Characterization and sources of black carbon in PM2.5 at a site close to a roadway in Gwangju, Korea, during winter

Abstract

Continuous measurements of black carbon (BC) concentrations in PM_2.5 were conducted using a single-wavelength aethalometer (@880 nm, Magee Sci., AE16) at a site close to a roadway (∼70 m from roadside) in Gwangju, Korea, during winter (December–February) to investigate the characteristics and sources of BC particles. The BC concentrations ranked in the order of January > December > February, probably due to lower boundary layer height, ambient temperature, and wind speed during January. Diurnal patterns in BC and carbon monoxide (CO) levels exhibited peak concentrations during the morning and evening hours coinciding with rush-hour traffic, with a strong correlation (R²) ranging from 0.52 (December) to 0.87 (January). It was found that wind speed was an important factor controlling BC concentrations at the site. Very high BC concentrations, up to ∼18.0 μg m⁻³, were observed at wind speeds < 1.5 m s⁻¹. The BC concentrations acquired under weak wind conditions are highly correlated with CO with ΔBC/ΔCO (the slope of BC and CO correlation) of 0.0063 (R² = 0.55, p < 0.01) and 0.0065 (R² = 0.59, p < 0.01) μg m⁻³ ppbv⁻¹ during day and night, respectively, suggesting no significant difference in the fraction of diesel vehicles between the daytime and nighttime periods. Two BC episodes, “A” and “B”, were classified based on BC, PM_2.5, and secondary SO₄²⁻ concentrations, and discussed to investigate the difference in the evolution of the BC observed. Episode “A” was associated with high BC and low PM_2.5 and SO₄²⁻ concentrations, while episode “B” was associated with high concentrations of BC, PM_2.5, and SO₄²⁻. Based on the temporal profiles of BC, NO, and NO_x concentrations, CO/NO_x ratio, and potential source contribution function map for BC, the BC observed during episode “A” was mostly attributed to locally produced emissions (e.g., traffic). However, the BC during episode “B” was influenced by long-range transport of air masses from China, as well as the local emissions.