Seasonal variations of atmospheric CH4 at Jingdezhen station in Central China: Understanding the regional transport and its correlation with CO2 and CO

Abstract Greenhouse gases observation networks have been extensively established in the eastern, southern, western, and northern China, but rare in the developing central China. We conducted in-situ measurement of atmospheric CH4 mixing ratios with high precision at JDZ (Jingdezhen) station (29.37°N, 117.22°E, 87.3 m a.s.l) in central China during December 2017 and November 2018. The “regionally representative data” and “polluted events” are separated from the complete dataset, based on the analysis of diurnal discrepancy among different levels, the impacts of wind directions and speeds, and the statistical identification procedure. The average CH4 mixing ratio of regionally representative compositions during the study period was 2037.4 ppb, lower than that of polluted conditions (2086.3 ppb). The lowest levels of CH4 for regionally representative conditions are observed in summer, while highest values are obtained during June–August for polluted events. Cluster analysis of backward trajectories indicated that air-mass transport arising from northwestern China, eastern China and Jiangxi made the significant contribution to the variability in regionally representative air. Air masses passing through Jiangxi province and Yangtze River Delta (YRD) regions carried the polluted air with higher CH4 mixing ratios to the observational station, probably owing to large quantities of CH4 emission from rice cultivation over these areas. △CH4 and △CO exhibit strong correlations in all month during the study period. Moreover, the △CO/△CH4 ratio showed the distinct seasonal characteristics, with the rapid decrease from the peak (2.74 ± 0.05, ppb/ppb) in January to the trough (0.40 ± 0.02, ppb/ppb) in July, and then the gradual increase to high levels in winter. It is possibly attributed to the maximum CH4 emission during summertime and the maximum CO emission during wintertime, as well as the significantly faster reaction rate of CO with OH in summer. The enhancements of CH4 (△CH4) and CO2 (△CO2) in polluted conditions relative to the regionally representative conditions present strong correlation only in winter, with the △CH4 /△CO2 slope of (3.92 ± 0.08) ppb/ppm, similar with the average △CH4 /△CO2 ratio observed in winter (4.15 ± 0.40, ppb/ppm) reported for LAN regional station during 2007 and 2013.