Seasonal Variations in the Sources and Influential Factors of Aerosol Dissolved Black Carbon at a Southeast Coastal Site in China

Aerosols are a significant source of dissolved black carbon (DBC) in the ocean. However, how anthropogenic activities, including fossil fuel (FF) burning and human-initiated biomass burning (BB), would affect the seasonal variations in the concentration and condensation degree of aerosol DBC was not well constrained. To fill this knowledge gap, we collected 33 aerosol samples from December 2018 to September 2019 at a southeast coastal site in China. Concentrations of DBC, water-soluble organic carbon (WSOC), and elemental carbon (EC) were determined, and air quality data were collected. DBC concentration was 0.02-0.23 mu gC m(-3), with significantly higher values in spring and winter, and was affected by sources, rainfall frequency, and secondary processes. The source of DBC, as indicated by DBC/WSOC (range: 0.009-0.045) and DBC/EC (range: 0.007-0.065), was dominated by FF burning. Furthermore, the significantly higher DBC/ WSOC value in late April to July indicated enhanced contribution from BB that was mainly associated with airmass transported from Southeast Asia (SEA). We further estimated that similar to 30% of aerosol DBC was from BB from May to July. The condensation degree of DBC was variable, with significantly higher values in winter and spring, which might be associated with prolonged photo-dissolution. Significant correlations were also observed between DBC and PM10 and PM2.5, which might be utilized to estimate DBC deposition flux, albeit requiring more studies. Our study emphasized the influence of human activities (including BB activities) on atmospheric DBC deposition, which might affect the regional and even global DBC pools. As a significant source of dissolved black carbon (DBC) to the ocean, the sources and temporal variation in its concentration and condensation degree were not well constrained. In this study, we collected 33 aerosol samples at a coastal site in China. We determined concentrations of DBC, water-soluble organic carbon, and elemental carbon (EC). Air quality and meteorological parameters were also collected. Our results showed that DBC at this coastal site was primarily derived from fossil fuel (FF) burning, with enhanced contribution from biomass burning that was brought by East Asian Monsoon. In addition, DBC concentration varied significantly among seasons, and was influenced by changes in sources, rainfall frequency as well as secondary processes. Moreover, we found that DBC concentration was significantly positively correlated with PM10 and PM2.5, which provided a potential way to predict DBC depositional flux in other regions that was mainly affected by FF burning. Our study indicated that human activities (FF burning and human-initiated biomass burning) could have significant impact on atmospheric DBC deposition, which might also affect the oceanic DBC pool in the future.

Automated summary

Aerosols are an important source of dissolved black carbon (DBC) in the ocean, and the impact of anthropogenic activities on the seasonal variations of aerosol DBC concentration and condensation degree remains unclear. In this study, we collected 33 aerosol samples from a coastal site in China and analyzed the concentrations of DBC, water-soluble organic carbon (WSOC), and elemental carbon (EC). Our results showed that DBC concentrations were higher in spring and winter, influenced by sources, rainfall frequency, and secondary processes. The dominant source of DBC was fossil fuel (FF) burning, with enhanced contribution from biomass burning brought by East Asian Monsoon.