Sedimentation aerosol particles

Davies, C.N., Diffusion and Sedimentation of Aerosol Particles from Poiseuille Flow in Pipes, Aerosol Sci. 4 317-328 (1973). 

One is compelled to pose the question if experimentally it will become possible to decide whether the 14C variations observed on tree-ring samples, peat bogs, sediments, etc., are primarily caused by an external forcing of the system (production rate variations) or by an internal one. Recent progress in detection of small numbers of nuclei of an isotope by mass spectrometry based on the use of a particle accelerator [57,58] make it possible to measure the cosmic ray produced 10Be or 36C1 deposited in only 1 kg of ice. These isotopes get attached to aerosol particles and deposited with them. 

Gschwend and Hites (1981) observed that the two closely related polycyclic aromatic hydrocarbons, phenanthrene and anthracene, occur in a ratio of about 3-to-l in urban air. In contrast, sedimentary deposits obtained from remote locations (e.g., Adirondack mountain ponds) exhibited phenanthrene-to-anthracene ratios of 15-to-l. You hypothesize that these chemicals are co-carried in aerosol droplets from Midwestern U.S. urban environments via easterly winds to remote locations (like the Adirondacks) where the aerosol particles fall out of the atmosphere and rapidly accumulate in the ponds sediment beds without any further compositional change (i.e., the phenanthrene-to-anthracene ratio stops changing after the aerosols leave the air). If summertime direct photolysis was responsible for the change in phenanthrene-to-anthracene ratio, estimate how long the aerosols would have to have been in the air. Comment on the assumptions that you make. What are your conclusions ... 

As Martell has pointed out (30), in the region of the stratospheric large particle layer near 18-20 km. altitude, radioactive aerosol particles become attached to natural sulfate particles in the size range of about 0.1-0.4 jumeter radius. Subsequent upward transport of the radioactive aerosols is opposed by gravitational sedimentation. This combination of processes affords an explanation for the observed accumulation of 210Pb near 20 km. in the tropical stratosphere (2). At higher latitudes where slow mean motions are directed poleward and downward, no such accumulation is possible. 

Bioavailability from Environmental Media. The bioavailability of elemental phosphorus following inhalation, oral, and dermal contact is poorly understood (see Section 2.3). The estimated log Koc for elemental phosphorus is 3.05 (See Table 3-2). Therefore, elemental phosphorus is moderately sorbed to aerosol particles in air, to sediment in water, and to soil. However, due to its high reactivity, elemental phosphorus may not be found in aerobic zones of soil and water, unless the element is protected from oxidation by unreactive oxide coating (Berkowitz et al. 1981). Its bioavailability in the sorbed state from inhaled air, ingested soil, and dermal contact with soil and water may be lower than the free form of the element under identical conditions. 

Aerosol particles deposit in the lung by three principal mechanisms inertial impaction gravitational sedimentation and Brownian diffusion. Particles with a larger MMAD are deposited by the first two mechanisms, while smaller particles access the peripheral region of the lung by diffusion. 

Smaller particles in the size range 0.5-5 pm may escape impaction in the upper airways and will deposit by impaction and sedimentation in the lower TB and A regions. If the aerosol particle size is between about 3 and 5 pm then deposition will predominantly occur in the TB region. If the particles are less than about 3 pm then appreciable deposition in the A region is likely to occur. 

As discussed previously, terminal settling velocities of aerosol particles are generally quite small. Under normal circumstances it is unreasonable to expect that simple sedimentation as such will be an effective removal mechanism. 

MIRAGE2 Bulk equilibrium with RH based on Kohler theory. Hysteresis is treated Mechanistic, parameterized activation based on Kohler theory bulk CCN only Modal activation. Brownian diffusion, autoconversion, precip. rate independent of aerosols Calculated modal scaveng. coeff using a parameterization of the collective efficiency of aerosol particles by rain drops with size dependence Two-moment sedimentation for aerosols, nosedimentation for cloud droplets/ices... 

Aerosol particles are removed from the atmosphere by wet deposition through incorporation into precipitation either in or below clouds, and by dry deposition through sedimentation and impaction on or diffusion to soil, leaves, or the like. 

