Acidic Deposition in the Northeastern United States: Sources and Inputs, Ecosystem Effects, and Management Strategies
Acidic deposition is the transfer of strong acids and acid-forming substances from the atmosphere to the surface of the Earth. The composition of acidic deposition includes ions, gases, and particles derived from the following: gaseous emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3), and particulate emissions of acidifying and neutralizing compounds. Over the past quarter century of study, acidic deposition has emerged as a critical environmental stress that affects forested landscapes and aquatic ecosystems in North America, Europe, and Asia. This complex problem is an example of a new class of environmental issues that are multiregional in scale and are not amenable to simple resolution by policymakers. Acidic deposition can originate from transboundary air pollution and can affect large geographic areas. It is highly variable across space and time, links air pollution to diverse terrestrial and aquatic ecosystems, and alters the interactions of many elements (e.g., sulfur [S], nitrogen [N], hydrogen ion [H+], calcium [Ca2+], magnesium [Mg2+], and aluminum [Al]). It also contributes directly and indirectly to biological stress and to the degradation of ecosystems. Despite the complexity of the effects of acidic deposition, North American and European management actions directed toward the recovery of damaged natural resources have resulted in recent decreases in both emissions and deposition of acidic S compounds. Accordingly, acidic deposition presents an instructive case study for coordinating science and policy efforts aimed at resolving large-scale environmental problems. Acidic deposition was first identified by R. A. Smith in England in the 19th century (Smith 1872). Acidic deposition emerged as an ecological issue in the late 1960s and early 1970s with reports of acidic precipitation and surface water acidification both within Sweden and around Scandinavia (Oden 1968). The first report of acidic precipitation in North America was made at the Hubbard Brook Experimental Forest (HBEF) in the remote White Mountains of New Hampshire, based on collections begun in the early 1960s (Likens et al. 1972). Controls on SO2 emissions in the United States were first implemented after passage of the 1970 amendments to the Clean Air Act (CAAA). In 1990, Congress passed Title IV of the Acid Deposition Control Program of the CAAA to further decrease emissions of SO2 and to initiate controls on NOx from electric utilities, which contribute to acidic deposition. The Acid Deposition Control Program had two goals: (1) By 2010, a 50% decrement from 1980 levels of SO2 utility emissions (amounting to 9.1 million metric tons per year, or 10 million short tons); (2) also by 2010, an NOx emission rate limitation (0.65 lbs NOx/m BTU in 1990 to 0.39 lbs NOx/m BTU in 1996), which will achieve a reduction of 1.8 million metric tons per year (2 million short tons) as NO2) in NOx utility emissions from the amount that would have occurred without emission rate controls. Both SO2 and NOx provisions focus on large utilities. The legislation capped total utility emissions of SO2 at 8.12 million metric tons per year (8.95 million short tons), whereas nonutility emissions of SO2 were capped at 5.08 million metric tons per year (5.6 million short tons). Because the legislation did not specify caps for NOx emissions, emissions may increase over time as the demand for electricity increases. The beginning of the 2lst century offers the opportunity to assess the effects of three decades of clean air legislation on emissions reductions; on air pollution levels, trends, and chemical effects of acidic deposition; and on ecosystem recovery. The opportunity also exists to look ahead to the anticipated reauthorization of the Clean Air Act and to the next national assessment of acidic deposition, scheduled for mid-2001. In this article we examine the ecological effects of acidic deposition in the study region of New England and New York and we the emissions and ecosystem by on three critical This on in the United with from the and United States and from on from a that the of precipitation and (Likens and Because the is in a region with that is to chemical and acidic surface are of of the that are to acidic deposition. we from the at with from the of in the which had the and Program of the had of the of et al. In had of and had to of and of the have of the of of and of in the water at of from and is critical for and for complex environmental problems. The to the problem of acidic deposition is made as the many acidic deposition and environmental issues are of the report that on has the 1990 the effects of acidic deposition on and surface and the of past of emissions on ecosystem recovery in the United are the and for emissions, precipitation and deposition of and across the United The United States has in emissions of SO2 over the past emissions of SO2 from million metric tons million short tons) in to a of million metric tons million short tons) in of which was from electric By total SO2 emissions for the United States had to million metric tons million short tons). 1970 to SO2 emissions from electric by as a of the 1970 and 1990 amendments to the Clean Air of NOx have from million metric tons million short tons) in to million metric tons million short tons) in have of SO2 in the United States are in the States around the and of the 10 with the SO2 emissions in the in for of the national SO2 emissions this of and were also the 10 with the total NOx emissions for and of national emissions emissions in this region from electric and from The 1990 CAAA in the emissions of SO2 from electric utilities, in with of the Acid Deposition Control This legislation to the of emissions and for in the United States the United SO2 emissions the whereas emissions by in the in the in emissions of NOx were and for the in the deposition of is derived from emissions of and can contribute to the acidification of and water are by to The has a national emissions for is on past and have identified as the of emissions of and from and is the from may as large are to et al. of also contributes to the the amount from this is of emissions from in the et al. of emissions and and Acidic deposition can as deposition; as or as deposition; as particles or or as or deposition, which is at and in areas. deposition is at by the Deposition which was in The study region has In precipitation has at (e.g., for an of deposition in the of the United States have by deposition with on and precipitation and deposition