Analysis of the trends in ambient methane in the Baltimore-Washington
region and comparison to model output
Abstract
We studied atmospheric methane observations from November 2016 to
October 2017 from one rural and two urban towers in the
Baltimore-Washington region (BWR). Methane observations at these three
towers display distinct seasonal and diurnal cycles with maxima at night
and in the early morning, reflecting local emissions and boundary layer
dynamics. Peaks in winter concentrations and vertical gradients indicate
strong local anthropogenic wintertime methane sources in urban regions.
In contrast, our analysis shows larger local emissions in summer at the
rural site, suggesting a dominant influence of wetland emissions. We
compared observed enhancements (mole fractions above the 5th percentile)
to simulated methane enhancements using the WRF-STILT model driven by
two EDGAR inventories. When run with EDGAR 5.0, the low bias of modeled
versus measured methane was greater (ratio of 1.9) than the bias found
when using the EDGAR 4.2 emission inventory (ratio of 1.3). However, the
correlation of modeled versus measured methane was stronger
(~1.2 times higher) for EDGAR 5.0 compared to results
found using EDGAR 4.2. In winter, the inclusion of wetland emissions
using WETCHARTs had little impact on the mean bias, but during summer,
the low bias for all hours using EDGAR 5.0 improved by from 63 to 23
nanomoles per mole of dry air or parts per billion (ppb) at the rural
site. We conclude that both versions of EDGAR underestimate the regional
anthropogenic emissions of methane, but version 5.0 has a more accurate
spatial representation.