Characteristics and drivers of carbonaceous aerosol pollution in a coastal city of northern China

• Coastal secondary organic aerosol (OA) exhibits lower concentrations but higher fractional contributions to total OA. • Sea-land breezes enhanced secondary organic carbon (SOC) mass concentrations. • Interpretable machine learning shows SOC drivers vary significantly by season. • O x drives SOC concentration during sea breezes, and CO dominates under land breeze conditions. Carbonaceous aerosols significantly impact air quality, human health and climate change, yet their concentration levels and influencing factors exhibit significant regional variability. This study examined the concentration levels and temporal variations of carbonaceous aerosols in Qingdao, a typical coastal city in China, using a year-long, high-time-resolution dataset of organic carbon (OC) and elemental carbon (EC) measurements. The impacts of meteorological conditions, primary emissions, atmospheric oxidants, and sea-land breezes were systematically analyzed by employing an interpretable machine learning model. The results revealed that atmospheric OC and EC concentration levels were relatively low in Qingdao, but secondary organic carbon (SOC) accounted for 43% of OC, emphasizing the substantial influence of secondary sources. SOC concentrations peaked in the evening, whereas primary organic carbon (POC) and EC concentrations peaked during morning rush hours. The elevated carbonaceous aerosol concentration observed in winter likely resulted from enhanced primary emissions coupled with unfavorable dispersion conditions, whereas intensive photochemical activities during summer facilitated SOC formation. Higher SOC levels were observed during sea-land breeze days than non-sea-land breeze days. The machine learning model indicated that atmospheric oxidants played an important role in SOC formation during sea-land breeze days, while combustion related emissions may be the key factor on non-sea-land breeze days. Furthermore, SOC levels were higher under land breezes compared to sea breezes, likely due to enhanced primary emissions from terrestrial sources coupled with confined pollutant dispersion. These findings revealed complex emission-meteorology-chemistry interactions affecting coastal air quality, informing targeted air pollution mitigation strategies.