Free tropospheric aerosols Above the planetary boundary layer, the number of aerosols decreases considerably. The influence of direct emissions at these heights are small, and the size distribution are close to the so-called back-ground aerosol size-distribution. The number distribution is dominated by the nucleation and Aitken mode, and the accumulation mode is relatively more important compared to the aerosol distribution in the lower troposphere. The reason is that the wet deposition, weak in this size range, is stronger in the lower troposphere.
1.6 Lifetime and sinks
Once in the atmosphere, aerosols have different residence times that depends on many factors: type, physical properties, size, altitude range... Residence times of aerosols vary significantly, from a few seconds for very large particles that soon after emission fall back on the ground, to years for sulphate aerosols stable at high altitudes in the stratosphere (e.g.Chazette et al.,1995).
The removal mechanisms can be divided into dry and wet removals. Dry mechanisms are the dry deposition at surface and the gravitational sedimen-tation, and wet mechanisms are the in-cloud scavenging and the below-cloud scavenging. The contribution and efficiency of these mechanisms is complex and depends on location, extent of these processes, physical and chemical properties of the aerosol particles and some other properties particular to each mechanism.
1.6.1 Dry sinks
Surface dry deposition The dry deposition at surface is an aerosol deposition process in which particles are removed from the atmosphere by the interaction with surface, or more precisely with the atmospheric surface layer and a thin layer of air next to the surface, so-called quasi-laminar sublayer. The dry deposition flux directly depends on the aerosol concentration:
Fdd=−vddn (1.19)
wherenis the aerosol concentration and vdd is the deposition velocity [m s−1].
The deposition velocity depends on size, shape, density of particles, properties of the surface, and the turbulence in the surface layer. In this process, the
particle first falls through the surface layer. After, it is transported in the quasi-laminar sublayer until it collides with an obstacle on the surface.
Particles near the surface are in the mean flow in the quasi-laminar sublayer. The smallest particles (<0.05 µm) are also subject to the Brownian motion and because of it they collide with the surface (Slinn,1982a). Larger particles, that are too big for the Brownian diffusion, flow following air stream-lines. When they encounter an obstacle (where air streamlines become denser) they can approach too close due to their size and collide with it. This process is called interception. The bigger particles (>2 µm) are not able to follow air streamlines close to the obstacle due to their inertia they leave the flow and collide with the surface (Slinn,1982a). This process is called impaction. The Brownian motion is effective for particles in the nucleation and accumulation mode. The impaction and interception are effective for the coarse mode. But, for particles in the accumulation mode the surface dry deposition is the least effective (Fig.1.12).
Gravitational Sedimentation Large particles are also the subject to the grav-itational settling. On a falling particle acts two forces, the gravgrav-itational force which makes the particle falling, and the drag force which slows down its fall. The sedimentation velocity, which determines the flux of particles that
Figure 1.12: Particle surface dry deposition as the function of particle size for deposition on a water surface. Figure adapted fromSlinn and Slinn(1980).
1.6. Lifetime and sinks 43
settle, depends directly on the mass of the particle. This means that for large particles the sedimentation is the dominant removal mechanism: particles with diameters>10 µm have sedimentation velocities>10 m h−1. This makes their atmospheric lifetime very short.
1.6.2 Wet sinks
Wet removal mechanisms are processes that act on aerosols via atmospheric hydrometeors (cloud droplets, rain, snow, fog) and depose them to the surface.
Aerosols can be scavenged when precipitation (cloud droplets) forms –in-cloud scavenging, or when precipitation fall –below-cloud scavenging. Both mecha-nisms can be efficient in the aerosol removing. Their efficiency is characterized by the scavenging coefficient Λ, and the change of the aerosol concentration due to the wet deposition is
∂n
∂t =−Λn (1.20)
The scavenging coefficient is a complex parameter that depends on the process of wet deposition involved, the properties of hydrometeors, the properties of aerosols and meteorological conditions. Wet deposition processes are reversible, because all hydrometeros that scavenged aerosols can also evaporate, releasing aerosols back into the air.
A number of synonyms are used for the wet deposition. Wet deposition is sometimes referred as wet removal, wet scavenging, or precipitation scavenging.
In-cloud scavenging is also known as rainout, while below-cloud scavenging is known as washout.
In-cloud scavenging Aerosols act as cloud condensation nuclei (CCN) and make the starting point of the formation of cloud droplet. This part of the in-cloud scavenging is known asnucleation scavenging. But, not all aerosols act as CCN. Their activation as CCN depends on: their type because more hydrofillic aerosols are more easily activated as CNN, their size because aerosols below a certain size cannot make CCN, and the state of supersaturation in the cloud because the aerosol threshold size depends on the magnitude of supersaturation. The nucleation scavenging can scavenge a large part of the aerosol mass in a cloud.
Besides the nucleation scavenging, aerosols can be scavenged inside of non-raining clouds by direct collisions with cloud droplets. Collision efficiencies depend on the size of the aerosols, and only the smallest aerosols can be
efficiently scavenged by this process. The lifetime of an aerosol particle larger than≈0.1 µm in a cloud due to this process is longer than the lifetime of clouds (Seinfeld and Pandis,1998). In this way only a small part of the aerosol mass gets incorporated inside the cloud droplets.
Below-cloud scavenging A hydrometeor that is falling (raindrop or snowdrop) can collide with an aerosol particle and collect it. A raindrop while falling, perturbs the air around it. As raindrops are usually significantly larger, aerosol particles follow these flow streamlines when approaching a raindrop. Similarly to the surface dry deposition, the interaction between an aerosol and a raindrop depends on their sizes. The smallest particles are subject to the Brownian motion, and larger particles to the interception and impaction (Slinn,1982a).
These processes are collectively the least effective for the particles in the accumulation mode, for the size range of 0.1 µm to 1 µm.
Figure 1.13: Interaction mechanisms of an aerosol particle with water droplet in the below-cloud scavenging process.
1.6.3 Other processes
Secondary aerosols can evaporate in response to the lost equilibrium between the gas and aerosol phase. Also, the coagulation is not strictly a removal mechanism, but it lowers the number of the particles and generally shortens the lifetime of particles.