Page 260 - Fundamentals of Air Pollution

III. Remote Sensing 221

condition for several hundred kilometers downwind. The presence of large
point sources of NO and SO 2 was observed by the changes in the O 3 and
l? scat profiles. Sharp decreases in ozone concentration occurred when large
amounts of NO were present, and rapid increases in b scat were caused by
the primary and secondary particulate matter from power plant plumes
embedded in the larger urban plume. The overall increasing level of ozone
and b scat at greater downwind distances was caused by the photochemical
reactions as the urban plume was transported farther away from St. Louis.
This type of plume mapping can be accomplished only by mobile monitor-
ing systems.

III. REMOTE SENSING

Remote sensing involves monitoring in which the analyzer is physically
removed from the volume of air being analyzed. Satellites have been used
to monitor light-scattering aerosol over large areas (8). Large point sources
such as volcanic activity and forest fires can be tracked by satellite. The
development and movement of hazy air masses have been observed by
satellite imagery (see Fig. 10-9). Remote sensing methods are available
for determining the physical structure of the atmosphere with respect to
turbulence and temperature profiles (see Chapter 17).
Differential absorption lidar (DIAL) is used for remote sensing of gases
and particles in the atmosphere. Lidar is an acronym for light detection and
ranging, and the technique is used for measuring physical characteristics
of the atmosphere. DIAL measurements consist of probing the atmosphere
with pulsed laser radiation at two wavelengths. One wavelength is effi-
ciently absorbed by the trace gas, and the other wavelength is less efficiently
absorbed. The radiation source projects packets of energy through the
atmosphere, which interact with the trace gas. The optical receiver collects
radiation backscattered from the target. By controlling the timing of source
pulses and processing of the optical receiver signal, one can determine the
concentration of the trace gas over various distances from the analyzer. This
capability permits three-dimensional mapping of pollutant concentrations.
Applications are plume dispersion patterns and three-dimensional gaseous
pollutant profiles in urban areas.
SO 2 and O 3 are detected by an ultraviolet DIAL system operating at
wavelengths near 300 nm (9). Tunable infrared CO 2 lasers are used in
applications of IR-DIAL systems which are capable of measuring SO 2 , CO,
HC1, CH 4, CO 2, H 2O, N 2O, NH 3, and H 2S (10). The components of this
type of system are shown in Fig. 15-3 (11). The laser source is switched
between the low-absorption and high-absorption frequencies for the trace
gas to be detected. The system is pointed toward a target, and focusing
lenses are used to collect the returning signal. The beam splitter diverts