is by the Clean Air and at as as by the and at in two are of the and around and are in the study Both and an to deposition. This on and on which can over short in complex et al. the of deposition in the United States are (e.g., et al. et al. and in the United States have for at and In recent the Acid Deposition Program as of the has the of water at in the United in the United and are not and contribute and 50% of the total deposition of S and to in the United States et al. from to in the to in New England and of the SO2 and NOx will to and to and which can and to which has a time at that of in precipitation from an has with the national decreases in SO2 emissions that the 1970 CAAA (Likens et al. of air and et al. identified the emissions region for deposition of S and study region in the United States of in precipitation at the were with SO2 emissions, based on both and (Likens et al. from and to have to the deposition in the study region the et al. In to for in of in precipitation at This of a is with the in NOx emissions over the past The of national clean air legislation is also in the strong the in air emissions from the region and the deposition of S the United SO2 emissions the and in to the a decrease in the geographic to deposition of S in of In after the of of the Acid Deposition Control emissions of SO2 in the and of in (1) deposition at and (2) deposition at in the were in the decreases were not from the (Likens et al. and the in precipitation across the region the and the of in S and deposition the of and the emissions and deposition et al. et al. The is also a of of the to the atmosphere is within of of SO2 and NOx can of the of and particles are is an of deposition. an of of in deposition at as deposition contributes a amount of S and to the are to as 10 across the United deposition of S was of total deposition deposition of was of total deposition, and deposition of was of total deposition. This is in to the of the to emission and to the of and are the effects of acidic deposition on terrestrial and aquatic ecosystems in the United and have ecosystems to in emissions and of the effects of acidic deposition on the rate at which acidifying are from the with the rate at which neutralizing is within the a of the of water or to of strong is the of terrestrial as and of and Acid neutralizing in the and are with the of water terrestrial and aquatic ecosystems. 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This to made the of in the with surface both within New York and around New at or with for have (e.g., et al. 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In acidic with we that in of strong will in in the of surface (Likens et al. et al. and in the as the will recovery of may in in which deposition has of S and that will or of In that chemical recovery will with controls on recovery can at (e.g., and of by (e.g., et al. The in ecosystem recovery is biological which can chemical recovery is to and of and The time for biological recovery is the and time for recovery of terrestrial ecosystems after decreases in acidic deposition, is to at decades after is of the of and the complex interactions of and that of may in to chemical whereas of are to 10 and may in to 10 after the recovery of of by is that aquatic ecosystems to the that in the chemical is to recovery of the ecosystem that and The rate and of ecosystem recovery is to the and of emissions SO2 and NOx emissions with the 1990 CAAA and with aimed at utility emissions which were in the of this to to a of acidic deposition and ecosystem effects at the The for in utility emissions of SO2 and the emissions for of the as by the and we levels of on for emission levels after of the 1990 CAAA the would utility SO2 emissions by and would decrease utility NOx emissions of the Acid Deposition Control Program of the 1990 and and would CAAA whereas would emissions of the we the is a of for utility emissions of SO2 and includes NOx from to to achieve a of the the and that to to the and the of which is to the and the of The of the the to with a of or to electric utility and a of or from the as to the we ecosystem at the to a of emissions for three S deposition without of the 1990 CAAA S deposition after of the 1990 CAAA S deposition after the 1990 with and in utility SO2 emissions in The and the of emissions in the would amount to and of the total emissions of that decreases in SO2 emissions will in and total S deposition in by et al. this SO2 emissions and deposition. Because the SO2 emissions have a on S deposition in the we on S controls. did not decreases in or deposition. Controls on emissions also the effects of acidic deposition. in surface deposition in the region within the the of with of will increase from to Because contribute of total NOx emissions is that in utility NOx emissions will to the or of ecosystems in the not deposition that from or NOx emissions, which are both of to the that in S deposition will in and surface water at the The that the Acid Deposition Control Program will in in water at the for the will decrease by will increase by and will increase controls on SO2 emissions, as in in and water The for that a reduction in utility emissions of SO2 from levels anticipated the 1990 CAAA would a decrease in S deposition in 2010, which would decrease by by In a decrease in S deposition decrease in utility SO2 would decrease by by Despite in S deposition over the past (Likens et al. water at the Because of the of of from S deposition the past the recovery of water after decreases in strong has were controls the 1990 CAAA the rate of increase by the for is of and in S deposition in increase the rate of to and The reduction in S deposition in would in in a decrease in S deposition, would by the that a reduction in S deposition of the 1990 CAAA would in over and in of at by the chemical and biological recovery not by with of the for utility that the the reduction in S deposition at the the the and rate of chemical recovery. for controls on S emissions will in chemical and biological recovery and in in the of a not for the that of the total and of the are acidic of the that recovery of surface would the for the that that and and will in North America and are in the of a large-scale and acids have and or terrestrial and aquatic It is critical to and to the recovery of complex ecosystems in to decreases in acidic deposition. the and are in to anticipated in air and in deposition. to in water are and to and are is an to assess the of resources to decreases in It would to assess the recovery of aquatic directly to in surface water and from the and across the United States to