Acid deposition and atmospheric chemistry

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... 

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). 

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. 

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. 

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. 

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. 

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. 

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). 

Atmospheric emissions of sulphur dioxide are either measured or estimated at their source and are thus calculated on a provincial or state basis for both Canada and the United States (Figure 2). While much research and debate continues, computer-based simulation models can use this emission information to provide reasonable estimates of how sulphur dioxide and sulphate (the final oxidized form of sulphur dioxide) are transported, transformed, and deposited via atmospheric air masses to selected regions. Such "source-receptor" models are of varying complexity but all are evaluated on their ability to reproduce the measured pattern of sulphate deposition over a network of acid rain monitoring stations across United States and Canada. In a joint effort of the U.S. Environmental Protection Agency and the Canadian Atmospheric Environment Service, eleven linear-chemistry atmospheric models of sulphur deposition were evaluated using data from 1980. It was found that on an annual basis, all but three models were able to simulate the observed deposition patterns within the uncertainty limits of the observations (22). 

Acid rain monitoring data in North America have been gathered by Environment Canada and stored in the National Atmospheric Chemistry (NatChem) Database, details of which can be found at www.airquality.tor.ec.gc.ca/natchem. Analysis of the deposition chemistry data has confirmed that wet sulfate deposition did indeed decline in concert with the decline in SO2 emissions in both eastern Canada and the... 

Part 1, Alpine Water Resources, examines the hydrological basics, the impacts of climate change in the Swiss Alps, and human interventions in mountain waters. Part 11, Biogeochemistry and Pollution of Alpine Waters, deals with the chemistry of mountain rivers, the effects of acid deposition on high elevation lakes, the glaciers as archives of atmospheric deposition, and the occurrence of persistent organic contaminants. 

Oxides of nitrogen play a central role in essentially all facets of atmospheric chemistry. As we have seen, N02 is key to the formation of tropospheric ozone, contributing to acid deposition (some are toxic to humans and plants), and forming other atmospheric oxidants such as the nitrate radical. In addition, in the stratosphere their chemistry and that of halogens interact closely to control the chain length of ozone-destroying reactions. 

James N. Pitts, Jr., is a Research Chemist at the University of California, Irvine, and Professor Emeritus from the University of California, Riverside. He was Professor of Chemistry (1954-1988) and cofounder (1961) and Director of the Statewide Air Pollution Research Center (1970-1988) at the University of California, Riverside. His research has focused on the spectroscopy, kinetics, mechanisms, and photochemistry of species involved in a variety of homogeneous and heterogeneous atmospheric reactions, including those associated with the formation and fate of mutagenic and carcinogenic polycyclic aromatic compounds. He is the author or coauthor of more than 300 research publications and three books Atmospheric Chemistry Fundamentals and Experimental Techniques, Graduate School in the Sciences—Entrance, Survival and Careers, and Photochemistry. He has been coeditor of two series, Advances in Environmental Science and Technology and Advances in Photochemistry. He served on a number of panels in California, the United States, and internationally. These included several National Academy of Science panels and service as Chair of the State of California s Scientific Review Panel for Toxic Air Contaminants and as a member of the Scientific Advisory Committee on Acid Deposition. 

Hoffmann, M. R., and Calvert, J. G., Chemical Transformation Modules for Eulerian Acid Deposition Models. Volume II The Aqueous-Phase Chemistry. National Center for Atmospheric Research, Boulder, Colo., 1985. 

Several areas in which chemical measurement technologies have become available and/or refined for airborne applications have been reviewed in this paper. It is a selective review and many important meteorological and cloud physics measurement capabilities of relevance to atmospheric chemistry and acid deposition (e.g., measurement of cloud liquid water content) have been ignored. In particular, we have not discussed particle size spectra measurements for various atmospheric condensed phases (aerosols, cloud droplets and precipitation). Further improvements in chemical measurement technologies can be anticipated especially in the areas of free radicals, oxidants, organics, and S02 and N02 at very low levels. Nevertheless, major incremental improvements in the understanding of acid deposition processes can be anticipated from the continuing airborne application of the techniques described in this review. 

Photochemistry of course plays a crucial role in the whole of tropospheric atmospheric chemistry, not just in locally confined events such as those involving acid deposition discussed above, and in the weU-documented photochemical smog of Californian cities, which is now a local problem of very widespread incidence throughout the world. 

Secondly I think one has to look very carefully at transport phenomena. Several speakers in this Study Week have referred to the effect of the introduction of tall stacks which permit an increased dilution of emissions from power plants. The inclusion of a tall stack at a power plant does not cut the deposition in the vicinity of that stack — and you can use the term vicinity in any way you like — to zero and the deposition at a distance of 500 kilometers to 100%. A very substantial fraction of the deposition associated with emission from a particular source, even with the tall stack, occurs relatively near to that source and again, the question of how near is one, that is extremely difficult to get solid answers for — one simply does not have that kind of information. If you want to take an applied mathematician and send him into shock, you ask him to model the flow from a tall smokestack over a distance of about ten or twenty kilometers — that is just something that is not done. The overall transport phenomenon in acid rain is an extraordinarily complex multi-scale phenomenon. So far as the chemistry is concerned, I think that, too, varies dramatically with the climate, with the season, with the presence of oxidants of various types in the atmosphere, and I fear that there can be no single generalization concerning acid rain and the mitigation of acid deposition worldwide. This is something that has to be handled on a scale which in fact I think will be much smaller. 

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