Page 357 - Fundamentals of Air Pollution 3E

312 19. Transport and Dispersion of Air Pollutants

Some measurements that are completely impractical for routine measure-
ment programs are useful during periods of intensive field programs. Winds
and temperatures can be measured through frequent releases of balloon-
carried sensors. Lidar is useful for determining plume dimensions. The
particle lidar measures backscatter of laser radiation from particles in the
plume and particles in the free air. The differential absorption lidar uses
two wavelengths, one with strong absorption by sulfur dioxide and the
other for weak absorption. The difference determines the amount of sulfur
dioxide in the plume. Positioning of the lidar and its scanning mode de-
termines whether vertical or horizontal dimensions of the plume are
measured.

C. Data Reduction and Quality Assurance
A meteorological measurement program includes data reduction, calcula-
tion of quantities not directly measured, data logging, and archiving.
Special-purpose minicomputers are used for sampling sensor output at
frequent intervals (down to fractions of a second), calculating averages,
and determining standard deviations. The output from the minicomputer
should go to a data logger so that the appropriate information can be
recorded on magnetic tape or disk or paper tape. If only hourly values
must be archived, a considerable period of record for all data from a site
can be contained on a single tape, disk, or cassette. Hard copy from a
printer is usually also obtained. Immediate availability of this copy can aid
in detecting system or sensor malfunctions. Sometimes analog charts are
maintained for each sensor to provide backup data recovery (in case of
reduction error or data logger malfunction) and to detect sensor mal-
function.
An extremely important part of a measurement program is an adequate
quality assurance program. Cost cutting in this part of the program can
result in useless measurements. A good-quality assurance program includes
calibration of individual components and of the entire system in the labora-
tory; calibration of the system upon installation in the field; scheduled
maintenance and servicing; recalibration (perhaps quarterly); and daily
examination of data output for unusual or unlikely values. More frequent
servicing than that recommended by manufacturers may be required when
sensors are placed in polluted atmospheres which may cause relatively
rapid corrosion of instrument parts.

V. ATMOSPHERIC TRACERS

A. Technique
Tracer studies are extremely important in furthering our knowledge of
atmospheric dispersion. These studies consist of release of a known quan-