Encyclopedia of Environmental Science and Engineering, Volume I and II

AIR POLLUTION INSTRUMENTATION 53

modifying the flue gas composition. This can even be

accomplished in the presence of particulate matter. Three

techniques, differential absorption, gas filter correlation

and second derivative spectroscopy, eliminate the problems

associated with a reduction in light transmission due to the

presence of particulates.

Two types of in-situ monitors exist: cross-stack and in

stack. Cross-stack monitors, which can either be single-pass

or double-pass systems (like transmissometers), measure the

gas concentration across the entire, or a majority, of the stack

diameter. In-stack systems (or short-path monitors) have a

shorter path length of 5 centimeters to a meter.

In-Situ Cross-Stack Analyzers

Cross-stack analyzers use either the principle of differential

absorption spectroscopy or gas-filter correlation spectros-

copy. Differential absorption analyzers utilize a technique

similar to that used by NDUV extractive analyzers; how-

ever, they operate in-situ and eliminate the particulate matter

interference. CO 2 , SO 2 , and NO can be monitored in this

manner.

Gas-filter correlation spectroscopy, used for CO, CO 2 ,

SO 2 , and NO analysis, is an NDIR in-situ method which,

like the differential absorption technique, eliminates par-

ticulate interference.

In-Situ In-Stack Analyzers

In-stack analyzers utilize second-derivative spectroscopy to

measure NH 3 , SO 2 , and NO concentrations. They are also

known as in-stack point, or short-path monitors. Ultraviolet

light is transmitted through the probe and the sensing area,

to a reflector and back.

Please refer to Table 3 for a list of some of the manufac-

turers of the various in-situ instruments available.

AMBIENT INSTRUMENTATION

Ambient monitoring requires the use of instrumentation

ranging in sophistication from the standard high volume par-

ticulate sampler to electronic systems incorporating several

different gaseous detectors and data loggers all maintained in

a temperature and humidity controlled remote sampling sta-

tion. The reasons for performance of an ambient monitoring

program are presented below:

1. Collection of background air quality data for
   preparation of air permits.


2. Verifi cation of the reduction of specifi c air quality
   impacts resulting from emission control programs.


3. Verifi cation of groundlevel downwind concentra-
   tions as determined by computer modeling.


4. To validate and refi ne models.

TABLE 3

Manufacturers of in-situ monitors

Cross-stack

Environmental Data Corporation Contraves Goerz Corporation

608 Fig Avenue 610 Epsilon Drive

Monrovia, CA 91016 Pittsburgh, PA 15238

In-stack

Lear Siegler, Inc.

Environmental Technology Division

74 Inverness Drive East

Englewood, CO 80110

Oxygen monitors only

Westinghouse Electric Corporation Corning Glass Works

Computer and Instrument Division Ceramic Products Division

Orville, OH 44667 Corning, NY 14803

Dynatron, Inc. Hays-Republic

Barnes Industrial Park Milton Roy Company

Wallingford, CT 06492 4333 South Ohio Street

Michigan City, IN 46360

Cleveland Controls, Inc.

1111 Brookpart Road

Cleveland, OH 44109

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