Black Carbon profiles from tethered balloon flights over the Southeastern Tibetan Plateau

Abstract

<p>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. 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. A cylindrical balloon with a diameter of 7.9 m and maximum volume of 1250 m<sup>3</sup> was used. A 7-channel Aethalometer 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/m<sup>3</sup>) on a on a 1-second timebase at 7 wavelengths ranging from 370 nm to 950 nm. Meanwhile, another Aethalometer 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/m<sup>3</sup>) was one order of magnitude higher than in RL and FT, which were 140±40 ng/m<sup>3</sup> and 120±40 ng/m<sup>3</sup>, 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 13±5% in FT. Analysis of 24-hour 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.</p>