Determining rainwater dissolved organic carbon to reveal atmospheric carbon deposition in China's karst city: Variations, origins, and deposition flux

A large amount of organic carbon (OC) in the atmosphere are transferred to the Earth's surface through dry (40%) and wet (60%) deposition. Therefore, wet deposition is an important physical pathway for the removal of atmospheric OC. Identifying the variations of DOC in wet deposition (rainwater) is essential for clarifying the earth-surface C balance. The present study detected the DOC content in 96 daily rainwater samples collected over an entire year in a karst city, with abundant rainfall in Southwest China. The DOC concentrations varied between 1.03–37.7 mg C L−1, while the VWM value stood at 2.76 mg C L−1, which is similar to the worldwide and Asian averages. The yearly DOC wet deposition flux was calculated as 2.88 g C m−2 yr−1. Both the VWM concentration and flux of rainwater DOC exhibit pronounced seasonal variations and regional differences, primarily controlled by precipitation and the external input of OC. The sources of rainwater DOC were multifaceted, including urban waste emission, agricultural activities, and fossil fuel combustion (especially coal combustion), while the contribution from natural sources was limited. Additionally, the C: N (2.08) ratio of rainwater is significantly lower than the Redfield ratio (6.625), indicating the importance of considering the impact of nutrient components in rainfall on the material cycling of surface water ecosystems. This work will benefit the knowledge of the present status of rainwater DOC and its deposition flux on the karst regional scale and further on a global scale, and contribute to a better understanding of the C geochemical cycle on the territorial system.