Atmosphere wet removal

As indicated in previous chapters, the atmosphere serves as the medium through which air pollutants are transported and dispersed. While being transported, the pollutants may undergo chemical reactions and, in addition to removal by chemical transformations, may be removed by physical processes such as gravitational settling, impaction, and wet removal. 

Fig. 7-13 Physical transformations of trace substances in the atmosphere. Each box represents a physically and chemically definable entity. The transformations are given in F, (from the ith to the /th box). Q, represents sources contributing to the mass or burden, M,> in the ith box. Rd, and Rw, are dry and wet removals from M,. The dashed box represents what may be called the fine-particle aerosol and could be a single box instead of the set of four sub-boxes (i = 1,2,3,4). The physical transformations are as follows ...

Heavy metals are removed from the atmosphere by means of surface uptake and precipitation scavenging. The ecosystem-specific dry deposition scheme is based on the resistance analogy approach and distinguishes 16 land use types. Wet removal by precipitation considers both in-cloud and sub-cloud scavenging. 

The advection scheme of the regional model is improved to take into account the surface orography. Terrain following vertical structure of the model domain with higher resolution was incorporated. Wet removal of heavy metals from the atmosphere was enhanced by developing newparameterizations of precipitation scavenging. Both in-cloud and sub-cloud wet removal were modified on the basis of the up-to-date scientific literature data. 

Dry deposition can also be a very important mechanism for removing pollutants from the atmosphere in the absence of precipitation. Indeed, even in such places as eastern England, the ratio of dry to wet removal of S02 has been estimated to be 2 1 (Davies and Mitchell, 1983). If this is the case, then in arid and semiarid regions such as much of the western United States dry deposition is clearly important. 

Hales, J. M., A Modelling Investigation of Nonlinearities in the Wet Removal of S02 Emitted by Urban Sources, in Atmospheric Chemistry Models and Predictions for Climate and Air Quality (C. S. Sloane and T. W. Tesche, Eds.), Chap. 8, pp. 117-130, Lewis Publishers, Chelsea, MI, 1991. 

Somewhat similar levels in air, between 0.5 and 5 ng/m (mean, 2 ng/m ) of di(2-ethylhexyl) phthalate have been found in the Great Lakes ecosystem (Canada and United States). The concentration of di(2-ethylhexyl) phthalate in precipitation ranged from 4 to 10 ng/L (mean, 6 ng/L). Atmospheric fluxes to the Great Lakes are a combination of dry and wet removal processes. The total deposition of di(2-ethylhexyl) phthalate into Lakes Superior, Michigan, Huron, Erie and Ontario was estimated to amount to 16, 11, 12, 5.0 and 3.7 tonnes per year, respectively (Eisenreich et al., 1981). 

Garland, J.A. (1978) Dry and wet removal of sulphur from the atmosphere. Atmospheric Environment, 12, 349-62. 

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) ... 

Air half-life is a few hours in the sunlit troposphere ty, = 19 and 50 h by dry deposition and wet removal, respectively ty, = 12 d when reacts with NO3 radical by H-atom abstraction. (Howard 1989) photooxidation ty, = 7.13-71.3 h, based on measured rate constant for the vapor-phase reaction with hydroxyl radical in air (Atkinson 1985 quoted, Howard et al. 1991) ti/j = 1.26-6.0 h, based on photolysis half-life in air (Howard et al. 1991) atmospheric transformation lifetime was estimated to be 1 to 5 d (Kelly et al. 1994) calculated lifetimes of 1.2 d, 80 d and > 4.5 yr for reactions with OH radical, NO3 radical and O3, respectively (Atkinson 2000). 

Formaldehyde is released to the atmosphere in large amounts and is formed in the atmosphere by the oxidation of hydrocarbons. However, the input is counterbalanced by several removal paths (Howard 1989). Because of its high solubility, there will be efficient transfer into rain and surface water, which may be important sinks (NRC 1981). One model has predicted dry deposition and wet removal half-lives of 19 and 50 hours, respectively (Lowe et al. 1980). Although formaldehyde is found in remote areas, it probably is not transported there, but is generated from longer-lived precursors that have been transported (NRC 1981). 

On the other hand, the effect of the wet removal can be practically neglected here.3 It is thus understandable that the residence time of trace constituents is greater in the stratosphere than in the troposphere. Above the tropopause the horizontal wind speed first decreases then increases with height. Consequently, a secondary maximum in the wind speed can be observed in this atmospheric layer. The increase of the temperature ends approximately at an altitude of SO km (stratopause), where the temperature is around 0 °C (see Fig. 1). Above this level, in the mesosphere, the temperature again decreases (third layer in the homosphere). For this reason the stratopause can be considered as an active heat-supplying surface similar to the Earth s surface. In this atmospheric region the distribution of the temperature makes possible the convection which, in favourable cases, results in a formation of so-called noctilucent clouds at an altitude of about 80 km (mesopause) where the temperature is only around — 80 °C. This is the coldest level of our atmosphere. 

The problems related to the water cycle will also not be considered in spite of the fact that, taking into account its quantity and atmospheric effects, water is one of the most important trace materials. This omission is explained by a historical precedent. The study of the atmospheric cycle of the water as well as the measurement of its concentration were included in the past in the program of other branches of atmospheric science. Thus, the formation of clouds and precipitation, the subject of the cloud physics (e.g. Mason, 1957, Fletcher, 1962), will only be discussed in relation with the wet removal of aerosol particles and water-soluble gases. 

Furthermore it seems appropriate not to discuss radioactive materials here, but to leave them to a separate volume. The atmospheric fate of radioactive aerosols and gases is similar in some respects to that of other trace substances (e.g. dry and wet removal). However, the presentation of their formation and decay would render the present volume too diffuse. Thus, results gained by radioactive tracers will only be mentioned in Chapter 5 dealing with the removal processes. Concerning atmospheric radioactivity, the reader is referred to other textbooks (Junge, 1963 Cadle, 1966 Israel and Israel, 1973). 

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