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Chinese Science Bulletin, Volume 64 , Issue 27 : 2949-2958(2019) https://doi.org/10.1360/TB-2019-0101

Black carbon profiles from tethered balloon flights over the southeastern Tibetan Plateau

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  • ReceivedMay 27, 2019
  • AcceptedJul 15, 2019
  • PublishedAug 29, 2019

Abstract

Black carbon (BC) can change the energy budget of the earth system by strongly absorbing solar radiation: both suspended in the atmosphere, incorporated into cloud droplets, or deposited onto high-albedo surfaces. BC’s direct radiative forcing is highly dependent on its vertical distribution. However, due to large variabilities and the small number of vertical profile measurements, there is still large uncertainty in this forcing value. Moreover, the vertical profile of BC and its relative elevation to clouds determine BC’s lifetime in the atmosphere and its transport and removal processes. Experimental measurements of BC vertical profiles over the Tibetan Plateau are very important: not only for studies of BC effects on regional climate, but also for studies of BC transport from surrounding regions with strong anthropogenic emissions. In November-December 2017, a series of tethered balloon flights was launched at the Southeast Tibet Observation and Research Station for the Alpine Environment of the Chinese Academy of Sciences (“SETORS”, located at 29°46′N, 94°44′E, 3300 m a.s.l.). A cylindrical balloon with a diameter of 7.9 m and maximum volume of 1100 m3 was used. A 7-channel Aethalometer (Model AVIO-33, Magee Scientific®, USA) was installed in the gondola attached to the balloon, together with several other instruments including a GPS for altitude, and sensors for temperature and relative humidity. The airborne Aethalometer measured BC mass concentration (ng/m3) on a on a 1-second timebase at 7 wavelengths ranging from 370 nm to 950 nm. Meanwhile, another Aethalometer (Model AE-33, Magee Scientific®, USA) was used to monitor BC mass concentration near the surface, at a height of about 10 m above the ground. From the tethered balloon flights, we derived three profiles designated as “F1”, “F3-ASC”, and “F3-DES”. The maximum height for the F1 flight was 500 m a.g.l., namely 3800 m a.s.l.; while the maximum height for the F3 flight was 1950 m a.g.l., namely 5250 m a.s.l. Based on the potential temperature and relative humidity data, the profiles were divided into three layers: the stable boundary layer (SBL), the residual layer (RL), and the free troposphere (FT). The vertical distribution of BC shows a prominent peak within the SBL. The mean BC concentration in SBL (1000±750 ng/m3) was one order of magnitude higher than in RL and FT, which were 140±50 ng/m3 and 120±50 ng/m3, respectively. The BC concentration measured in the present study in FT over the southeastern Tibetan Plateau is comparable to measurements in Arctic regions, but lower than values in South Asia. Analysis of the wavelength dependence of the data yields an estimate of the biomass burning contribution. This showed a maximum value in SBL of 44%±37%, and was 16%±6% in RL and 14%±5% in FT. Analysis of 24-h isentropic back trajectories showed that BC in SBL and RL was dominated by local sources, while in the FT, BC is mainly influenced by mid- to long-distant transport by the westerlies. In addition, analysis of the variations of BC concentration and biomass burning contribution on a high-resolution time scale showed that BC concentrations and the nature of their sources are largely influenced by air mass origins and transport. To our knowledge, this is the first ever in situ measurement of BC concentration over the Tibetan Plateau in the atmospheric boundary layer and free troposphere up to 5000 m a.s.l.


Funded by

国家自然科学基金(41771091)

中国科学院战略性先导科技专项(XDA20070100)

中国科学院前沿科学重点研究项目(QYZDJ-SSW-DQC037)


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  • Figure 1

    Profiles of BC concentration (BC), potential temperature (θ) and relative humidity (RH) during the flight segments F3-ASC (a), F3-DES (b) and F1 (c). The horizontal gray dash lines show the corresponding tops of the stable boundary layers (SBL) and residual layers (RL). The vertical gray dash lines show the corresponding mean BC concentrations in the SBL, RL and FT (free troposphere). The black inverted triangles mark the mean BC concentrations measured at ground level during these flight segments

  • Figure 2

    Profiles of biomass burning contribution (BB), potential temperature (θ) and relative humidity (RH) during the flight segments F3-ASC (a), F3-DES (b) and F1 (c). The horizontal gray dash lines show the corresponding tops of the stable boundary layers (SBL) and residual layers (RL). The vertical gray dash lines show the corresponding mean BB in the SBL, RL and FT (free troposphere). The black inverted triangles mark the mean BB contributions measured at ground level during these flight segments

  • Figure 3

    24-h backward trajectories starting at 6:00 local time on December 3rd, 2017, computed using the HYSPLIT model at 150, 500 and 2000 m a.g.l. The experimental location is marked with a black star

  • Figure 4

    Time series of (a) biomass burning contribution (BB), (b) BC concentration (BC), (c) relative humidity (RH), (d) potential temperature (θ) and (e) elevation (m) when the balloon was above the stable boundary layer (SBL) in the 3rd Flight. P2 and P4 with obviously sharp changes of potential temperature and relative humidity are noted by gray shadow boxes. Five time periods (P1–P5) are described in detail in Section 2.2 in the present study. Horizontal dash lines in (a) and (b) show the mean BB contribution and mean BC concentration above the SBL

  • Table 1   Heights (H) of stable boundary layers, residual layers and free troposphere, and corresponding mean black carbon concentrations () and mean biomass burning contributions () during the flight segments F3-ASC, F3-DES and F1

    F3-ASC过程

    F3-DES过程

    F1过程

    高度(m)

    BC(ng/m3)

    BB(%)

    高度(m)

    BC(ng/m3)

    BB(%)

    高度(m)

    BC(ng/m3)

    BB(%)

    SBL

    210

    470±170

    34±11

    140

    1750±590

    57±15

    180

    640±210

    14±7

    RL

    950

    130±30

    16±6

    950

    150±50

    16±6

    N.A.

    220±80

    13±4

    FT

    N.A.

    120±50

    11±5

    N.A.

    120±40

    16±5

    N.A.

    N.A.

    N.A.

    N.A.表示无数据

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