12 ACID RAIN
Midwest and Northeast (or other regions) cannot be pre-
cisely quantified on the basis of the historical precipitation
chemistry data.
The longest continuous precipitation chemistry record
is for the Hubbard Brook site in New Hampshire, where the
record began in 1963 (Likens et al. , 1984). The sampling
method was to continuously expose a funnel and bottle,
i.e. bulk sampling. From 1964 to 1982 sulfate decreased
quite regularly, which seems to be consistent with the trend
of combustion sulfur emissions for this area of the coun-
try. Values for pH did not show a significant change. The
National Research Council (1986) tabulated the published
trends for the Hubbard Brook data set to indicate that the
results are sometimes sensitive to the specific type of anal-
ysis. For example, one publication indicated that nitrate
increased from 1964 to 1971, and then remained steady
through 1980. A second publication included the nitrate data
for 1963 to 1983, and found no significant overall trend.
A third publication, including data for 1964 to 1979, found
a significant overall increase in nitrate. Bulk data should
not generally be compared with wet-only data, however,
comparisons have shown that the dry deposition component
is relatively small for the Hubbard Brook site and thus it
appears valid to suggest that the bulk trends are probably
representative of wet-only trends.
The NADP/NTN weekly wet deposition data provides
the best data set for trend analysis because of the compre-
hensive quality assurance program for the network and
because of the good spatial coverage across the 48 states.
Lynch et al. (1995) reported the most recent comprehensive
summary of temporal trends in precipitation chemistry inthe United States using data from 58 NADP/NTN sites from
1980 through 1992. Results showed widespread declines in
sulfate concentrations accompanied by significant decreases
in all of the base cations, most noticeably calcium and mag-
nesium. As a result of the decreases in both acids and bases,
only 17 of the 42 sites with significantly decreasing sulfate
trends had concurrent significant decreasing trends in hydro-
gen ion (acidity). The decline in precipitation sulfate during
this period is consistent with the known declines in sulfur
dioxide emissions from electric power plants. The decline
in base cations does not yet have a definitive explanation
since the strengths of the various emission sources are not
well known.
Phase I of Title IV of the 1990 Clean Air Act
Amendments required specific reductions in sulfur diox-
ide emissions on or before 1 January 1995 at selected
electric utility plants, the majority of which are located
in states east of the Mississippi River. As a result of this
legislation, large reductions in sulfur dioxide emissions
were likely to have occurred in 1995, which should have
affected sulfate and hydrogen ion concentrations in pre-
cipitation in this region. Lynch et al. (1996) compared
the 1995 concentrations to those expected from the 1983–
1994 trends and indeed found that sulfate and hydrogen
ion decreased much more than expected due to just the
1983–1994 trends. Thus they concluded that acid rain in
the eastern United States had decreased as a result of the
Phase I emission reductions. Additional major emission
reductions in sulfur dioxide are required in Phase II by the
year 2000 so it will be important to look for corresponding
additional reductions in acid rain.TABLE 3
Seasonality of Ion Concentrations in Precipitation as Shown By Average Ratio Values (Warm Period/Cold Period
Precipitation Concentrations) for Four Regions of the United States**********Mean 2 Std. Dev. of Period Ratios**********Regiona Nb SO 42 NO 3 NH 4 Ca^2 HMW 20 1.35 0.64 1.00 0.47 1.67 1.45 1.63 1.02 1.03 0.88
SE 15 1.52 0.60 1.73 0.92 1.87 0.92 1.57 0.62 1.52 0.87NE (^23) 2.19 0.80 1.36 0.88 2.45 1.48 1.44 0.72 1.89 0.64
RM 16 2.15 1.11 2.63 2.87 2.65 1.54 2.39 1.30 2.58 2.37
**Mean 2 Std. Dev. of Period Ratios**
Regiona N Mg^2 K Na Cl
MW 20 1.40 0.67 1.55 0.68 0.79 0.58 0.92 1.21
SE 15 1.23 0.69 1.53 0.54 0.95 0.73 0.87 0.51
NE (^23) 1.17 0.65 1.43 0.67 0.67 0.53 0.64 0.36
RM 16 1.82 0.90 2.67 1.58 1.30 0.84 1.51 1.05
a MW is Midwest, SE is Southeast, NE is Northeast, and RM is Rocky Mountain.
b N is the number of sites in the region used in the analysis. States bordering the Pacific Ocean and states in the
Great Plains were not included in this analysis.
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