(Accepted by the AMS Council on 18 September 1997)
Since its identification as an environmental problem, acid deposition has been studied extensively over the past two decades. Legislation has been implemented to mitigate the undesirable effects associated with these pollutants. The following statement summarizes the present state of knowledge and uncertainty about atmospheric aspects of acid deposition and provides recommendations.
An "acid" is a substance that releases hydrogen ions (H+) when dissolved in water. Several atmospheric trace constituents dissociate into positive and negative ions when dissolved in water, and some are acidic to varying degrees. The strongest acids in atmospheric waters are dissolved sulfuric acid (H2SO4) and nitric acid (HNO3). Numerous other acidic substances have been identified in the atmosphere, such as sulfur dioxide (SO2), organic acids, hydrochloric acid (HCl), carbon dioxide (CO2), and even water itself, but these substances are either relatively weak acids, or are present at relatively small concentrations, and thus do not usually contribute appreciably to measured acidity. For any solution, there is an equal concentration of dissolved positive and negative ions, and a typical ion balance in cloud water and precipitation is
[H+] + [Na+] + [NH4+] + [soil ions] = (positive ions) = 2[SO4=] + [NO3] + [Cl] + [HCO3] (negative ions).
Sodium (Na+) and chlorine (Cl) ions arise from dissolved sea-salt aerosols and are usually present in approximately equal concentrations. Dissolved ammonia is NH4+, "soil ions" refers to calcium and magnesium ions that are typically associated with carbonates (HCO3) in soil dust, and SO4= and NO3 are sulfuric and nitric acid, respectively. Since an ion balance is always maintained in atmospheric waters, the concentration of H+ is
[H+] = 2[SO4=] + [NO3] + [HCO3] [NH4+] [soil ions].
Thus the concentration of hydrogen ion is proportional to the concentrations of sulfates, nitrates, bicarbonate (dissolved CO2), ammonia, and carbonate-laden soil dust dissolved in cloud water and precipitation. Measured concentrations of H+ vary over several orders of magnitude, and therefore the following logarithmic pH scale is used to quantify acidity levels in water:
pH = log10[H+].
Pure water in equilibrium with atmospheric CO2 has a pH near 5.6, but the concentrations of sulfate, nitrates, ammonia, or soil cations in cloud and rainwater usually greatly exceed the concentrations of dissolved CO2, even in remote areas. Typical "clean" atmospheric waters have a pH of 4.55.5. In more polluted areas, pHs in precipitation range from 3 to 4, and in some low liquid water content clouds, pHs as low as 23 have been measured. The lower the pH, the higher the concentration of H+ and acidity. A decrease in one pH unit means a 10-fold increase in acidity or H+ concentration.
Sulfuric and nitric acids are produced by reactions between atmospheric oxidants and emitted sulfur and nitrogen oxides (SO2 and NOx), which are by-products of fossil fuel combustion and other industrial activities. The dominant reactions converting SO2 to sulfuric acid include reactions with hydrogen peroxide (H2O2) in clouds and hydroxyl radical (HO) in air. Nitric acid is produced by the oxidation of NO2 by HO radical and also a heterogeneous reaction involving NO3 radicals and ozone at night. Atmospheric oxidants responsible for acid formation are produced via a complex sequence of photochemical reactions, and some acid-generating chemical reactions occur among dissolved gaseous constituents in atmospheric clouds or aerosols. Typically, emitted NOx is converted to nitric acid within a day or less, and SO2 is converted to sulfuric acid within several days following emission. The concentrations of the oxidants, and the timescale for chemical reactions vary strongly with season, latitude, time of day, sunlight intensity, background concentrations of NOx and organic compounds, and many other chemical and meteorological factors.
Strong acids have an affinity for water, and therefore hygroscopically grow or combine with water vapor to form "haze" aerosols containing sulfuric acid, nitric acid, and varying degrees of neutralizing ammonia (NH3), especially when atmospheric relative humidities are above 60%70%. Typically, ammonia and nitric acid are present as both gases and aerosols in the atmosphere, while sulfate partitions predominantly into condensed aerosols. These sulfate-, nitrate-, and ammonium-containing aerosol particles constitute a significant fraction of cloud condensation nuclei (CCN), and thus acid-containing aerosols are readily incorporated into clouds. Precipitation forming within clouds therefore contains dissolved CCN together with other soluble gases such as HNO3 and NH3.
Since the timescales for the chemical formation and wet and dry removal of atmospheric acids and their precursors are in the range of several days or less, a wide range of physical and dynamical meteorological processes affect the location and extent of acid impacts. Impacted areas extend hundreds to thousands of kilometers from the regions where precursor SO2 and NOx pollutants are emitted. The spatial extent of impacts is strongly influenced by virtually all scales of atmospheric motions including turbulence, cloud-scale convective processes, and larger-scale prevailing synoptic weather systems.
