Particles are eventually removed from the atmosphere by two mechanisms: deposition at Earth's surface, so-called dry deposition, and scavenging by droplets, so-called wet deposition (Seinfeld and Pandis, 1998). Because wet and dry deposition lead to relatively short residence times in the troposphere and because the geographical distribution of particle sources is highly nonuniform, tropospheric aerosols vary widely in concentration and composition over Earth. Whereas atmospheric trace gases have lifetimes ranging from less than a second to a century or more, the residence times of particles in the troposphere vary only from a few days to a few weeks.
The dry deposition flux of particles to the surface, Fd, is assumed to be proportional to the particle concentration at a reference height, C, i.e., Fd = vdC, where the proportionality constant vd, the deposition velocity, depends on the meteorological state of the atmosphere and the size of the particles. Three processes serve to deliver particles to Earth's surface: gravitational settling, turbulent transport, and Brownian diffusion. Although virtually any atmospheric flow is turbulent, a very thin laminar sublayer exists immediately adjacent to the surface. Turbulence brings particles down to the laminar sublayer, through which Brownian diffusion and settling govern transport. Small particles have a relatively large Brownian diffusivity, so move efficiently through the sublayer, whereas larger particles transfer primarily via inertia or settling. Those in between, in the size range of 0.1 to l pm diameter, are deposited about an order of magnitude slower than those at either the small or large extremes because none of the mechanisms is relatively effective in this intermediate size range.
Wet deposition involves the scavenging of particles by droplets and the subsequent removal by precipitation. Scavenging is necessary, but not sufficient, for wet deposition to occur since cloud or rain drops can evaporate, and if this occurs, the scavenged particle is returned to the air mass. As opposed to dry deposition, which operates only at Earth's surface, wet deposition serves to remove particles from the entire air mass. The rate of particle collection by falling drops is proportional to the number of drops, their settling velocity, their cross-sectional area, and a collection efficiency. The efficiency with which a particle is collected by a falling drop depends on the mechanics of particle motion in the vicinity of the drop. As with dry deposition, there is a minimum in the total collection efficiency in the O.l to l pm size range—small particles diffuse to the drop surface, larger ones collide with it, while in between neither process is very efficient.
Particles that become activated to grow to fog or cloud droplets are termed cloud condensation nuclei (CCN). At a given mass of water-soluble material in the particle, there is a critical value of the ambient water supersaturation, above which the particle undergoes an unstable process of spontaneous water accretion, leading to a cloud droplet (Seinfeld and Pandis, 1998). The critical water supersaturation for activation results from a combination of the curvature increase in and the solute concentration lowering of the water vapor pressure over a droplet. The number of particles that can act as CCN thus depends on the water supersaturation. For marine stratiform clouds, for example, supersaturations are in the range of 0.1 to 0.5%, which corresponds to a minimum CCN particle diameter of 0.05 to 0.14 pm. CCN number concentrations vary from fewer than 100/cm3 in remote marine regions to a few thousand per cubic centimeter in polluted urban areas. Once activated, fog and cloud droplets grow to sizes exceeding 10 pm diameter. Particles that are not activated to form droplets may remain as airborne aerosol or be removed by falling drops.
Aerosol lifetimes in the atmosphere depend primarily on the size of the particle and the height in the atmosphere at which the particle resides. The residence time r can be viewed as an exponential half-life, the time required for a population of particles of a given size to decay to 1/e of its initial concentration.
An empirical expression for atmospheric particle residence time as a function of particle size and altitude that is useful for estimates is (Jaenicke, 1988)
where K = 1.28 x 108 s (constant), Z)max = 0.6 pm (the diameter of particle with maximum residence time), and rwet is the lifetime for removal of particles by wet deposition. The first term on the right-hand side of Eq. (5) represents dry removal at Earth's surface; rwet depends mainly on the altitude in the atmosphere. Roughly three altitude regions can be distinguished by the frequency with which precipitation scavenging occurs:
Height < 1.5 km (lower troposphere) Twet % 6.9 x 105 s (8 days) Middle troposphere to tropopause rwet % 1.8 x 106 s (3 weeks)
Tropopause and above Twet % 1.7 x 107 s (200 days)
Figure 1 shows atmospheric particle residence time r as a function of particle radius, Dp/2. Dry removal predominates for particle radii either much smaller or larger than 0.3 pm; in the region around 0.3 pm wet scavenging is the most effective removal mechanism.
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