Analysis Of Mixing State And Optical Properties Of Black Carbon Aerosols In The Residual Layer Over Beijing

Black carbon（BC）is the most important light-absorbing aerosol in the atmosphere,which is considered to be one of the most important components of global warming.It has a great impact on the global atmospheric radiation balance and atmospheric stability by absorbing shortwave radiation and continuously heating the atmosphere.Fresh BC tends to mix with other materials in the aging process in the atmosphere.The mixing state of BC will change significantly,which will alter its hygroscopicity and optical properties,and have a great impact on climate change and the environment.As the largest city in northern China,Beijing is also one of the regions with the largest emission of BC.At present,although many observational studies were conducted on BC in Beijing,most of the observation sites were located on the ground and the continuous observation of BC at the upper level of Beijing urban area was relatively lacking.And the study of its mixing state in the residual layer is still insufficient.The observations were conducted at the CITIC（China International Trust and Investment Corporation）station during February 23 to 28,2021,which is located on the roof of the CITIC Tower with a height of 528 m above ground.A Single-Particle Soot Photometer（SP2）was used to observe BC.This study focused on the analysis of the source,size distribution of BC,BC’s mixing state and the optical properties during the observation period.The main conclusions are as follows:During the observation period,the meteorological conditions in Beijing presented stable state,with low wind and poor diffusion conditions.The pollutants accumulated rapidly and reached the peak on the 26th.Through simulation of the wind fields and pollutants by model and the analysis of potential sources,the result show that there was no regional transport of pollutants during the pollution period,and the main source of pollutants in Beijing was local emissions.The size of BC core was concentrated between 70-300 nm,and the mass median diameter（MMD）of BC was 160nm.The relative coating thickness of the BC（D_p/D_c,the diameter ratio between BC-containing particle and BC core）was 1.5±0.09（D_c=180nm）.The number fraction of thickly-coated BC particles(f _BC)was 55.3%±8%.The variation of the mixing state of BC at CITIC station was determined:（1）The diurnal variation of the mixing state of BC showed a clear bimodal distribution.There were two peaks of f _BC（D_p/D_c）,one peak appeared at 07:00 in the morning,and the other peak appeared at 15:00 in the afternoon,which was significantly different from the unimodal peak observed on the ground（the peak appeared at 15:00）.（2）The BC was still aging at a fast rate in the nighttime in residual layer.It was observed that the f _BC of BC increased from 51.3%at 00:00 to 61.5%at 07:00 at the CITIC station,with an increasing rate of 1.4%per hour.Such an increase rate was even higher than that during noontime（11:00 to 15:00,0.7%per hour）.Similar trend also reflected in D_p/D_c of BC particles,which increased from 1.52 at 00:00 to 1.63 at 07:00.During the observation period,the height of the boundary layer in Beijing was less than 1.5 km in the daytime and less than 400 m at night.The CITIC station experienced a conversion from inside the convective boundary layer（CBL）in daytime to inside the residual layer（RL）at night,with conversion time around 09:00 and 18:00.The vertical transport of pollutants was strong in daytime when the CITIC station located in the CBL,but significantly weaken at night when the CITIC station located in the RL.In the daytime,the aging rate of BC was fast due to the strong photochemical reactions.However,due to the high O₃ concentration in the residual layer,a large number of secondary process reactions（such as the formation of nitrate particles）might be a major factor that contributes to nocturnal aging of r BC particles in the RL.The aging of BC in the RL significantly enhanced its light absorption correspondingly.The light absorption enhancement(E_abs)increased from 1.64 at 00:00to 1.79 at 07:00,and the mass absorption cross section（MAC）increased from 7.05m² g^-1 to 7.64 m² g^-1.Further analysis indicated that the E_abs depends not only the mixing state of BC particles,but also on its size.An increase in the size of BC core can also significantly enhance E_abs.