ACID RAIN 3
and fog are technically included but in practice are
poorly measured, except by special instruments.
The automatic devices currently in use to sample
precipitation for acid rain studies collect rain and
“wet” snow very efficiently; collect “dry” snow
very inefficiently; and collect some fog water, frost
and dew, but these usually contribute very little to
the annual chemical deposition at a site.
3) Acid Rain A popular term with many meanings;
generally used to describe precipitation samples
(rain, melted snow, melted hail, etc.) with a pH
less than 5.6. Recently the term has sometimes
been used to include acid precipitation, ambient
acid aerosols and gases, dry deposition of acid
substances, etc., but such a broad meaning is con-
fusing and should be avoided.
4) Acid Precipitation Water from the atmosphere in
the form of rain, sleet, snow, hail, etc., with a pH
less than 5.6.
5) Wet Deposition A term that refers to: (a) the
amount of material removed from the atmosphere
by rain, snow, or other precipitation forms; and
(b) the process of transferring gases, liquids, and
solids from the atmosphere to the ground during a
precipitation event.
6) Dry Deposition A term for (a) all materials depos-
ited from the atmosphere in the absence of precipi-
tation; and (b) the process of such deposition.
7) Atmospheric (or Total) Deposition Transfer
from the atmosphere to the ground of gases, par-
ticles, and precipitation, i.e., the sum of wet and
dry deposition. Atmospheric deposition includes
many different types of substances, non-acidic as
well as acidic.
8) Acid Deposition The transfer from the atmo-
sphere to the earth’s surface of acidic substances,
via wet or dry deposition.PROCEDURES AND EQUIPMENT FOR WET
DEPOSITION MONITORINGFor data comparability it would be ideal if all wet deposi-
tion networks used the same equipment and procedures.
However, this does not happen. Therefore, it is important to
decide which network characteristics can produce large dif-
ferences in the databases. The following discussion outlines
procedures and equipment which vary among networks, past
and present.Site LocationSites are selected to produce data to represent local, regional,
or remote patterns and trends of atmospheric deposition of
chemicals. However, the same site may produce a mixture of
data. For example, the measured calcium concentrations at a
site might represent a local pattern while the sulfate concen-
trations represent a regional pattern.Sample ContainersThe containers for collecting and storing precipitation must
be different, depending on the chemical species to be mea-
sured. Plastic containers are currently used in most networks
in measuring acidic wet deposition. Glass containers are
considered less desirable for this purpose because they can
alter the pH: For monitoring pesticides in precipitation, plas-
tic containers would be unacceptable.Sampling ModeThere are four sampling modes:
Bulk Sampling A container is continuously exposed to
the atmosphere for sampling and thus collects a mixture of
wet and dry deposition. The equipment is simple and does
not require electrical power. Thus bulk sampling has been
used frequently in the past, and it is still sometimes used
for economic reasons. For many studies an estimate of total
deposition, wet plus dry, is desired, and thus bulk sampling
may be suitable. However, there is a continuing debate as to
precisely what fraction of dry deposition is sampled by open
containers. The fraction collected will probably depend on
variables such as wind speed, container shape and chemi-
cal species. The continuously exposed collectors are subject
to varying amounts of evaporation unless a vapor barrier
is part of the design. When one objective of a study is to
determine the acidity of rain and snow samples, bulk data
pH must be used with great caution and ideally in conjunc-
tion with adequate blank data. For wet deposition sites that
will be operated for a long time (more than one year), the
labor expenses for site operation and the central laboratory
expenses are large enough that wet-only or wet-dry collec-
tors should certainly be purchased and used instead of bulk
collectors in order to maximize the scientific output from
the project.
Wet-Only Sampling There are a variety of automatic
wet-only samplers in use today that are open only during
precipitation events. Side-by-side field comparison stud-
ies have documented differences in the reaction time for
the sensors, in the reliability of the instruments, and in the
chemical concentrations in the samples from the different
sampling devices. Wet-only sampling can also be achieved
by changing bulk samples immediately (within minutes) at
the beginning and end of precipitation events, but this is very
labor-intensive if done properly.
Wet-Dry Sampling With this device, one container is
automatically exposed during dry periods and the second
container is exposed during precipitation periods. If the
sample in the dry deposition container is not analyzed, the
device becomes a wet-only collector.
Sequential Sampling A series of containers are con-
secutively exposed to the atmosphere to collect wet depo-
sition samples, with the advance to a new container being
triggered on a time basis, a collected volume basis, or both.
These devices can be rather complicated and are usually
operated only for short time periods during specific research
projects.C001_001_r03.indd 3C001_001_r03.indd 3 11/18/2005 10:07:26 AM11/18/2005 10:07:26 AM