Vertical distribution of black carbon and its mixing state in the 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 meteorological tower in the Beijing urban area during June and July. A total of 112 vertical profiles (0–240 m), including the concentrations of BC, Oinline-formula₃, NOinline-formula_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 and 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 inline-formula∼ 80 % of profiles were uniform during the daytime and inline-formula∼ 40 %–90 % of profiles were of the gradual and sharp decrease types at night. Oinline-formula₃ is an exception, and its concentration generally increases with height, even under strong vertical mixing conditions. The size distribution of the BC core varied slightly along the vertical direction, and the coating thickness, denoted by the diameter ratio between the BC-containing particle and BC core (inline-formula $M6inlinescrollmathml{D}_{\mathrm{normal\; p}}/D_{\mathrm{normal\; c}}$ 33pt16ptsvg-formulamathimgd50dc121304216e862e76b48739cef5d acp-23-7225-2023-ie00001.svg33pt16ptacp-23-7225-2023-ie00001.png ), of BC increased with height under stable conditions. Although the coating thickness could increase the absorption ability with an average absorption enhancement of 1.25 at 23:00 LT (local time: UTCinline-formula+8), the vertical difference of inline-formula $M8inlinescrollmathml{D}_{\mathrm{normal\; p}}/D_{\mathrm{normal\; c}}$ 33pt16ptsvg-formulamathimg8fde848d69408fed46a6c17c5016a2f9 acp-23-7225-2023-ie00002.svg33pt16ptacp-23-7225-2023-ie00002.png (2 %) was much lower than that of BC concentration (inline-formula∼ 35 %). The vertical variation in absorption ability was mainly caused by the variation in BC concentration. In addition, Oinline-formula₃ and inline-formula $M11inlinescrollmathml{D}_{\mathrm{normal\; p}}/D_{\mathrm{normal\; c}}$ 33pt16ptsvg-formulamathimg05521d5d0259e1ef7a227fc3b24358d1 acp-23-7225-2023-ie00003.svg33pt16ptacp-23-7225-2023-ie00003.png occasionally increased during 06:00–08:00 but remained stable during 08:00–10:00. 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.