Atmospheric deposition as a direct source of particulate organic carbon in region coastal surface seawater: Evidence from stable carbon and nitrogen isotope analysis

To assess the source characteristics of coastal aerosols and evaluate the contribution of atmospheric deposition to particulate organic matter in surface seawater, total suspended particulates (TSP) were collected at a shore-based site on the south coast of North Yellow Sea from December 2019 through November 2020. The samples were analyzed for total organic carbon (TOC) and nitrogen (TN) as well as stable carbon and nitrogen isotope (δ¹³C and δ¹⁵N). The results showed that the annual mean concentrations of TOC and TN were 5.36 ± 4.74 and 5.12 ± 6.52 μg m⁻³, respectively. δ¹³C fluctuated between −25.1 ‰ and −19.2 ‰ with an annual mean of −24.0 ± 1.0 ‰ and a significant seasonal variation (P < 0.05) characterizing by the enrichment in winter (−23.4 ± 0.6 ‰) compared to other seasons, which was probably related to the massive coal combustion. Besides, δ¹⁵N ranged from 7.9 ‰ to 21.1 ‰ with an annual mean of 12.5 ± 2.9 ‰ and a less pronounced seasonal pattern (P = 0.23). The Bayesian isotope-mixing model showed that, annually, the most important source of TSP was biogenic and biomass source (55.5 ± 10.8 %), followed by fossil fuel combustion (31.9 ± 9.0 %), while the marine contribution was less (12.6 ± 2.3 %). For TOC and TN, the dominated sources were fossil fuel combustion (47.7 ± 3.4 %) and biogenic and biomass source (57.3 ± 11.7 %), respectively. Furthermore, the model results indicated that the contribution of atmospheric deposition to suspended particulate matter in surface seawater was 18.0 ± 11.0 %, 17.1 ± 6.7 % and 10.2 ± 2.0 % in autumn, spring and summer, respectively. For particulate organic carbon in surface seawater, the contribution of atmospheric deposition was 35.2 ± 3.5 % in spring, highlighting the huge impact of atmospheric deposition on particulate carbon cycling in coastal waters.