Water‐Insoluble Components in Rainwater in Suburban Guiyang, Southwestern China: A Potential Contributor to Dissolved Organic Carbon

Dissolved organic carbon (DOC) in rainwater affects climate change and global carbon cycles. Relatively little work has focused on DOC and the remaining water‐insoluble components (WICs) simultaneously; moreover, the potential transformation of WICs to DOC remains questionable. The WICs (i.e., nonleachable particulates) were extracted from rainwater collected in suburban Guiyang (southwestern China) over a 1‐year period. Correlations of DOC with indicative parameters (e.g., potassium, calcium, and sulfate ions) showed that DOC (271 ± 152 μmol L−1) was partly derived from biomass burning, soils, and nitrate‐ and sulfate‐related secondary processes. Moreover, DOC concentration tended to increase with decreasing particulate amino acid‐based degradation index, likely suggesting that the biodegradation of WICs can enhance DOC abundance. Further, a laboratory simulation with WICs from rainwater and particulate matter (PM2.5) demonstrated the microbe‐driven rapid increase in DOC. The organic matter in rainwater was compositionally similar to that in concurrently collected PM2.5, suggesting that rainwater DOC in this area was largely controlled by in‐cloud scavenging of fine particles. This consideration was confirmed by insignificant differences in volume‐weighted DOC concentrations between forepart and posterior (cloud water with little below‐cloud scavenging effect) rainwater. Since the time for rainwater to reach the ground is fairly short, the overall results can indicate that the in‐cloud processing of WICs is a potential source of rainwater DOC. Moreover, this source contribution was estimated to be less than 2% per day. Therefore, the study provides novel insights into the carbon cycle in the atmosphere and surface ecosystems.