Atmospheric Particulate Matter Retention Time Measurement Method And Its Application

The significant meanings, backgrounds, methods and present conditions of research for Residence Time of Atmospheric Aerosol(RTAA) were systematically expounded in this paper. Relations between activities of ²¹⁰Po/²¹⁰Pb, ²¹⁰Bi/²¹⁰Pb and RATT were mainly discussed, in which the formula to calculate RTAA with activity ratios of ²¹⁰Po/²¹⁰Pb,²¹⁰Bi/²¹⁰Pb,²¹⁴Bi/²¹⁴Pb and theα,βactivity growth characteristics of ²¹⁰Po and ²¹⁰Bi was the key parts. Ambient aerosols with different diameters were collected with a high-volume sampler (Staplex, Model235, <0.49( m) , 0.49～0.95( m) , 0.95～1.50( m) , 1.50～3.00( m) , 3.00～7.20( m),>7.20( m) of particle aerodynamic cutoff diameters). Electrical low pressure impactor (ELPI) was used to investigate the size distributions of particulates. The activities of ²¹⁰Po,²¹⁰Pb and ²¹⁰Bi were detected by anαspectrometer workstation, a low-background-γ-spectrometer and low-background detectors. Put the results into the formulas and then get RATT. At last, the influences on RATT and environmental of size distributions of ²¹⁰Po,²¹⁰Pb and ²¹⁰ activities were discussed.In the study of RTAA estimation, firstly, the method of estimating RTAA based on activity quotient was established by studying the relationship between airborne particle's 222Rn daughter activity quotient and particle's mean life, and computing formula was deduced. Then above mentioned were verified in the radon chamber under relative optimal conditions. Through the experiment, it was found that, when the radon concentration is steadily equal to 1.816 kBq/m³, the RTAA calculated by the activity quotient of ²¹⁴Bi/²¹⁴Pb is 112.17 minutes, which is equivalent to the mean ventilation time of radon chamber(104.17 minutes). Consequently, it was proved that it is right to estimate the method and formula of RTAA by using ²¹⁴Bi/²¹⁴Pb, ²¹⁰Bi/²¹⁰Pb or ²¹⁰Po/²¹⁰Pb.In the deep research, first of all, aiming at the international problems that the RTAA is lower measured and calculated by the activity quotient of ²¹⁴Bi/²¹⁴Pb, but higher when using the activity quotient of ²¹⁰Bi/²¹⁰Pb, new measuring thinking and method were proposed. Secondly, the mainly research about the new method of estimating RTAA was deduced by assuming and establishing new measuring model using the Betman principle. Thirdly, the formula was simulatively computed and proved by using decay data. Then typical experimental data abroad was analyzed by using the new formula. In the end, it was found that the value calculated by using the method improved is more correct and simple.To prove the practicability of the method with actual data from measurement and calculation, we selected reasonable technique routine, experimental media and experiment design for the experiment according to those specific requirements of the method.We collected atmospheric particulate matter with Staplex M235 Impact Large Flow Particulate Samplers, obtained the size distribution by ELPI aerosol measuring instrument, and then got the Specific Activity-particle size distribution of atmospheric particulate matter and relevant data of RTAA by high-purity-Geγ-spectrometer, BH1216-typeα,βlow-background measuring instrument andα-spectrometer workstation.The volume specific activity of ²¹⁰Pb in the TSP(Total Suspended Particle) collected in Shanghai JiaDing is 1.0629³.6324mBq/m³ with a mean value 1.7720 mBq/m³, and the range of different particle size is 0.02995 0.7740 mBq/m³.The mass specific activity of ²¹⁰Pb in the TSP is 6.4798¹3.4339mBq/mg with a mean value 8.6511mBq/m³, and the range of different particle size is 0.8898¹5.4429 mBq/m³.It is found that the maximum value of volume specific activity and mass specific activity of ²¹⁰Pb appeared in particle size 0.49～0.95um, with the volume specific activity accounting for 37% in TSP. The concentration of ²¹⁰Pb in particle size less than 0.49um accounts for 26.2% in TSP, so 63.2% of ²¹⁰Pb in TSP appears in particle size less than 0.95um(approximately PM1.0), 91.97% in PM1.5 and 94.97% PM3.0. The activity property of ²¹⁰Po and ²¹⁰Bi were detected by low-backgroundα,βCounter or anα-spectrometer workstation. Put the results into the new formulas and then get RATT. The mean RATT is found to be 25 days in the TSP collected in ShangHai JiaDing and RATT increases obviously with particle size decreasing. The mean RATT is 16.95 days in particle size more than 7.2um and 32.33 days in particle size less than 0.49um.The error of the whole experiment is given in the last part of the paper.