Trace Study On The Resident Time And Deposition Velocity Of Airborne Particulates Using Multi-Nuclides In Shenzhen

Cross-border transported airborne particles have a great impact on the regional environment and ecosystem,it is essential to understand the transport pathways of airborne particles if we are to know the pollution source and for environmental impact assessment.Radio-nuclides serve as good tracer for the study of airborne particles,on their source,transport and scavenging,they provide not only the direct information of air pollution source,but will also the residence time and deposition velocity of aerosols.In this study,airborne particles and atmospheric total deposition samples in Shenzhen were collected and analyzed for ²¹⁰Pb,²¹⁰Po,and ⁷Be,aiming to understand the concentration variations as well as atmospheric deposition fluxes of these radio-nuclides in this region.Using radio-nuclide as tracer,the residence time and deposition velocities of aerosol were determined,their variations during haze weather and the relationship with meteorological factor were also examined.The achievement of this study will helps to understand the transport pathways of airborne pollutants in a subtropical monsoon region,and provide scientific basis on air pollution prevention and control as well as human health protection in megacities.The main results of this study are as follow:（1）The specific activity of radionuclides in the airThe specific activity of ⁷Be in TSP in Shenzhen ranged from 0.29 to 14.97mBq/m³,with an average of 3.18mBq/m³.The specific activity of ²¹⁰Pb in TSP and PM_2.5 ranged from 0.71to 4.78mBq/m³（averaged 1.99mBq/m³）and 0.55 to 5.10mBq/m³（average 2.31mBq/m³）,respectively.The specific activity concentration of ²¹⁰Po in TSP and PM_2.5 ranged from0.04⁰.23mBq/m³（averaged 0.10mBq/m3）and 0.04⁰.25mBq/m³（averaged 0.11mBq/m³）,respectively.The content of ⁷Be in atmospheric particles showed seasonal variation with higher in spring and winter and lower in summer and autumn The higher value of ⁷Be during spring may due to the influence of"spring leak"in the stratosphere,and the long-range transported air mass which contain“rich”⁷Be may contributed to the high level of ⁷Be in winter seasons.The specific activity concentration of ²¹⁰Pb in the air were higher in winter and sring but lower in sumer and autumn,and no apparent sesonal variation was observed for²¹⁰Po.The concentration of ²¹⁰Pb and ²¹⁰Po showed positive correlation with the content of airborne particle,whereas,²¹⁰Po correlated less with its parent nuclide ²¹⁰Pb.（2）The depositional fluxes and velocity of atmospheric ⁷Be and ²¹⁰PbThe depositional fluxes of ⁷Be and ²¹⁰Pb ranged from 0.25 to 3.54Bq/m²?d and 0.45 to1.31Bq/m²?d,with an average value of 1.55Bq/m²?d and 0.77Bq/m²?d,respectively.The deposition velocities of ⁷Be and ²¹⁰Pb were calculated from the depositional fluxes and the monthly average concentration of ⁷Be and ²¹⁰Pb during the sampling period.The deposition velocity of ⁷Be and ²¹⁰Pb ranged from 0.05 to 5.10cm/s and from 0.22 to 1.44cm/s,respectively,with average values of 1.14cm/s and 0.78cm/s,respectively.The deposition fluxes as well as the velocity of ⁷Be and ²¹⁰Pb show significant positive correlation with the amount of rainfall,indicating that precipitation plays an important role in the removal of⁷Beand ²¹⁰Pb in the air.By comparison,the deposition velocity of ²¹⁰Pb was slightly lower than that of ⁷Be,it is because that ²¹⁰Pb is a terrestrial origin radionuclide which mainly derived from the ²²²Rn in the soil.（3）The residence time of airborne particles based on the activity ratio of ²¹⁰Po/²¹⁰PbThe activity ratio of ²¹⁰Po/²¹⁰Pb in TSP and PM_2.5 were 0.007⁰.168 and 0.029⁰.233,repecitvely,with an average value of 0.071.The residence time of airborne particles（RTA）were calculated by the activity ratio of ²¹⁰Po/²¹⁰Pb.The RTA of TSP in Shenzhen was4.2⁴7.9d,with an average of 21.2d,whereas the RTA of PM_2.5 was 10.3⁶9.5d,with an average of 21.4d.The RTA varied with aerosol particle size and meteorological conditions.Precipitation played an important role in reducing RTA of coarse particles,higher RTA was observed under inversion weather conditions.The apparent residence time derived from ²¹⁰Po/²¹⁰Pb ratios is a consequence of both residence time and transit time of aerosols.