Constraining Methane Emissions from Beijing and the North China Plain Using Open-path, Mobile Measurements and FLEXPART-WRF

Abstract:

Methane is the second most important anthropogenic greenhouse gas. Although the global atmospheric methane budget is well constrained, the magnitude and trends of methane emissions from different source types and source regions are highly uncertain. Many studies have shown that bottom-up methane emission inventories are inconsistent with top-down approaches. China is the top emitter of greenhouse gases, and effective characterization of methane emissions is the key to cap Chinese carbon emissions by 2030. However, so far there have been very few top-down constraints on Chinese methane emissions at local to regional scales, due to a lack of in-situ measurements. In this study, mobile measurements of methane and other important tracers (NH₃, CO₂) were conducted on the North China Plain during the CAREBeijing/NCP campaigns in both 2013 and 2014. Simultaneous detection of multiple gas species provides chemical fingerprinting of methane emission sources. The high resolution (10 Hz) and large spatial coverage (5000 km) enable fine-grained mapping of methane concentrations over large area. Extended plumes with high methane mixing ratio (> 2.1 ppmv) were observed over the Beijing metropolitan area. Continuous mobile sampling over the 4th Ring Road in Beijing indicates that the urban background methane mixing ratio (1st percentile of each loop around the Ring Road) peaked at night due to the accumulation of emissions under the stable, shallow mixing layer. The on-road methane mixing ratio was 2.3±0.1 ppmv at noontime and 2.7±0.5 ppmv at night. To simulate atmospheric transport affecting the mobile measurement locations, we use a Lagrangian backward particle dispersion model (FLEXPART) driven by the WRF model with a nested spatial resolutions of 1 km near Beijing and 3 km in the entire North China Plain. Footprints given by FLEXPART-WRF and measurements of other tracers indicate that the Beijing urban area is a strong source of methane. Inverse methods will be applied to quantify and attribute the surface fluxes of methane on the North China Plain with a special focus on the Beijing metropolitan area.