Atmospheric removal Deposition processes

Deposition is the mass transfer from the atmosphere to the earth s surface it is the opposite of emission (the escape of chemical species from the earth s surface into the air). It is a flux given in mass per unit of area and unit of time. According to the various forms and reservoirs of atmospheric chemical species (molecules in the gas, particulate and liquid phases), different physical and chemical processes are distinguished  

From a measurement point of view, there are difficulties in approaching the physically correct removal processes. Dry deposition strongly depends on the surface characteristics correct estimation is only possible by flux measurements (eddy correlation and gradient methods). Any so-called deposition sampler can be installed with a wet-only/dry-only cover-plate to avoid bulk sampling, but it is never possible to avoid the collection of sedimentation dust in dry-only samplers or dry deposition or sedimentation by wet-only samplers. However, with enough accuracy all other deposition processes contribute negligibly to the deposition flux under wet-only and dry-only conditions, respectively. 

Dry deposition is generic and very similar process to the mass transfer by scavenging described in Chapter 4.3.7 where the transport axis is vertically downwards and the uptaking object is the fixed earth s surface. The situation, however, is complicated by the possible upward flux of the substance A due to plant and soil emission. Only the net flux is measurable (Foken et al. 1995). The case that downward and upward fluxes (so-called bidirectional trace gas exchanges) are equal Fdry = Fq) is called the compensation point. It only depends on the concentration gradient Ch — Co). Additionally, the net flux can be influenced by a fast gas phase reaction 

Chamberlain (1953) first introduced the concept of dry deposition and the dry deposition flux is considered of first-order concerning atmospheric concentration (Slinn 1977)  

The reference height h is normally considered the mixing height (there are other 

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Atmospheric removal Deposition processes airtxime species... 

Sulfur oxides (SO,) are compounds of sulfur and oxygen molecules. Sulfur dioxide (SO2) is the predominant form found in the lower atmosphere. It is a colorless gas that can be detected by taste and smell in the range of 1, (X)0 to 3,000 uglm. At concentrations of 10,000 uglm , it has a pungent, unpleasant odor. Sulfur dioxide dissolves readily in water present in the atmosphere to form sulfurous acid (H SOj). About 30% of the sulfur dioxide in the atmosphere is converted to sulfate aerosol (acid aerosol), which is removed through wet or dry deposition processes. Sulfur trioxide (SO3), another oxide of sulfur, is either emitted directly into the atmosphere or produced from sulfur dioxide and is readily converted to sulfuric acid (H2SO4). 

Deposition is the atmospheric removal process by which gaseous and particulate contaminants are transferred from the atmosphere to surface receptors - soil, vegetation, and surface waters (22,27,28, 32). This process has been conveniently separated into two categories dry and wet deposition. Dry deposition is a direct transfer process that removes contaminants from the atmosphere without the intervention of precipitation, and therefore may occur continuously. Wet deposition involves the removal of contaminants from the atmosphere in an aqueous form and is therefore dependent on the precipitation events of rain, snow, or fog. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

The atmospheric and chemical processes controlling the spatial and temporal variability of psychoactive substances in urban atmospheres are largely uncertain, mostly due to the fact that the atmospheric residence time of these compounds is so far unclear. The transport, transformation and deposition/atmospheric removal... 

Wet removal processes are further controlled by precipitation types and rates. Dry deposition processes on surfaces are affected by atmospheric transport rates that mix fresh pollutant into the surface boundary layers and by the physical properties of particles. For the Eastern U.S., the approximate annual deposition rates of sulfate can be compared as follows (Table III), considering that deposition flux is the product of a concentration and a velocity of deposition (Vd) (20) ... 

Atmospheric aerosols are multicomponent particles ranging from 0.001 to 10 pm in diameter. Particles are introduced into the atmosphere by combustion processes and a variety of other anthropogenic and natural sources. They evolve by gas-to-particle conversion and coagulation, are augmented due to the formation of fresh particles by nucleation, and are removed by deposition at the earth s surface and scavenging by airborne droplets. Atmospheric aerosols are the main cause of the visibility degradation accom-... 

