Determining Methane Emission Rates from Urban Compressed Natural Gas Stations Using Mobile Measurements and a New Urban Vertical Dispersion Parameterization

This study introduced a high-resolution mobile measurement method and a new Gaussian vertical dispersion parametrization for plume dispersion in urban surface, with an application to CH₄ emission measurements of compressed natural gas (CNG) stations in urban Beijing, China. Tracer release experiments were conducted at two urban sites, and the results showed that the vertical plume dispersion coefficient in urban surface has an exponential relationship with plume travel distance, despite the surface topology. The results were fitted to a new vertical dispersion parametrization: ln(D_z) = −2.42 × ln(x) + 6.28. Using the new vertical dispersion parametrization along with lateral dispersion widths mapped through mobile measurements, the CH₄ emission rates of CNG fueling stations were determined, with an average of 2.5 (95% CI [0.21,30]) kg h^–1 for normal operating conditions, and 0.56 (95% CI [0.07,4.4]) kg h^–1 for stations under nonoperating conditions. A CH₄ emission factor (EF) of CNG fueling stations in Beijing was calculated based on the measured CH₄ emission rate and the annual CNG consumption in transportation, with an average of 0.010 (95% CI [0.001,0.087]) kg/kg. This indicates that the CH₄ emission of CNG fueling stations is nonnegligible compared with the emission of natural gas vehicles and should be considered in an inventory study.