Page 355 - Fundamentals of Air Pollution 3E

310 19. Transport and Dispersion of Air Pollutants

interest. The specifications are response time of 1 min, accuracy of Q.1°C,
and resolution of 0.02°C. Both sensors should use good-quality aspirated
radiation shields to give representative values. Sensor sampling about every
30 sec yields good hourly averages.
Radiation instruments are useful in determining stability such as F. B.
Smith's (17) stability parameter P. Although somewhat similar to the Pas-
quill stability class (Table 19-3), P is continuous (rather than a discrete class)
and is derived from wind speed and measurement of upward heat flux or,
lacking this, incoming solar radiation (in daytime) and cloud amount at
night. Pyranometers measure total sun and sky radiation. Net radiometers
measure both incoming (mostly shortwave) radiation and outgoing (mostly
longwave) radiation. Data from both are useful in turbulence characteriza-
tion, and the values should be integrated over hourly periods. Care should
be taken to avoid shadows on the sensors. The net radiometer is very
sensitive to the condition of the ground surface over which it is exposed.
B. Measurements above the Surface
Measurements above the surface are also important to support pollutant
impact evaluation. The radiosonde program of the National Weather Service
(Fig. 19-10), established to support forecast and aviation weather activities,
is a useful source of temperatures and data on winds aloft, although it has
the disadvantage that measurements are made at 12-hr intervals and the
surface layer is inadequately sampled because of the fast rate of rise of the
balloon. Mixing height, the height aboveground of the neutral or unstable
layer, is calculated from the radiosonde information (see Chapter 17, Section
II).
Measurements of wind, turbulence, and temperature aloft may also be
made at various heights on meteorological towers taller than 10 m. Where
possible, the sensors should be exposed on a boom at a distance from the
tower equal to two times the diameter of the tower at that height.
Aircraft can take vertical temperature soundings and can measure air
pollutant and tracer concentrations and turbulence intensity. Airborne lidar
can measure plume heights, and integrating nephelometers can determine
particle size distributions.
Since operating aircraft, building towers, and establishing instruments
on towers are extremely expensive, considerable attention has focused on
indirect upper-air sounding from the ground. Mixing height within the
range of measurement (approximately 500-600 m aboveground) can be
determined by either the Doppler or the monostatic version of sodar (sound
direction and ranging) with a spatial resolution of about 30 m. Data on
wind and turbulence can be determined by Doppler sodar, FM-CW radar,
and lidar. Doppler sodar measurements of wind components are within
1
approximately 0.5 m s" of tower measurements. Measurements represent
30-m volume averages in the vertical. A height of 500 m aboveground, and
sometimes over 1000 m, can be reached routinely.