Acidic substances in the atmosphere are deposited on the earth's surface by two mechanisms: wet and dry deposition. Wet deposition involves the dissolution of acidic substances into cloud water, and the subsequent fallout of acidified precipitation to the earth's surface ("acid rain"). Acidic substances and their precursors can also dry deposit directly to the earth's surface through particle settling, turbulent mixing, and dissolution and reaction on biological and other surfaces. Dry-deposited substances ultimately release acids into the environment when they dissolve into subsequent precipitation or otherwise migrate into surface waters. The direct impaction of cloud or fog droplets on trees and other vegetation also contributes to acidic deposition, particularly in mountainous areas that are immersed in clouds. Dry- or wet-deposited acidic substances are transported through soils into lakes, streams, and groundwater.
Transport and dispersion of pollutants in the lower troposphere, the radiative and chemical impacts of the emitted compounds, and the microphysics of aerosol, cloud nucleation, and precipitation formation all play roles in determining the concentrations of acidic substances in the troposphere. A comprehensive understanding of acid deposition requires a quantitative description of all of these meteorological and chemical processes. While many features of acid formation and deposition in the atmosphere are understood, a large number of uncertainties remain, whose resolution will require additional atmospheric measurements and substantial combined efforts of atmospheric scientists.
An area of significant quantitative uncertainty in our current understanding of acid deposition in the atmosphere involves cloud-scale processes. Chemical reactions in clouds are probably the most important mechanism for the formation of sulfuric acid in the atmosphere, and other cloud-scale dynamic and radiative processes affect the formation of numerous oxidants in the atmosphere. In addition, the formation of precipitation constitutes a major atmospheric removal mechanism. All of these cloud-scale phenomena are only semiquantitatively estimated in current models of tropospheric chemistry, and measurements of the regional-scale chemical influences of clouds are lacking.
At high concentrations or exposures, acidic solutions induce numerous undesirable reactions with surfaces. In conjunction with other pollutants, acid deposition contributes to potentially deleterious effects on aquatic, agricultural, and forest ecosystems. Chemical changes attributed to the deposition of acidity from the atmosphere have been measured in forest ecosystems and surface waters. Concentrations of acids in lakes has been correlated with the concentrations and deposition rates of atmospheric acids, and high concentrations of acids in lakes and streams can adversely affect fish populations. Health effects associated with exposure to acid-containing particulates in humans remains an area of uncertainty, since current studies of these effects are too limited to unambiguously discern dose-response relationships in humans. Acid deposition from the atmosphere has been shown to accelerate the deterioration rate of exposed metals, painted finishes, and concrete or stone surfaces.
In industrialized areas, concentrations of sulfuric and nitric acids in cloud water and precipitation are up to 50100 times greater than values measured in areas that are not influenced by upwind emissions of anthropogenic pollutants. The relative concentrations of deposited sulfur and nitrogen acids correlates with the relative sulfur and nitrogen emission rates over a larger-scale area.
In response to the Acid Precipitation Act of 1980, a National Acid Precipitation Assessment Program (NAPAP) was established as a 10-year integrated research effort to coordinate federally funded research and assessment activities to facilitate the development of a firm scientific basis for policy decisions pertaining to acid rain.
In 1990, NAPAP produced a quantitative assessment of the undesirable effects associated with atmospheric acidity, and also summarized our current understandings of the emission, transport, transformation, and deposition of acids in the atmosphere. Integrated numerical modeling systems of atmospheric chemistry and transport were refined and validated during NAPAP.
Emissions of reactive sulfur and nitrogen over the United States have been steadily declining by 6%10% per decade during the 1980s. In the industrialized eastern United States, the concentrations of sulfuric and nitric acid in precipitation also declined during the 1980s.
With the adoption of the 1990 Clean Air Act Amendments, emissions of SO2 are ultimately mandated to be reduced to about 40% of 1980 values. Reductions in sulfur emissions are to be achieved using a market-based trading and banking system of emission allowances. Emissions of nitrogen oxides should be initially reduced by about 10% from 1980 values after various provisions of the U.S. Clean Air Act are implemented, although these NOx emissions may increase after the turn of the century.
As pollution-control technologies improve and legislative policies are implemented, air quality and the concentrations of acidic substances should improve with time. However, there is an important need to design, implement, and maintain a system for providing an accurate measure of the influence of control measures taken in response to air pollution legislation.
It is essential that research and monitoring continue to ensure compliance with existing public policy objectives and to quantitatively improve and verify our understanding of the complex processes involved in the formation and deposition of atmospheric acidity as embodied in current models of acidity in the atmosphere. The scientific basis for current and future pollution control strategies should be continually tested and validated in the light of our evolving understandings of the physics and chemistry of atmospheric acidity, particularly in areas where there is significant uncertainty in quantifying processes affecting concentrations of acidity in the atmopshere. Further and more sophisticated analysis of alternate emission reduction scenarios using reliable models of atmospheric processes could assist in designing the optimum pollution reduction strategy.
The American Meteorological Society will continue to provide a forum for ongoing collaboration between the meteorological and chemical communities to further understand atmospheric acidity.
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