Residence time of aerosol particles

The residence time of atmospheric aerosol particles in the lower atmosphere, assuming that the troposphere is considered a well-mixed reservoir (closed system), is a function of various removal processes, the most important being  

A plot of a gamma-ray spectrum of an atmospheric aerosol sample (air filter) obtained by a Ge detector is illustrated in Eigure 3.1, in which the 477.6 keV-gamma peak of Be and 

Activity median aerodynamic diameter (AMAD) of atmospheric aerosol particles (pm) 

Residence time of tropospheric aerosol particles associated with the cosmic ray 

A method for estimating the residence time of tropospheric aerosol particles associated with the cosmic-ray produced radionuclides, such as Be, is based on the aerosol particle growth rate, which is the change of particle diameter with time, which was estimated to be 0.004 to 0.005 pmh (McMurry and Wilson, 1982) and the difference between the activity median aerodynamic diameter, AMAD, of a radionuclide, e.g. Be, and the size of the Aitken nuclei in the size distribution of the aerosol particles, which is 0.015 pm (NRC, 1979). The AMAD of all radionuclides is in the accumulation mode of the size distribution of atmospheric aerosol particles which ranges between 0.1 and 2.0 pm (NRC, 1979 Papastefanou and Bondietti, 1987). 

---

All aerosol particles are formed by condensation of gases or vapours or by mechanical processes. They may be transformed by coagulation or condensation at the same time as they are transported by air movement and dilution. They might disappear from the atmosphere and settle on some surfaces which act as a sink. The residence times of aerosol particles in the atmosphere vary from some days near the earth s surface in the troposphere to a year or more in the stratosphere. 

The first purified and characterized drug substances were administered as aerosols as a topical treatment for asthma approximately 50 years ago. More recently, drugs have been evaluated for systemic delivery. For each category of drug the mechanism of clearance from the airways must be considered. These mechanisms may be listed as mucociliary transport, absorption, and cell-mediated translocation. The composition and residence time of the particle will influence the mechanism of clearance. 

As a result, these aerosols follow the motions of the atmosphere and are responsible for the transport of much of the nonvolatile products of photochemical and oxidative reactions. Pb is a minor constituent of this aerosol population (one aerosol particle in or more will carry an atom of Pb) and is a useful tracer of the transport, deposition, and residence time of aerosols. 

Physical interferences may arise from incomplete volatilization and occur especially in the case of strongly reducing flames. In steel analysis, the depression of the Cr and Mo signals as a result of an excess of Fe is well known. It can be reduced by adding NH4C1. Further interferences are related to nebulization effects and arise from the influence of the concentration of acids and salts on the viscosity, the density and the surface tension of the analyte solutions. Changes in physical properties from one sample solution to another influence the aerosol formation efficiencies and the aerosol droplet size distribution, as discussed earlier. However, related changes of the nebulizer gas flows also influence the residence time of the particles in the flame. 

Fig. 7-28. Combined residence lifetimes of aerosol particles as a function of size. [Adapted from Jaenicke (1978c, 1980).] Important removal processes, active in various size ranges, are indicated. Coagulation and sedimentation time constants were calculated the time constant for wet removal is the residence time derived from 2,0Bi/210Pb and 222Rn/210Pb ratios (Martell and Moore, 1974). Curves 1 and 2 represent the background aerosol for rwel equal to 12 and 3 days, respectively. Curve 3 represents the continental aerosol with rwel = 6 days. The dashed line is calculated from a simple model for sedimentation equilibrium, as described in Section 7.6.3.

In addition to the differences in geographical distribution of the greenhouse gases compared to the aerosol particles and the day-night differences, there are also differences in their temporal behavior. As discussed earlier, typical residence times for sulfate particles are about a week, whereas that of C02 is about 100 years. As a result, the impacts of sulfate aerosols are almost immediately manifested, whereas those due to C02 occur over decades to centuries (Schwartz, 1993). 

Because of their atmospheric residence times of days to weeks, aerosol particles can be transported... 

---