Physical and optical properties of black carbon observed at the 303-meter-high Tower in the urban environment.

Urban air pollution has consistently captured social and scientific attention due to its significant health and climatic impacts. Among the short-lived climate pollutants (SLCPs) targeted for reduction, black carbon (BC) stands out as a critical component. BC is a carbonaceous primary aerosol emitted from fossil fuel and biomass combustion, with an atmospheric lifetime of approximately five days.

This study involved a tower-based field campaign conducted in late spring in Incheon, a South Korean city adjacent to the Yellow Sea. Using a Single-Particle Soot Photometer (SP2; Droplet Measurement Technology, Boulder, CO, USA), refractory black carbon (rBC) properties, including concentrations, size distribution, and mixing state, were monitored for two weeks in May 2023 at the 303-meter-high Posco Tower-Songdo (PT).

The average ± standard deviation mass concentration of rBC was 0.2±0.1 μg m⁻³, with the mass median diameter (MMD) ranging from 133 to 227 nm. The highest mass concentration and the lowest MMD and R_shell/core (the ratio of shell-to-core diameter of rBC) were observed at 10 a.m. daily, indicating the arrival of freshly emitted local rBC particles. During pollution events characterized by elevated PM_2.5 and O₃ levels, R_shell/core increased to 1.4–2.0. The mass absorption cross-section (MAC) at 550 nm, estimated using the BHCOAT implementation of Mie theory with input of measured diameter and coating thickness of individual rBC particle, was enhanced by a factor of 1.7 (E_abs). E_abs was positively correlated with O_x (NO₂+O₃) and aerosol liquid water content (ALWC). Notably, the highest E_abs coincided with relative humidity (RH) exceeding 70% and ALWC reaching ~30 μg m⁻³. These results suggest that under high atmospheric oxidation states, coating formation on the rBC surface is enhanced, promoting the development of hygroscopic aerosols on BC particles in this urban area. More detailed analysis will be presented in the meeting.

 

This research was supported by the National Institute of Environ- mental Research (NIER) grants funded by the Korean government (NIER-2023-01-02-083) and the National Research Foun- dation of Korea (NRF) from the Ministry of Science and ICT (NRF- 2021R1C1C2011543 & RS-2023-00249553). We thank POSCO International for establishing and maintaining the site.