Dissolved black carbon (DBC) is the largest known slow-cycling organic carbon pool in the world’s oceans. Atmospheric deposition could significantly contribute to the oceanic DBC pool, but respective information is lacking. Here we estimate that, during the dust outbreak season, the atmospheric dry deposition of water-soluble black carbon (WSBC) is ~ 40% of the riverine input to the China coastal seas. The molecular composition of atmospheric WSBC determined by ultrahigh-resolution mass spectrometry, reveals similar soil-derived sources as for riverine discharge. WSBC is significantly positively correlated with water-soluble organic carbon (WSOC) in marine aerosols, and water-soluble black carbon contributes on average 2.8 ± 0.65% to the total WSOC. Based on this relationship, the global atmospheric deposition of DBC to the ocean is estimated to be 1.8 ± 0.83 Tg yr⁻¹. Anticipated future changes in biomass burning and dust mobilization might increase these numbers, with consequences for regional ecosystems and global carbon reservoirs.

In order to evaluate impacts of different source emission on marine atmospheric particles over the South China Sea (SCS), major inorganic ionic concentrations (Na⁺, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, K⁺, NH₄⁺ and NO₃⁻) were determined in total suspended particulates (TSPs) at Yongxing Island, from March 2014 to February 2015. The annual average concentration of TSPs was 89.6 ± 68.0 µg m⁻³, with 114.7 ± 82.1, 60.4 ± 27.0 and 59.5 ± 25.6 µg m⁻³ in cool, warm and transition seasons, respectively. Cl⁻ had the highest concentration, with an annual average of 7.73 ± 5.99 µg m⁻³, followed by SO₄²⁻ (5.54 ± 3.65 µg m⁻³), Na⁺ (4.00 ± 1.88 µg m⁻³), Ca²⁺ (2.15 ± 1.54 µg m⁻³), NO₃⁻ (1.95 ± 1.34 µg m⁻³), Mg²⁺ (0.44 ± 0.33 µg m⁻³), K⁺ (0.33 ± 0.22 µg m⁻³) and NH₄⁺ (0.07 ± 0.07 µg m⁻³). Concentrations of TSPs and the major ions showed seasonal variations, which were higher in the cool season and lower in the warm and transition seasons. Factors of influence were wind speed, temperature, relatively humidity, rain and air mass source region. Back trajectories, concentration-weighted trajectories (CWTs), and positive matrix factorization (PMF) of chemical compositions were analyzed for source apportionment, source contribution and spatiotemporal variation of major ions. Back trajectories and CWTs showed that air masses at Yongxing Island were mainly from the northeast, southwest and southeast in the cool, warm and transition seasons, respectively. The PMF results showed that 77.4 % of Na⁺ and 99.3 % of Cl⁻ were from sea salt; 60.5 % of NH₄⁺ was from oceanic emission. Anthropogenic sources were very important for atmospheric aerosols over the island. Secondary inorganic aerosol of SO₂ and NO_x from fossil fuel combustion (especially coal in Chinese coastal regions) was the dominant source of NO₃⁻ (69.5 %) and SO₄²⁻ (57.5 %).