Encyclopedia of Environmental Science and Engineering, Volume I and II

10 ACID RAIN

less than 80 percent of the four-year interval (98 percent

or 176 of the 180 selected sites operated for more than 95

percent of the interval). Second, sites with a low number of

valid weekly samples were excluded. That is, if at least two

hundredths of an inch of liquid precipitation would have

fallen every week and if valid chemical measurements were

obtained for each weekly sample, then 205 samples would

have been available. In fact for the semi-arid western states,

a large fraction of the weekly samples are completely dry.

A decision was made to include in the analysis only those

western sites with at least 100 valid samples and those east-

ern sites with at least 129 valid samples. For the 180 sites

meeting all of the selection criteria, the median number of

valid samples was 152.

Shown in Figures 2–6 are lines (isopleths) of median

ion concentration or median pH. The isopleths are computer

generated and include some automatic smoothing, but are

very similar to hand-drawn contours. The concentrations are

for the ion, i.e., for sulfate it is milligrams per liter of sulfate,

not sulfur.

Sulfate concentrations in precipitation, shown in

Figure 2, are highest in the Northeast with values exceed-

ing 2.5 mg/L at sites in eastern Illinois, Indiana, Ohio, and

western Pennsylvania. This is consistent with known high

emissions to the atmosphere of sulfur from coal burning

electrical power plants in this region. The sulfate levels

decrease to the west of this area, with West Coast values

being less than 0.5 mg/L.

The major anthropogenic sources for the nitrogen pre-

cursors which become nitrate in precipitation are high tem-

perature combustion sources, which includes power plants

and automobiles. The known locations for these sources are

consistent with the observed nitrate concentrations in pre-

cipitation shown in Figure 3. Nitrate concentrations are high

in the Northeast, from Illinois to New York. The high values

of nitrate in southern California are reasonable considering

the high density of people and automobiles in this area. The

lack of high sulfate values in this California area reflects the

lack of intensive coal combustion in the area.

Figure 4 shows the concentrations of calcium in pre-

cipitation. With respect to sources of the calcium, Gillette

et al. (1989) have indicated that dust from soils and dust

from traffic on unpaved roads are the major sources of

calcium in the atmosphere. Dust devils in the southwest-

ern states, wind erosion of agricultural fields, and crop

5.70

5.70

6.00

0

50

FIGURE 6 Median pH in precipitation for 180 NADP/NTN sites

for the period 1990–1993.

0.15

0.30

0.30

0.60

0.15

FIGURE 5 Median concentration (mg/L) of ammonium in pre-

cipitation for 180 NADP/NTN sites for the period 1990–1993.

0.75

0.751.00

1.00

0.25 1.50

1.25 1.75

3.25

FIGURE 3 Median concentration (mg/L) of nitrate in precipita-

tion for 180 NADP/NTN sites for the period 1990–1993.

0.15

0.250.35

0.15

0.250.35

0.250.15

FIGURE 4 Median concentration (mg/L) of calcium in precipita-

tion for 180 NADP/NTN sites for the period 1990–1993.

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