Aerosol particles used for inhalation deposit within the lower respiratory tract mainly by inertial impaction, sedimentation, and diffusion. Loose fractal aerosols were found to settle slower and therefore had more time to increase gravitational coagulation with other floes leading to much more rapid particle growth. This will increase the chance of the aerosol floes settling on the airway walls before reaching the end of the airways. 

The measurements showed that major masses of voleanic aerosol in the first period after die volcano cmption were located in layers of 16-18 and 23-25 km (Chen and Lelevkin, 2000). During the period following die sedimentation of particles, the formation of aerosol from sulphur dioxide in the layer of maximum stratospherie ozone concentration (26-28 km) occurred. Therefore, the reduction of total ozone oeeurred (Figure 2), due to the photooxidation reaetion of SO2 (Toktomyshev and Semenov, 2001) ... 

This division is very convenient from the point of view of particle characterization and measurement. Thus, in the range of Aitken particles diffusion effects are significant and particle coagulation is rapid. However, in case of giant particles these phenomena can be neglected and the behaviour of aerosol particles is mostly determined by their sedimentation due to gravitation. The large particles constitute... 

The removal of aerosol particles under dry weather conditions is caused by turbulent diffusion and gravitational sedimentation, which transport particles to the Earth s surface, as well as by impaction on vegetation, buildings and other objects. Turbulent diffusion itself does not remove particles (Twomey, 1977). Soil and other surfaces are bordered by a thin laminar layer ( 1 mm thick) across which particles must be transported by other processes (e.g. phoretic forces, molecular diffusion, sedimentation). 

Dry deposition velocity (A) and sedimentation velocity (B) of aerosol particles. Curve A refers to flow over grass (Hidy, 1973). (By courtesy of Plenum Press)... 

Bernhard et al., 1986 Landner, 1987 Patterson Passino, 1987 Batley, 1989). Problems of "speciation" became particularly complex in heterogenous systems, e.g. in soils, aerosol particles and sediments thermodynamic models may give suggestions as to the possible species to expect, but due to the important role of kinetically controlled processes in biogeochemistry, the actual speciation is often different from what can be expected. 

The concept of air as a colloid and the term aerosol for air containing an assembly of suspended particles were originally introduced by Schmauss and Wigand (1929). Colloids are inherently stable because fine particles are subject to Brownian motion and resist settling by sedimentation. The individual aerosol particles may be solid, liquid, or of a mixed variety, and all types are found in the atmosphere. Solid particles in the air are called dust. They are primarily formed by the erosion of minerals at the earth surface and enter the atmosphere by wind force. Sea spray from the ocean surface provides a prolific source of liquid droplets, which upon evaporation produce sea-salt crystals or a concentrated aqueous solution thereof. Solid and liquid particles also arise from the condensation of vapors when the vapor pressure exceeds the saturation point. For example, smoke from the open and often incomplete combustion of wood or agricultural refuse arises at least in part from the condensation of organic vapors. 

As for other constituents of the atmosphere, it is possible to set up a mass budget of the aerosol and to calculate its residence time. The main problem is to characterize the global distribution of particulate matter in order to determine its total mass in the troposphere. One may then apply the emission estimates of Table 7-11 to calculate the tropospheric residence time ta with the help of Eq. (4-11). This approach will be discussed in the first part of this section. Subsequently, we consider an independent method for estimating the residence time, which results from the use of radioactive tracers. Finally, the removal of aerosol particles by sedimentation and impactation at the Earth surface will be discussed. 

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.

Most of the methods used for aerosol degradation are based on intensifying the processes of coagulation, coalescence, adhesion of aerosol particles on different surfaces (on solid walls of filters, or water drops, as in artificial irrigation), and sedimentation (by changing the velocity and direction of aerosol streams during the inertial settling e.g. in so called cyclones). 

The mechanisms of annihilation of cluster ions are ion-ion recombination (on the average 3%) and sedimentation on aerosol particles (on the average 97% of cluster ions at ground level). The result of the combination of a cluster ion and neutral particle is a charged particle called an aerosol ion. In conditions of detailed thermodynamic equilibrium the probability that a spherical particle of diameter d carries q elementary charges is calculated from the Boltzmann distribution ... 

---