基于系留气球的藏东南大气黑碳垂直分布

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

在青藏高原东南部地区开展大气黑碳垂直分布的探测对研究黑碳跨境传输过程及其气候环境影响具有重要的意义.本研究利用系留气球观测平台获得青藏高原大气黑碳浓度的垂直廓线.观测结果显示,稳定大气边界层黑碳浓度为1000±750 ng/m^3,生物燃烧源贡献为44%±37%;剩余层黑碳浓度为140±50 ng/m^3,生物燃烧源贡献为16%±6%;自由大气黑碳浓度为120±50 ng/m3,生物燃烧源贡献为14%±5%.此外,本项研究表明稳定大气边界层和剩余层大气黑碳主要来源于局地排放,而自由大气黑碳则主要受西风急流长距离传输的影响.

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.,namely3800 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 were140±50 ng/m3 and 120±50 ng/m^3,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 longdistant 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.