Rapid decline in atmospheric organic carbon deposition in rural Beijing, North China between 2016 and 2020

Abstract

Atmospheric deposition of dissolved organic carbon (DOC) is equivalent to the magnitude of riverine export of organic carbon (OC) to the oceans. Interpreting the fluctuations of DOC in precipitation is therefore essential in understanding the global carbon balance. This study measured DOC in 137 rain events collected from November 2016 to November 2020 at a rural site in agricultural areas of Beijing on the North China Plain. Concentrations of DOC ranged from 0.09 to 19.69 mg C L−1 with a volume weighted mean value of 2.76 ± 0.27 mg C L−1, which is comparable with those measured in North China a decade ago (3.0 ± 0.4 mg C L−1) and the global mean level (2.64 ± 2.04 mg C L−1). While differences in these values were small, their overall mean values were statistically different (p < 0.05). When classified via air mass back-trajectories, precipitation events with trajectories from south demonstrated higher DOC concentrations than those from north, matching with the spatial distribution of OC emission in the region. The annual precipitation DOC fluxes varied from 6.60 to 15.85 kg C ha−1 yr−1, with an average of 11.05 ± 3.83 kg C ha−1 yr−1 in this study. Both the annual mean concentrations and fluxes of precipitation DOC decreased by approximately 60% between 2017 and 2020, coinciding with the declining trend in OC emissions and aerosol OC concentrations due to the clean air actions. These unique data will allow for a better understanding of current status of rainwater DOC deposition on the global scale and will be critical for evaluating the impacts of future changes, e.g., replacing coal with natural gas, on atmospheric composition.