Encyclopedia of Environmental Science and Engineering, Volume I and II

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ACID RAIN

OVERVIEW OF THE PROBLEM

Acid rain is the general and now popular term that pertains

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 sur-

face. Water (H 2 O) is the chemical compound deposited on

the earth’s surface in the greatest amount. The major atmo-

spheric 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 cloud; (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 below-

cloud scavenging, often called washout.

Carbon dioxide gas, at the levels present in the atmo-

sphere, 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) par-

ticle 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 atmo-

sphere 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, indi-

rect information is obtained about the removal rates and

processes, the transport/transformation rates and processes,

and the source characteristics.

A great number of projects have been carried out to

measure various chemicals in precipitation. For example,

Gorham (1958) reported that hydrochloric acid should be

considered in assessing the causes of rain acidity in urban

areas. Junge (1963) summarized research discussing the role

of sea salt particles in producing rain from clouds. Even as

far back as 1872, Robert Anges Smith discussed the rela-

tionship between air pollution and rainwater chemistry in his

remarkable book entitled Air and Rain: The Beginnings of

A Chemical Climatology (Smith, 1872). These three exam-

ples indicate that the measurement of chemicals in precipita-

tion is not just a recent endeavor. Certainly one reason for

the large number of studies is the ease of collecting samples,

i.e., the ease of collecting rain or snow. Over time and from

project to project during a given time period, the purpose for

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