Deposition from the atmosphere to the ground, vegetation or water surface can occur via three deposition processes. The direct interaction of a material with the surface is called dry deposition and occurs continuously. The second process is called wet deposition and includes the removal of material from the atmosphere in any falling hydrometeors, such as raindrops, snowflakes or hailstones. The third process includes deposition in fog droplets which do not readily fall under gravity due to size limitations. Fog deposition is a process which falls midway between dry and wet deposition and includes mechanisms which are important in both the wet and dry removal processes. 

Atmospheric fate If released to the atmosphere, 2-heptanone is expected to undergo a gas-phase reaction with photochemically produced hydroxyl radicals the estimated half-life for this process is 1.9 days. 2-Heptanone has relatively high water solubility (4300 mgl at 25°C), which indicates that it may undergo atmospheric removal by wet deposition processes. Although 2-heptanone has the potential of being removed from the atmosphere by direct photochemical degradation, the rate of this process is not expected to be able to compete with atmospheric removal by the reaction with hydroxyl radicals. 

A passing cold front is heralded by clouds, a drop in temperature, and precipitation cooler air and clear skies occur behind the cold front (see Fig. 4-15b). The slower moving warm front is characterized by a more gradual lowering of cloud heights, followed by rain or snow (Fig. 4-15a). As air masses pass, so do their burdens of airborne chemicals. The clouds and precipitation formed along the front act as sinks for certain atmospheric chemicals because of rainout and washout processes. These processes, which remove particles, gases, and dissolved chemicals from the atmosphere and deposit them on Earth s surface, are discussed in Section 4.5. 

Particulate chemicals also may be removed from the atmosphere through wet deposition processes. The major mechanism by which particles are incorporated into precipitation is by serving as nucleation sites for condensation at the onset of water droplet or ice crystal formation. [If nucleation sites were absent, water droplets would not form unless the air temperature were significantly below the dew point. In the rainmaking practice of cloud seeding, condensation nuclei in the form of silver iodide (Agl) crystals are dispersed into air masses to increase the formation of ice crystals, which in turn accumulate additional water from the cloud and thus promote precipitation ] Particles also can be incorporated into already-formed water droplets within a cloud by collision. 

In the atmosphere, ammonia can be removed by rain or snow washout. Reactions with acidic substances, such as H2SO4, HCl, or HNO3 (all produced in high concentrations from anthropogenic activities) produce ammonium aerosols, which can undergo dry or wet deposition. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process. The best estimate of the half-life of atmospheric ammonia is a few days. 

Crutzen 1983 Dawson 1977 Galbally and Roy 1983 Moller and Schieferdecker 1985). The reaction of atmospheric ammonia with acidic substances in the air results in the formation of ammonium aerosols that can subsequently be removed from the atmosphere by dry or wet deposition. In general, dry deposition processes predominate where there are high amounts of NH3 emissions where NH3 emissions are lower, wet deposition of particulate NH/ predominates (Asman et al. 1998). 

This chapter is devoted to chemical removal processes in the troposphere. In this section, however, we briefly discuss physical removal processes. Gases and particles are physically removed from the atmosphere by deposition at the earth s surface (so-called dry deposition) and by absorption into droplets followed by transfer of the drops to the surface in the form of precipitation (so-called wet deposition). 

PAN acts as a reservoir species for both CH3C(0)02 radicals and NO. Because of this, the atmospheric lifetime of PAN is important if its lifetime is relatively long, PAN can act as an effective reservoir for NO. Potential atmospheric removal processes for PAN include thermal decomposition (reaction 4 above), UV photolysis, and OH reaction. PAN is not highly water-soluble it is more soluble than NO or N02 but considerably less soluble than HNO3. Thus, wet deposition is a minor removal process. Dry deposition is also unimportant. The PAN-OH rate constant is <3 x 10 14 cm3 molecule-1 s 1, and OH reaction is not an effective removal process. PAN absorbs UV radiation up to 350 nm (Libuda and Zabel 1995 Talukdar et al. 1995). Thus, thermal decomposition and photolysis are the principal removal processes for PAN. 

Extensive investigation of this process showed that the substrate temperature and the pressure of the decomposition products have a pronounced influence on the composition of the resulting deposit. The pyrolysis can be performed only in a carbonyl atmosphere, but the mixture of H2 was found to be virtually necessary in any practical deposition process, both as a carrier gas and as a carbon-removing agent... 

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