What is acid rain?
The term "acid rain"
is used to describe rain, mist or snow that is unusually acidic. A pH
value is the measure of acidic or alkaline material. The lower the pH,
the higher the acid reading. Rain and snow are naturally slightly acidic
due to naturally occurring chemical reactions in the atmosphere. Compared
to normal rainwater with a pH readings of 5.6, the Eastern U.S. suffers
from some of the most severe acid rain, with levels typically reading
at 4.4, though some locations in the west also face severe impacts.
The burning of fossil
fuels generates air pollution that scientists have determined is the major
cause of acid rain. Power plants, along with factories and vehicles that
also burn fossil fuels, all emit sulfur dioxide (SO2) and oxides of nitrogen
(NOx). When combined with moisture in the atmosphere, these pollutants
are returned to the earth as acids. This process is known as "deposition"
and occurs when it rains or snows, but it can also occur when dust settles
out of the atmosphere during dry periods.
Acid precursors can
be carried in the atmosphere for several days and travel several hundred
miles downwind of the power plant stack before being deposited on the
earth's surface. Because of prevailing winds, the northeastern United
States and Canada receive significant quantities of acid precursors from
coal-fired power plants in states stretching from Missouri to the west
and Pennsylvania to the east.
What are the consequences of acid rain?
Acid rain is linked
to a range of negative impacts on the natural world as well as human environments:
believe that acid rain is responsible for the dramatic disappearance of
brook trout and other fish species from pristine lakes and streams. These
treasured water bodies receive acid directly from the atmosphere and from
runoff from the surrounding watershed. Of the lakes and streams studied
in a National Surface Water Survey conducted by the US Environmental Protection
Agency, acid rain was determined to cause acidity in 75 percent of the
acidic lakes and 50 percent of the acidic streams analyzed. Some lakes
are particularly susceptible to acid rain since the underlying soil has
limited ability to neutralize, or "buffer," the acids. Lakes suffering
from chronic acidity can be found in several regions of the United States
and Canada, including the Adirondacks, the mid-Appalachian highlands,
the upper Midwest and the high elevation West.
vary in their tolerance to elevated levels of acidity. The acid interferes
with reproduction much sooner in some especially sensitive species than
with others. Generally speaking, acid rain fosters a shift in fish population
from acid-sensitive to acid-tolerant fish and other aquatic plant and
Acid rain may render intense impacts on the health of forest ecosystems.
According to the National Assessment Precipitation Assessment Program's
1998 Biennial Report to Congress, the current mortality and decline of
high elevation red spruce populations in the Northeast, and decline in
growth rates for Appalachian red spruce, "are the only cases of significant
forest damage for which there is strong scientific evidence that acid
deposition is a primary cause." Nonetheless, several recent studies conducted
by the United States Geological Survey and others point to acid rain as
contributing to long-range damage to forests by depleting calcium, a nutrient
vital to plant growth.
Acid rain affects many types of materials, from objects of particular
historical artistic or cultural value -- buildings and monuments -- to
more ordinary objects such as cars and trucks. Acid rain, especially in
the "dry" form, corrodes metal, and accelerates the deterioration of stone
Sulfur dioxide emissions reduce visibility when they form sulfate particles
in the atmosphere. Visibility reductions are most pronounced in the eastern
part of the United States, particularly in and around national parks.
How does electricity production contribute to acid rain? Electricity generation
accounts for the lion's share of air pollutants that spawn acid rain.
Every year, the nation's fossil fuel power plants spew roughly 70 percent
of SO2 emissions and 30 percent of NOx emissions that are critical ingredients
in making acid rain.
Of course, not all
power plants generate the same level of air pollutants contributing to
acid rain. Emissions rates vary widely depending upon factors as the precise
fossil fuel type used, the nature of the combustion process, pre- and
post-combustion air emission controls, as well as vintage of the power
plant. Older coal plants exempt from modern clean air standards under
"grandfathering" provisions of the Clean Air Act (especially those designed
to burn high sulfur content coal) are at one extreme and are the most
significant source of acid rain pollutants. These power plants are highly
concentrated in the Ohio Valley and Midwest. Given the prevailing winds,
these older, largely uncontrolled pollution sources exacerbate the acid
rain experienced in the Northeast.
On the other end
of the spectrum are new natural gas-fired generation fitted with best
available control technology. They release a fraction of the SO2 produced
by coal-fired power plants. However, the performance of natural gas plants
is decidedly more mixed in the area of NOx emissions, the other major
precursor of acid rain. Although possible to mitigate NOx emissions using
advanced technologies, many gas-fired power plants now in service use
older, more polluting technologies.
How can consumer electricity choice address acid rain?
Competition in the
electricity industry offers consumers for the first time the opportunity
to directly influence the environmental footprint of electric power production.
In several states, suppliers are assembling electricity resource portfolios
that are significantly cleaner than the status quo. By selecting one of
these resource portfolios, which boost the amount of renewable energy
sources in the fuel mix, consumers can help ensure that the emissions
of pollutants that cause acid rain are reduced. Consumers can send a powerful
signal to electricity suppliers that they demand their supply not include
power from older coal power plants exempt from the nation's federal air
quality standards. These dirty power plants have increased their power
production recently in response to wholesale competition. Between 1995
and 1995 [typo in the draft REPP report - need to find actual date - I
assume the date should be 1996], a single Midwestern utility increased
coal-fired generation by 10 percent, which increased its share of NOx
emissions by over 50,000 tons. That increase in NOx emissions from a single
utility surpasses the total NOx emissions from all fossil power plants
operating in Massachusetts and New Hampshire combined.