Size distributions, mixing state, and morphology of refractory black carbon in an urban atmosphere of northeast Asia during summer

Black carbon (BC) exerts profound impacts on air quality, human health, and climate. Here, we investigated concentrations and size distributions of refractory BC (rBC) and mixing state and morphology of rBC-containing particles in urban Seoul for 2019 summer. Mass concentrations of rBC ranged from 0.02 μgm⁻³ to 2.89 μgm⁻³, and daily maximums of rBC mass, daily minimums of rBC mass median diameter (MMD) (110–130 nm), and shell-to-core ratio (R_shell/core) occurred with NO₂ maximums during morning rush hour. As the first report of ground observations on rBC mixing state, these results indicate that vehicle emission is a major local source of rBC in Seoul. MMDs of 127–146 nm and the greatest mass loadings of ≥1 μg m⁻³ were accompanied by high O₃ and PM_2.5 concentrations, in contrast to the largest MMDs (135–165 nm) associated with transport from upstream regions. The average R_shell/core was 1.25 for the rBC mass-equivalent diameter (D_rBC) of 140–220 nm. R_shell/core increased gradually through the day and was positively correlated with O_x concentration, indicating photochemical aging of rBC particles. Co-emissions of rBC and volatile organic compounds from vehicles facilitated internal mixing during the daytime. However, R_shell/core tended to be low at temperature >∼30 °C, while 58 % of rBC particles with R_shell/core exceeding 1.25 were found at nighttime under relative humidity >75 %. These results demonstrate that the mixing state of freshly-emitted rBC particles was altered through coating by photochemically oxidized vapors during the day and hygroscopic growth at night. Additionally, the delay-time approach revealed rBC morphological characteristics, the most common being the bare type (74 %), and the attached type (6 %) was relatively large in numbers during morning rush hour. Therefore, it is suggested that during summer, rBC particles from traffic emissions should be considered in parallel to winter pollution mitigation strategies in urban atmosphere of northeast Asia.