Acid Rain Program

Atmospheric Deposition

Atmospheric deposition is composed of both wet and dry deposition. It includes acidic deposition, deposition of toxic compounds and toxic trace elements, and the deposition of non-toxic compounds including nutrients. Atmospheric deposition occurs throughout the year, although weather patterns alter the seasonal and annual amount and type of deposition.

Of all the components of atmospheric deposition, acid rain is probably the most commonly known in New England. In this section most of the discussion will focus on acid rain, or acidic precipitation, which includes acid rain and snow, acidic fog and ice. In addition to the discussion on acid rain, in the future, there will be brief reports on dry deposition in New England and deposition of toxic trace elements and toxic compound deposition (in particular mercury).

Acid Precipitation

There are ten (10) acid precipitation monitoring stations in New England. These sites have measured acid rain and acid snow on a weekly basis (Tuesday to Tuesday) since the early 1980's. The longest continuous acid precipitation monitoring record in the United States is from a site located in New Hampshire, the Hubbard Brook site, which was established in 1978. This site and the other nine New England sites are a part of a national network of 200+ sites that compose the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). The NADP was established in 1978 to provide a national network of sites to monitor trends in the chemistry of wet and dry deposition throughout the United States. The NADP merged with the National Trends Network in the early 1980's to form the NADP/NTN. The current national network is operated and maintained by a consortium of federal and state agencies, private industries, and universities.

The New England map, presented in Figure 1, depicts the approximate location of the 10 NADP/NTN sites in the region. The sites are equipped with al Belford 5-780 Universal Rain Gage, and an Aerochem Metrics Wet/Dry Precipitation Collector. Most sites are located well inland, with the exception of the Waltham/Boston, the Cape Cod National Seashore, and the Acadia National Park monitoring sites, which are located near the coast. Seasonal and annual differences in weather patterns influence precipitation and chemical deposition patterns at the inland and coastal sites. Differences in the annual and seasonal amount of precipitation measured at the seven New England sites, over the past nineteen years (1979-1997), are presented in Figures 2a and 2b.

Figure 1 - 10 National Acid Precipitation Monitor Sites in New England*

Map of 10 National Acid Precipitation Monitor Sites in New England *National Atmospheric Deposition Program/National Trends Network

The data presented in Figures 2a and 2b indicate that the amount of precipitation and seasonal pattern of precipitation changes over time and by site. These patterns influence the concentration and deposition of pollutants in precipitation. Inland sites generally record the most precipitation during the summer (and fall), and the least precipitation during the winter. Coastal sites (Cape Cod National Seashore, Acadia National Park, and Waltham/Boston), on the other hand, generally record more precipitation during the spring. The is clearly evident during the late 1980's and early 1990's.

Figure 2a - Changes in the amount of precipitation (cm) recorded during the spring at three coastal NADP/NTN sites (bold line) and four inland sites in New England (1979-1997)

Graph of changes in the amount of precipitation recorded during the spring at three coastal NADP/NTN sites and four inland sites in New England (1979-1997)

Figure 2b - Changes in the amount of precipitation (cm) recorded during the summer at three coastal NADP/NTN sites (bold line) and four inland sites in New England (1979-1997)

Graph of changes in the amount of precipitation recorded during the summer at three coastal NADP/NTN sites and four inland sites in New England (1979-1997)

Over the past 19 years, the precipitation data collected at the New England NADP sites suggest to multi-year intervals with enhanced precipitation. Two-year (paired) averaged data are presented in Figure 3. These data show enhanced precipitation in New England during the early 1980's and again during the early 1990's. Less precipitation fell during the period 1985-1988.

Figure 3 - Average amount of precipitation (cm) recorded for paired years from ten (10) NADP/NTN monitoring sites in New England (1979-1997)

Graph of average amount of precipitation recorded for paired years from ten NADP/NTN monitoring sites in New England (1979-1997)

Precipitation Chemistry

Almost all forms of precipitation are efficient scavengers of pollution in the atmosphere. Hence, the chemistry of precipitation reflects changes in the level of air pollutants in the atmosphere and can be used to provide evidence of trends in air pollution. The NADP/NTN sites in New England provide a unique view of changes in concentration of pollutants in precipitation that falls on New England, in particular, acid rain.

