Vertical distribution of black carbon and its mixing state in the urban boundary layer in summer
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Published:2023-06-30
Issue:12
Volume:23
Page:7225-7239
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Liu HangORCID, Pan XiaoleORCID, Lei Shandong, Zhang Yuting, Du Aodong, Yao Weijie, Tang Guiqian, Wang Tao, Xin Jinyuan, Li Jie, Sun YeleORCID, Cao Junji, Wang Zifa
Abstract
Abstract. 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, O3, NOx 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
∼ 80 % of profiles were uniform during the daytime and
∼ 40 %–90 % of profiles were of the gradual and 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 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 (Dp/Dc), 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: UTC+8), the vertical difference of Dp/Dc (2 %) was much
lower than that of BC concentration (∼ 35 %). The vertical
variation in absorption ability was mainly caused by the variation in BC
concentration. In addition, O3 and Dp/Dc 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.
Funder
National Natural Science Foundation of China National Key Research and Development Program of China China Postdoctoral Science Foundation
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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