Vertical distribution of black carbon and its mixing state in urban boundary layer in summer

The vertical distribution of black carbon (BC) as well as its mixing state is of great concern due to BC’s strong regional climatic and environmental effects. In this study, vertical measurements were conducted through a moveable container based on a meteorology tower in an urban area. A total of 112 vertical profiles (0-240 m), including the concentrations of BC, O₃, NO_x and the optical properties of aerosols, were obtained. Based on BC concentration, the vertical profiles could be classified into four categories: uniform, gradual decrease, sharp decrease, and sudden increase. The uniform type indicates strong vertical mixing with similar pollutant concentrations along the vertical direction. The gradual/sharp decrease types indicate stable vertical conditions with higher pollutant concentrations on the ground and lower concentrations at higher altitudes. Due to the strong radiation in summer, the vertical profiles exhibited a clear diurnal variation in which ~80% of profiles were uniform during the daytime and ~40%-90% of profiles were gradual/sharp decrease types at night. O₃ is an exception, and its concentration generally increases with height even under strong vertical mixing conditions. The size distribution of BC core varied slightly along the vertical direction, and the coating thickness of BC increased with height under stable conditions. However, the vertical variation in BC concentration is much more significant than BC coating thickness and plays a more critical role in the determination of BC absorption ability vertical distribution. In addition, O₃ and D_p/D_c occasionally increased during 6:00-8:00 but remained stable during 8:00-10:00. Vertical mixing and transportation from upper heights, such as the residual layer, could significantly influence the pollutant properties on the surface during early mornings. This study exhibits a continuous vertical picture of BC and its mixing state in urban areas, which would be helpful for understanding BC’s regional environmental effect.