摘要
CO2 and CH4 are the two most potent greenhouse gases (GHGs) in the atmosphere. They are frequently monitored at background observatories, but less so in urban areas, especially in China. In this study, the temporal variations of urban atmospheric CO2 and CH4, including seasonal, weekly, and diurnal characteristics, were obtained based on one year of continuous measurement in the megacity Shanghai. Probable anthropogenic sources were further investigated through relationships among GHGs and atmospheric pollutants. Potential sources regions and transport pathways of GHGs were studied in association with meteorological conditions. The annual average concentrations of CO2 and CH4 were 428.36 +/- 13.96 ppm and 2154 +/- 190 ppb, respectively, which were higher than those at global and regional background observatories. The highest CO2 concentration appeared in winter and the lowest in autumn, whereas CH4 values were highest in summer and lowest in spring. Both CO2 and CH4 showed strong diurnal variations, but only CO2 exhibited seasonal and weekly differences due to the variation of emission sources and plant photosynthesis in Shanghai and the surrounding Yangtze River Delta (YRD). CO2 and combustion-derived air pollutants were co-emitted from similar sources and co-transported to the research site, as demonstrated by correlation analysis. Furthermore, the correlation analysis also suggested some neglected anthropogenic combustion sources of CH4 in Shanghai, besides the traditional biogenic emissions from landfills, wetlands, paddy fields, and dairy farms. The anthropogenic CO2 concentration in urban Shanghai estimated from a CO2-CO regression model was 14.98 +/- 5.09 ppm, together with a background concentration of 413.31 +/- 7.07 ppm, which was higher than at global background sites but lower than at the nearby regional background Lin'an station. GHGs were accumulated from local sources under stable weather conditions and transported from remote sources at high wind speeds. Air masses from super-emitters north and south of the observation site contributed most of the elevation in GHGs concentration above their annual averages. Considering the co-emissions of GHGs and air pollutants from carbon-containing material combustion and use, and the potential of methane to generate O-3 in the urban environment, reduction of CO2 and CH4 together with air pollutants could benefit both climate and air quality in Shanghai and the surrounding YRD.
