Sulfate aerodynamic size distribution

This paper summarizes part of the results of an investigation designed to characterize the aerodynamic size distributions of natural radioactivity and to evaluate the results in the context of sulfate distributions and recent advances in the understanding of aerosol growth mechanisms. This paper, while emphasizing our results on Pb-212 and Pb-214, also summarizes our initial data for longer-lived radionuclides. 

The aerodynamic size distributions of Pb and SO were quite different, reflecting the different dependencies of surface area and volume on aerosol diameter (Friedlander, 1977). Lead-212, like the other radionuclides, becomes associated with atmospheric aerosol particles by condensation or coagulation processes which are surface-area related. Sulfate, on the other hand, is the main solute in the accumulation mode aerosol so that its steady-state distribution is proportional to volume, even though condensation of H2SO4 may dominate its initial aerosol association. The difference also reflects residence times of 16.7 h for Pb -i- Bi and of about a week for sulfate. 

Figure 4.1 shows the mass aerodynamic size distribution of sulfate aerosols obtained by a low-pressure cascade impactor (LPI) (Bondietti and Papastefanou, 1993). In Figure 4.1 a is the frequency distribution of sulfate aerosols, in which are indicated the Aitken nuclei mode (I), the accumulation mode (II) and the coarse mode (HI), while in Figure 4.1b is the log-normal distribution of sulfate aerosols. This distribution was selected from 12 measurements made... 

Pb-212 vs S0%- LPI Distributions. Figure 3 presents a summary of the average Pb-212 AMADs and SOj MADs (mass median aerodynamic diameters) determined from a series of LPI measurements made during the period January to October, 1985. The Pb-212 data were derived from collections made at the same time as S0jj and from measurements made to compare Pb-212 vs Pb-214. The mean aerodynamic diameter of Pb-212 was about three times smaller than SoJ . Much less sulfate was found in the aerosol fraction below 0.08 um, compared with Pb-212. While Pb-212 was largely absent above 0.52 um, about 20% of the SoJ occurred above this size. 

---