Reducing acid rain Continuation of the Pimentel report
Acid rain is one of the more obvious and pressing results of degradation of air quality. Acidic substances and their precurms are formed when fossil fuels are burned to generate power and provide transportation. These substances are principally acids derived from oxides of s u l k and nitrogen. There are some natural sources of these compounds such as lightning, volcanos, burning biomass, and microbial activity, but, except for rare. volcanic erup tions, tbese are relatively small com-
pared with emissions from power plams, smelters, and vehicles in indnstrial regions. The effects of acidic rainfaU are most evident and highly publicized in Europe and the northeastern UNted States, but a r p at risk include Canada and perhaps the California Sierras, the Rocky Mountains, and China. In some places precipitation as acidic as vinegar has occasionally been observed. The extent of the eof acid rain is the subject of continuing controversy. Damage to
aquaticlife. in lakes and streams was the original focus of attention. More recently, damage to buildings, bridges, and equipment has been recognized as another costly consequence of acid rain. The effect of polluted air on human health is the most difficult to quan-
tie.
Greatest damage is done to lakes that are poorly buffered. When ~ l ~ ~ aal-l l y kaline buffers are present, the acidic compounds in acid rain, largely sulfuric acid, nitric acid, and smaller
Emimn. Sci. Technol.. MI. M ,No. 5. iM 49S
amounts of organic acids, are neutralW, at k t until this auralinity is wn-
sumed. However, lakes lying on granitic (acidic) strata are susceptible to immedmte ' damage because acids in precipitation can dissolve metal ions,
such as aluminum and manganese, causing reduction in biological productivity and, in some lakes, the decline or elhination of fish populations. Damage to plants from pollution ranges from adverse effects on foliage to d e Stnlction of f i i RYN systems. In a region such as the northeastern United states the principal candidates for pollutant reduction are the power plants burning wal with high sulfur content. chemical ~mblxrsthat prevent the emission of the pollutants offer one of the possible remedies. Catalysts that reduce oxides of nitrogen emissions from both stationary and mobde sources offer yet another example of the role that chemistry can play in improving air quality. The various strategies for reducing acid rain involve possible investmems of billions of dollars annually. With the stakes so high, it is imperative that the atmospheric prooegses determining the transport, chemical transformation, and fate of pollutants he well understood.
Acid deposition consists of both "el" precipitation (as in rain and snow) and dry depsition (in which aerosols or gaseous acidic compounds are depositedon surfaces such as soil particles, plant leaves, etc). what is finally deposited has usually been injected into the atmosphere in a quite F a example, differentchemical h. sulfur in wal is oxidized to s u l h diox440 Emlmn. W. Technol.,MI. 20, No. 5.1886
ide, the gasews form in which it is emitted 6um smkstacks. As it moves uyough the ahnosphere, it is slowly oxidized and reacts with water to form
oxides of nitrogen and sulfiv as well as hydrogen pwoxide, ozone, forma& hyde, and other p i e s in cloud drople& and in the vapor state. The detailed sulfivic acid-the form in which it may pathways for oxidation of precursors to he depdted hundreds of miles down- prcduc& are not yet e s t a h l i i . Rob Wind. lems are sometimes encountered in exThe pathways by which oxides of ni- trapolating from laboratory experitrogen are formed,undergo chemical ments and computer-based models to transformation, and are eventually rs dfield wnditions. Frequently, the moved from the amcs&re are also rate laws and equilibrium wmtants esvery complex. Nitmgen and oxygen, tablished under ideal laboratory dwhen heated at high semperahurs in tions hecome difficvlt to apply to the power plants, home furnaces, and vehi- more complex mixtures and conditions cle engines, form nihic oxide, NO, present in the. a m p h e r e . The role of which reacts with oxidants to form ni- aqueous-phase photochemistry in trogen dioxide, Na,and evenlually ni- chemical'onswithin clouds tric acid, H N a .Quantitative estimates is not known. Atmospheric chemisay of the glohal budget for the oxides of that OCCUH in daylight drives Itactions nitrogen still contain umwptably widc to favor pmducts diffemlt from those ullcertainty ranges. formed at night, so more resegTdl is It can readily he sem that without a needed at field stations around the thorough knowledge of the biogeo- clock, as well as in the laboratory. chemical cycles for the various chemiTheroles of reactive species, such as cal forms of nitrogen, sulflq and car the radicals OH, OOH, and Na,in hon, and of the global compartments oxidation reactions leading to scavengfrom which these s p i e s arise and are ing of pollutants from the. atmosphere partitioned into, it will be diffidt to need tohe understood in the betselect air pollution control strategies while systems with confidence. Atmospheric and en- are easlcr to sludy in the gas phase, vironmental chemistry are central to a much impaant chemistry pmhbly occleaner and more healthful environ- CUIS at the li@d droplet-gas phase inment. Developmt of reliihle methods terfaces, or within the droplets. Most of measurement of hbce species in air, earlier work has focused on the inorkinetia ofilllpmnt BmEo8phericreac- ganic camtituents of acid precipitation, tions, and the discovery of new, more butcertainorganicspeciesmaypotentieffdve, chemical processes for redue ate or inbiii d o n s . For example, ing pollutant emission are goals that organic compouads in surface organic should receive a national wmtnitmnt microlayers covering water droplets for the coming decade. may alter mass aaasport of reactantsor 'Ib minimize acid rain in awst-eEec- act as either catalysts or inhiitom. tive m m m , we must develop a hemr understsnding of the chemistry of the lkh is thefowrh in a seven-pan series.
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