Mobile observations of intracity variations in atmospheric CO₂ and CH₄

The spatial variability in the atmospheric CO₂ and CH₄ concentrations in urban land is affected by the source type, source distribution and emission intensity in the cityscape. In this study, we analyze vehicle-mounted measurements of street-level CO₂ and CH₄ concentrations in Hangzhou, a large metropolitan area in the Yangtze River Delta in eastern China. The results revealed that CO₂ and CH₄ emission hotspots did not overlap geographically, with the former occurring as linear features at elevated road intersections and expressways and the latter occurring at waste treatment facilities (sewage treatment plants and landfills). The CH₄:CO₂ emission ratios (ppb ppm^-1) were ranked in increasing order as follows: traffic (1.01 ± 1.82; mean ± 1 SD), overall (3.46 ± 2.71), sewage treatment (12.76 ± 2.50) and landfill (36.50 ± 10.15). Waste treatment was largely responsible for the increased overall emission ratio, supporting this source category as a major contributor to the CH₄ budget in this city and suggesting a negligible role of domestic appliances (cookstoves and water heaters). A two-source mixing model calculation indicates that 99.9% of nonelectric vehicles in Hangzhou were gasoline-powered, revealing a recent shift in vehicle fuel composition from gasoline/natural gas to gasoline/electricity. The methodology established in this study is applicable to cities elsewhere.