to both acid rain and acid snow. This article discusses the
physical and chemical aspects of the acid rain phenomenon,
presents results from a U.S. monitoring network to illustrate
spatial and seasonal variability, and discusses time trends
of acid rain during recent decades. A chemical equilibrium
model is presented to emphasize that one cannot measure only
pH and then expect to understand why a particular rain or
melted snow sample is acidic or basic.
Monitoring networks are now in operation to characterize the time trends and spatial
patterns of acid rain. Definitions, procedures, and results from
such measurement programs are discussed. The monitoring
results are necessary to assess the effects of acid rain on the
environment, a topic only briefly discussed in this article.
Chemicals in the form of gases, liquids, and solids are
continuously deposited from the air to the plants, soils,
lakes, oceans, and manmade materials on the earth’s surface.
Water (H 2 O) is the chemical compound deposited on
the earth’s surface in the greatest amount.
The major atmospheric removal process for water consists of these steps:
(1) air that contains water vapor rises, cools, and condenses
to produce liquid droplets, i.e., a visible clou
(2) in some clouds the water droplets are converted to the solid phase,
ice particles;
(3) within some clouds the tiny liquid droplets
and ice particles are brought together to form particles that
are heavy enough to fall out of the clouds as rain, snow, or
a liquid–solid combination.
When these particles reach the ground, a precipitation event has occurred.
As water vapor enters the base of clouds in an air updraft in step (1) above,
other solid, liquid, and gaseous chemicals are also entering the clouds. The chemicals that become incorporated into the cloud water (liquid or ice) are said to have been removed by in-cloud scavenging processes often called rainout. The
chemicals that are incorporated into the falling water (liquid or ice) below the cloud are said to be removed by belowcloud scavenging, often called washout.
Carbon dioxide gas, at the levels present in the atmosphere,
dissolves in pure water to produce a carbonic acid solution with a pH of about 5.6. Therefore, this value is usually considered to be the neutral or baseline value for rain and snow. Measurements show that there are always additional
chemicals in rain and snow. If a salt (sodium chloride) particle
in the air is scavenged (captured) by a raindrop or snow
flake, it does not alter the acidity. If an acid particle, such as
one composed of sulfuric acid, is scavenged, then the rain
or snow becomes more acid. If a basic particle, such as a
dust particle composed of calcium carbonate, is scavenged
then the rain or snow becomes more basic. It is important that
both pH as well as the major chemicals that alter the pH of
rain and snow be included in routine measurement programs.
The adverse or beneficial effects of acid rain are not related
only to the hydrogen ion concentration (a measure of acidity
level), but also to the other chemicals present.
In following the cycle of chemicals through the atmosphere
one considers (1) the natural and manmade sources
emitting chemicals to the atmosphere, (2) the transport and
transformation of the chemicals in the atmosphere, and
(3) the removal of the chemicals from the atmosphere.
Therefore, when one regularly measures (monitors) the
quantity of chemicals removed from the atmosphere, indirect
information is obtained about the removal rates and
processes, the transport/transformation rates and processes,
and the source characteristics
source :
FIFTH EDITION VOLUME 1 A-L E N C Y C L O P E D I A O F ENVIRONMENTAL SCIENCE and ENGINEERING (EDITED BY
JAMES R. PFAFFLIN
EDWARD N. ZIEGLER)
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