estimates to hourly natural gas usage data for one year at the urban sites. A diurnal pattern with larger daytime emissions is present at all sites and lags a similar diurnal pattern in natural gas usage at sites where usage data is available, suggesting the diurnal trends are related to postmeter emissions. The estimates show a seasonal trend and higher emissions at the urban sites which we expect is related to natural gas distribution pipeline leaks and leak maintenance schedules. These seasonal trends are absent at the suburban sites which are in areas where pipelines have likely been upgraded. Our results reinforce that postmeter gas emissions are likely an important source of urban CH4 emissions. We expect that gas pipeline leak maintenance schedules play an important role driving seasonal CH4 emissions trends, and differences in pipe material can cause within-city variability in CH4 emissions magnitudes and trends.