Acid deposition and atmospheric

Acid Deposition and Atmospheric Chemistry at Allegheny Mountain... 

Air pollutants, transport, 4 Air sampling—See Interstitial air sampling Aldehydes, determination in atmospheric samples, 299 Allegheny Mountain acid deposition and atmospheric chemistry, 28-36 deposition budgets for sulfate and nitrate, 33... 

A variety of models have been developed to study acid deposition. Sulfuric acid is formed relatively slowly in the atmosphere, so its concentrations are beUeved to be more uniform than o2one, especially in and around cities. Also, the impacts are viewed as more regional in nature. This allows an even coarser hori2ontal resolution, on the order of 80 to 100 km, to be used in acid deposition models. Atmospheric models of acid deposition have been used to determine where reductions in sulfur dioxide emissions would be most effective. Many of the ecosystems that are most sensitive to damage from acid deposition are located in the northeastern United States and southeastern Canada. Early acid deposition models helped to estabUsh that sulfuric acid and its precursors are transported over long distances, eg, from the Ohio River Valley to New England (86—88). Models have also been used to show that sulfuric acid deposition is nearly linear in response to changing levels of emissions of sulfur dioxide (89). 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

It is known from studies carried out over many decades that oxides of nitrogen at high concentrations dissolve in aqueous solution and react to form species such as nitrate and nitrite. With the focus on acid deposition and the chemistry leading to the formation of nitric and sulfuric acids during the 1970s and 1980s, a great deal of research was carried out on these reactions at much lower concentrations relevant to atmospheric conditions (for reviews, see Schwartz and White, 1981, 1983 and Schwartz, 1984). 

The CASTNET provides atmospheric data on the dry deposition component of total acid deposition, ground-level ozone, and other forms of atmospheric pollution. CASTNET is considered the nation s primary source for atmospheric data to estimate dry acidic deposition and to provide data on rural ozone levels. Used in conjunction with other national monitoring networks, CASTNET is used to determine the effectiveness of national emission control programs. Established in 1987, CASTNET now comprises over 70 monitoring stations across the United States. The longest data records are primarily at eastern sites. The majority of the monitoring stations are operated by EPA s Office of Air and Radiation however, approximately 20 stations arc operated by the National Park Service in cooperation with EPA. 

In the last decade many experiments have been made in stratosphere and troposphere chemistry, as well as in the laboratory on models, on the causes of acid depositions and on the role of fluorocarbons on the ozone layer and the impact of the COi increase in the atmosphere on the climate and on oceans. The studies have given important results for our knowledge and for the advancement of science. 

Before passing on to other aspects of atmospheric photochemistry, we might just anticipate later discussion of the effects of acid deposition, and indeed the location of this. From the parochial UK viewpoint the region most affected is southern Scandinavia, Tables 2 and 3 giving the total contributions by country to deposition in this region [3]. 

Calcite and dolomite are the common carbonate materials of sedimentary (limestone, sandstone) and metamorphic (marble) rocks used as building stones. These materials are highly susceptible to attack by acid deposition and by the presence of atmospheric SO2 according to the following reactions ... 

To establish the effects on human health of the fallout of the chemical species formed in the atmosphere, we have to distinguish between acid depositions and substances like ozone, PAN or other peroxides. In this case we have to follow a different approach, that is, we must establish, as proposed by WHO, the limits for some characteristic contaminants. 

Water droplets and particulate matter often influence the rates of chemical transformations in the atmosphere. Whereas homogeneous reactions involve only gaseous chemical species in the atmosphere, reactions involving a liquid phase or a solid surface in conjunction with the gas phase are called heterogeneous reactions. Reactions that occur much more rapidly in water than in air may occur primarily in droplets, even though the droplets constitute only a small fraction of the total atmospheric volume. Solid surfaces also can catalyze reactions that would otherwise occur at negligible rates specific examples are discussed in the following sections on acid deposition and stratospheric ozone chemistry. 

Sherwood, S. I, "Acidic Deposition and the Corrosion and Deterioration of Materials in the Atmosphere A Bibliography. 1880-1982" PB83-126091 National Technical Information Service, July 1983. 

Figure 3. Stone briquettes arrayed for exposure to atmospheric acid deposition and weathering at field test site. (a) side and (b) front views.

The section on metallic corrosion clearly indicates that the conventional method of classification of environments into marine, industrial, and rural no longer is adequate. More specific information is needed about the actual chemical components in the atmosphere as well as humidity and other factors. Specific environments also are addressed in the metallic corrosion section. For example, the automotive environment in the northeastern United States is particularly severe because of the combination of acid deposition and the use of road de-icing salts. These factors exert a synergistic effect on the corrosion behavior of auto-body steel and on exterior anodized aluminum automobile trim. 

Atmospheric chemical transport models are used for the simulation of a wide range of atmospheric phenomena (urban smog, regional ozone, acid deposition, global atmospheric chemistry) over a variety of spatial and temporal scales. Much of the field s history can be found in reviews by Te.sche (1983), Seinfeld (1988), Roth et al. (1989), and Peters et al. (1995). In this section we di.scuss a few selected applications. 

Husar R. B., Sullivan T. J. and Charles D. F. (1991) Historical trends in atmospheric sulfur deposition and methods for assessing long-term trends in surface water chemistry. In Acidic Deposition and Aquatic Ecosystems (ed. D. F. Charles), pp. 65-82. Springer, Berlin. 

Acid Deposition. Acid deposition, the deposition of acids from the atmosphere to the surface of the earth, can be dry or wet. Dry deposition involves acid gases or their precursors or acid particles coming in contact with the earth s surface and thence being retained. The principal species associated with dry acid deposition are S02(g), acid sulfate particles, ie, H2SO4 and NH HSO, and HN02(g). Measurements of dry deposition are quite sparse, however, and usually only speciated as total and total NO3. In general, dry acid deposition is estimated to be a small fraction of the total... 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

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