Acid Deposition Atmospheric Processes in Eastern North America (1983) / Chapter Skim
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Several studies have documented the potentially harmful effects of the deposition of acids on ecosystems, which are of particular concern in areas with low geochemical capacities for neutralizing the acidic inputs (such as parts of the northeastern quadrant of North America, the Appalachian Mountains, and some of the mountainous areas of western North America)
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The central issues of concern in this report are the adequacy of current scientific understanding about the relationships between emissions and deposition, the extent to which the relationships are strongly nonlinear, and the extent to which distant sources contribute to deposition in ecologically sensitive, remote areas. We have reviewed the available scientific evidence that pertains to the issues of nonlinearity in the relationships between emissions and deposition and long-range transport.
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From page 3... ...
Available data on precipitation chemistry and on annual average ambient concentrations of SC2, NOk, sulfate, ammonium, and nitric acid indicate elevated levels of pollutants in the air and acidic substances in precipitation over much of eastern North America. Ambient concentrations are much higher than can be accounted for by emissions from natural sources on a regional scale.
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From page 4... ...
These processes include the large-scale transport of air masses, atmospheric mixing near the Earth's surface, physical and chemical reactions among pollutants and naturally present species, deposition of gases and suspended particles, and cloud processes leading to precipitation. Transport, mixing, physical and chemical reactions, and cloud processes are responsible directly or indirectly for the distribution and rate of deposition of pollutants to the ground.
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From page 5... ...
Nonetheless, empirical analyses suggest that many of the precipitating air masses -- and therefore most of the pollution-related ions dissolved in precipitation -- reaching several sensitive, remote areas of the northeastern United States and southeastern Canada have their origins in unwind regions to the south and southwest. Because of the high variability in synoptic-scale meteorological phenomena affecting sensitive areas, however, all sources in eastern North America must be considered as contributing in one degree or another to the phenomenon of acid deposition.
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Because of the simplifying assumptions made in order to develop practical, economical regional-scale airquality models and because data are not available to validate or verify the models, workers in the field generally have only limited confidence in current results. The models and their results are useful research tools; but given the state of knowledge of the physics and chemistry of the atmosphere in the context of long-range transport in air pollution, we advise caution in using deterministic models to project chances in patterns of deposition on the basis of changes in patterns of emissions of precursor gases.
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From page 7... ...
However, on the basis of analysis of currently available data in eastern North America and within the limits of uncertainty associated with errors in the data and in estimating emissions, we conclude that there is no evidence for a strong nonlinearity in the relationships between long-term average emissions and deposition. This conclusion is based on analysis of available data on historical trends (mainly at the Hubbard Brook Experimental Forest in New Hampshire)
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From page 8... ...
Analysis of the spatial distributions of the molar ratio of SO2 to NOx in emissions and the molar ratio of sulfate to nitrate in precipitation provides additional though indirect evidence that there is no strong nonlinearity in the relationships between long-term average emissions and deposition in eastern North America (see Chapter 4)
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From page 9... ...
indicate that much of the acidity in precipitation -- as well as much of the precipitation -- comes from air masses arriving from the South and Southwest. Based on the analysis of spatial distributions of the annual average molar ratios of pollutants in emissions and deposition, it appears that the atmospheric processes in eastern North America lead to a thorough mixing of pollutants, making it difficult to distinguish between effects of distant and local sources (see Chapter 4)
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From page 10... ...
On the basis of analysis of the spatial distributions of the molar ratios of pollutants in emissions and deposition and assuming that all other emissions and conditions remain unchanged, we would expect that if the molar ratio in emissions in eastern North America were changed by changing SO2 emissions, a similar change would occur in the ratio of sulfate to nitrate in wet deposition. If, as described in Chapter 4, dry deposition is linearly proportional to emissions, then the average annual ratio in total deposition in the region should also respond to changes in the emission ratio.
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From page 11... ...
Although the results of such field studies may not yield complete detailed descriptions of the interactions of all the processes involved, the studies are likely to provide basic phenomenological evidence with sufficient reliability to form a basis for improving the near-term strategy for dealing with the problem of acid deposition in eastern North America. Indeed, the data are essential to enhance theoretical understanding and to develop improved deposition models.
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