Seasonal Variations of Carbonaceous Aerosols of PM2.5 at a Coastal City in Northern China: A Case Study of Qinhuangdao

Carbonaceous aerosols exert significant impacts on human health and climate systems. This study investigates the seasonal variations of carbonaceous components in fine particulate matter (PM2.5) in Qinhuangdao, a coastal city in northern China, throughout 2023. The mass concentrations of organic carbon (OC) and elemental carbon (EC) averaged 9.44 ± 4.57 μg m−3 and 0.84 ± 0.33 μg m−3, contributing 26.49 ± 8.74% and 2.81 ± 1.56% to total PM2.5, respectively. OC exhibited a distinct seasonal trend: winter (12.02 μg m−3) > spring (11.96 μg m−3) > autumn (8.15 μg m−3) > summer (5.71 μg m−3), whereas EC followed winter (1.31 μg m−3) > autumn (0.73 μg m−3) > spring (0.70 μg m−3) > summer (0.63 μg m−3). Both OC and EC levels were elevated at night compared to daytime. Secondary organic carbon (SOC), estimated via the EC-tractor method, constituted 37.94 ± 14.26% of total OC. A positive correlation between SOC/OC ratios and PM2.5 concentrations suggests that SOC formation critically influences haze events. In autumn and winter, SOC formation was higher at night, likely driven by aqueous-phase reactions, whereas in summer SOC formation was more pronounced during the day, likely due to enhanced photochemical reactions. Source apportionment analysis revealed that gasoline and diesel vehicles were major contributors to carbonaceous aerosols, accounting for 27.35–29.06% and 14.97–31.83%, respectively. Coal combustion contributed less (10.51–21.55%), potentially due to strict regulations prohibiting raw coal use for domestic heating in surrounding regions. Additionally, fugitive dust was found to have a high contribution to carbonaceous aerosols during spring and summer.