CO2 and CH4 observations surrounding Beijing to to distinguish possible emission processes and locations

CO₂ and CH₄ are radiatively important trace gases closely associated with human activity particularly in urban emission hotspots. Through rapid development and economic growth China has become a major source of CO₂. CO₂ emission inventories for China are becoming increasingly accurate. CH₄ emissions in China are not as well characterized, though for various reasons, including; Chinese policies mandating conversion from coal to natural gas for district heating, intensification of agriculture, and the large volumes of urban waste that must be managed, it is likely CH₄ emissions from Chinese urban centers could be significant. As part of an ongoing Tsinghua – Harvard collaboration we have set up a pair of atmospheric observatories to the north and south of Beijing. The northern site (Miyun) is 75km NNE and the southern site, Dashiwo, is 63 km SSW of the center of Beijing. Miyun has been in semi continuous operation since 2005. Miyun was located to sample Beijing urban outflow as well as clean airmasses depending on wind direction. Dashiwo is located primarily to capture the polluted air coming into Beijing from Hebei province, though it will also be influenced at times by cleaner airmasses coming over the mountains on the western edge of the basin. The high accuracy and precision measurements of CO₂ and CH₄ that are the focus of this presentation started in May 2018. Observations at Dahsiwo started in November 2019.For this presentation we focus on quantifying the magnitudes of CO₂ and CH₄ in urban-influenced air masses and their enhancements relative to clean background air. The correlations between CO₂ and CH₄ and their relationships to other air pollutant tracers including SO2, NOx/NOy, and CO provide constraints on potential sources for these gases. Through back trajectory analysis the source regions can be distinguished. As expected, both sites have enhanced mixing ratios of CO₂ and CH₄. Median CO₂ during the overlapping period Nov. Dec. 2019 is 430, and 459 ppm at Miyun and Dashiwo. Median CH₄ is 2036 and 2228 ppb. Outside the growing season when CO₂ is influenced by vegetation uptake the CH₄:CO₂ ratio is 6.1 ppb:ppm. The Dashiwo data are bounded by the same slope, but have more scatter due to periods with elevated CH₄ but not CO₂. A tight correlation for CO₂ and CH₄ at Miyun suggests a single predominant combustion or respiration source type, while variability in the Dashiwo observations suggests multiple sources including some rich in CH₄ that are not combustion or respiration. Identification of major CH₄ sources is a starting point for choosing mitigation options.