A Regional Source Study Of Black Carbon In Beijing And Surrounding Areas Based On Aircraft Observations And Simulations

Observations suggest that the vertical distributions of air pollutants,such as black carbon(BC),present as various types depending on the emission sources and meteorological diffusion conditions.However,the formation process and source appointment of some special BC profiles are not fully understood.In this paper,by using the Weather Research and Forecasting model coupled with chemistry(WRF-Chem)with BC-tagging and process analysis technique to explore the effect of weather situation on the distribution of BC,to quantify the contribution of different source regions to the near-surface BC in Beijing(BJ),to calculate the contribution of different physical processes to the variation of BC concentration,and to deeply explain the formation mechanism and regional sources of a BC peak in the free troposphere observed by aircraft flight in BJ on May 5th,2018.The results are shown as follows:(1)The synoptic situation has an important impact on BC pollution in BJ.In this case,BJ was in front of the weak low pressure in the early period of pollution and there was convergence near the surface,and air pollutants in the surrounding areas were likely to accumulate,leading to an increase in BC concentration in BJ.At the same time,there was an obvious an obvious “transport channel” from SX to BJ in the free troposphere.To the late period of pollution,BJ was controlled by the uniform pressure field,which was not beneficial for the diffusion of BC in the horizontal and vertical directions.Meanwhile,BC was transported southward due to clean north winds at an altitude of approximately 4000 m.(2)The source region of BC in BJ varies markedly at different heights.BC near the surface mainly originated from BJ,HB,HN,TJ,and SD,whose mean contribution rates were52.0%,23.9%,6.1%,5.7%,and 4.4%,respectively.The near-surface BC contribution rate of the BTH region to BJ exceeded 80%,while the source of BC in the free troposphere(? 4000m)showed different characteristics.The total contribution rate of BJ and HB below 700 m was as high as 96.7%,while the BC peak in the free troposphere was almost transported from external regions,including SX,NWCHN,SWCHN,HB,HN,and even SCHN,and the contribution rates were 24.7%,23.5%,10.5%,9.4%,8.9%,and 7.1%,respectively.(3)Horizontal advection(HADV)and vertical advection(VADV)processes played an important role in the convergent and upward movement and the transport of BC.Near-surface BC that originated from SX,HB,HN,and NWCHN had been uplifted by a cyclone system,was transported to a height of approximately 3000 m above BJ,and was then lifted by the VADV process to approximately 4000 m.At the surface,BJ and its surroundings were in the field of a weak pressure gradient,leading to the accumulation of BC.(4)The results indicate that cyclone systems can quickly lift air pollutants,such as BC,up to the free troposphere,extend their lifetimes,and further affect the regional atmospheric environment and climate.