Precipitation has complex chemistry. Both beneficial and harmful pollutants are present. Acid precipitation illustrates this. In general, the dilute chemical constituents of acid precipitation are composed of beneficial plant and soil nutrients (calcium, magnesium, potassium, nitrogen) and harmful acids, principally sulfuric and nitric acids. Nitrates play several roles; they contribute to the acidity of the precipitation and to freshwater acidification, and they can be a plant nutrient. As a plant nutrient they are important to New England forests, but enhance the eutrophication of some New England coastal marine environments, including Long Island Sound and Casco Bay (These are New England ecosystems where scientific research is focusing on the potential environmental effects for nitrogen and enhanced eutrophication).

The following anions and cations are measured from precipitation samples set to the NADP/NTN central analytical laboratory:

anions cations
Sulfate (SO4 Hydrogen (H)
Nitrate (NO3 Calcium (Ca)
Orthophosphate (PO4 Magnesium (Mg)
Chloride (Cl) Potassium (K)
Sodium (Na)
Ammonium (NH4

In addition to these anions and cations, the conductance of each sample (a secondary measure of ionic strength) is also measured. Precipitation samples from all of the NADP/NTN collectors are shipped every Tuesday to the Central Analytical Laboratory (CAL) at the Illinois State Water Survey for analysis.

Spatial and Temporal Chemistry of Precipitation

Both concentration and deposition of beneficial and harmful constituents of precipitation are environmentally important. In this section, the yearly deposition patterns (Kg/ha) of key chemical components of precipitation are presented for the 1979-1997 period. These chemical components include sulfate, nitrate, chloride, and potassium ions.

Figure 4 presents the average composite yearly deposition of sulfates, nitrates, and chlorides for the ten NADP/NTN sites in New England. Nitrate deposition, in New England, shows no significant long-term increase or decrease in deposition over the period of record. The decrease in 1987-1988 may be attributed to lower precipitation. Sulfate deposition, on the other hand, significantly drops during the early 1980's and again during the 1990's. The decreased deposition during the 1987-1988 period is attributed to reduced precipitation. However, the long-term trend, and the most recent declines in sulfate deposition can only partially be attributed to changes annual precipitation.

Figure 4 - Average composite sulfate and nitrate (bold line) deposition (Kg/ha) for ten New England NADP/NTN precipitation monitoring sites (1979-1997)

Graph of average composite sulfate and nitrate deposition for ten New England NADP/NTN precipitation monitoring sites (1979-1997)

The final series of figures provide sub-regional views of changes in precipitation chemistry over the past 19 years. Coastal and inland sites show different patterns of deposition, over the years. Chloride and potassium enrichment is evident at the coastal sites, indicative of precipitation that has been influenced by sea spray. This enrichment is especially evident during 1983 and 1984 (Figure 5a and 5b). Inland sites, on the other hand, do not show similar patterns of chloride or potassium enrichment. Sulfate, which is also a major constituent of seawater, is also enriched at coastal sites, as noted at the Acadia NP site in Maine and the Cape Cod National Seashore site (Figure 6a). The Acadia site also records generally higher deposition of nitrates than the other Maine sites (Figure 7a). This is unexpected, since this site is located far from major point and mobile emission sources of nitrogen oxides in New England, and ocean spray does not contain significant concentrations of nitrate.

Figure 5a - Wet deposition (Kg/ha) of potassium measured at two coastal precipitation monitoring sites (bold lines) and four representative inland precipitation monitoring sites in New England (1979-1997)

Graph of wet deposition of potassium measured at two coastal precipitation monitoring sites and four representative inland precipitation monitoring sites in New England (1979-1997)

Figure 5b - Wet deposition (Kg/ha) of chloride measured at two coastal precipitation monitoring sites (bold lines) and four representative inland precipitation monitoring sites in New England (1979-1997)

Graph of wet deposition of chloride measured at two coastal precipitation monitoring sites and four representative inland precipitation monitoring sites in New England (1979-1997)

The final set of figures indicate a south to north and west to east gradient of sulfate and nitrate deposition (except in those instances where a sea spray enhances sulfate deposition). As expected, the highest deposition of sulfate generally occurs west of Maine (Figures 6a-6c). The highest nitrate deposition occurs at the New Hampshire, Vermont and Quabbin Reservoir-Massachusetts monitoring sites (Figures 7b and 7c).

Figure 6a - Wet deposition (Kg/ha) of sulfate measured at the Acadia NP (bold line), Greenville (dotted line), Caribou (solid line), and Bridgton (dashed line) NADP/NTN precipitation monitoring sites in Maine (1979-1997)

Graph of wet deposition of sulfate measured at the Acadia NP, Greenville, Caribou, and Bridgton NADP/NTN precipitation monitoring sites in Maine (1979-1997)

Figure 6b - Wet deposition (Kg/ha) of sulfate measured at the Hubbard Brook-NH (bold line), Underhill-VT (dotted line), and Bennington-VT (solid line) NADP/NTN precipitation monitoring sites in Northern New England (1979-1997)

Graph of wet deposition of sulfate measured at the Hubbard Brook-NH, Underhill-VT, and Bennington-VT NADP/NTN precipitation monitoring sites in Northern New England (1979-1997)

Figure 6c - Wet deposition (Kg/ha) of sulfate measured at the Truro-CCNS (bold line), Waltham (dotted line), and Quabbin Reservoir (solid line) NADP/NTN precipitation monitoring sites in Massachusetts (1979-1997)

Graph of wet deposition of sulfate measured at the Truro-CCNS, Waltham, and Quabbin Reservior NADP/NTN precipitation monitoring sites in Massachusetts (1979-1997)

Figure 7a - Wet deposition (Kg/ha) of nitrate measured at the Acadia NP (bold line), Greenville (dotted line), Caribou (solid line), and Bridgton (dashed line) NADP/NTN precipitation monitoring sites in Maine (1979-1997)

Graph of wet deposition of nitrate measured at the Acadia NP, Greenville, Caribou, and Bridgton NADP/NTN precipitation monitoring sites in Maine (1979-1997)

Figure 7b - Wet deposition (Kg/ha) of nitrate measured at the Hubbard Brook-NH (bold line), Underhill-VT (dotted line), and Bennington-VT (solid line) NADP/NTN precipitation monitoring sites in Northern New England (1979-1997)

Graph of wet deposition of nitrate measured at the Hubbard Brook-NH, Underhill-VT, and Bennington-VT NADP/NTN precipitation monitoring sites in Northern New England (1979-1997)

Figure 7c - Wet deposition (Kg/ha) of nitrate measured at the Truro-CCNS (bold line), Waltham (dotted line), and Quabbin Reservoir (solid line) NADP/NTN precipitation monitoring sites in Massachusetts (1979-1997)

Graph of wet deposition of nitrate measured at the Truro-CCNS, Waltham, and Quabbin Reservior NADP/NTN precipitation monitoring sites in Massachusetts (1979-1997)

The most recent data for 1993 through 1997 show no marked systematic trend of pollutants in precipitation for the New England sites. Sulfate deposition in 1995 showed a marked decline at every monitoring site except Acadia NP in Maine. The average 1995 sulfate deposition in New England was the lowest ever recorded during the 17 years of sulfate deposition monitoring. The 1996 data indicate that sulfate deposition increased in eight of the ten sites. Nonetheless, sulfate levels in 1996 were ~16% below historical averages (1979-1995). Similar increases occur for nitrate at most sites, and appear to be enhanced in 1997. Nitrate deposition during 1996 and 1997 were virtually the same (~5+%) as the historical average (1979-1995). This trend for nitrate and to some extent sulfate continued in 1997, suggesting the possibility that the deposition pattern for New England 1995 data may be more related to year-to-year changes in atmospheric chemistry and weather conditions than other deposition modifiers, such as emission reductions.

The year-to-year variation that is recorded at the NADP sites can be attributed to differences in precipitation and prevailing wind patterns, as well as emission sources. This is characteristic of precipitation data and should be viewed as normal. Trends in pollutant deposition within New England, whether due to long-term changes in pollutants emitted to the atmosphere or trends in climate, will be evident only after many years (10-20+ years).

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Steven McDougall / swmcd@theworld.com / 1998 September 27