2.7 Radiation
Solar
and/or net radiation data are used to:
- determine atmospheric stability
(Section 6.4.2),
- for calculating various
surface-layer parameters used in dispersion modeling (Section 6.6),
- for
estimating convective (daytime) mixing heights, and
- for modeling
photochemical reactions.
Solar
radiation refers to the electromagnetic energy in the solar spectrum (0.10
to 4.0 µm wavelength); the latter is commonly classified:
- as ultraviolet
(0.10 to 0.40 µm),
- visible light (0.40 to 0.73 µm),
- and near-infrared
(0.73 to 4.0 µm) radiation.
Net radiation includes both solar radiation
(also referred to as short-wave radiation) and terrestrial or long-wave
radiation; the sign of the net radiation indicates the direction of the flux
(a negative value indicates a net upward flux of energy).
Pyranometers
are a class of instruments used for measuring energy fluxes in the solar spectrum. These instruments are configured to
measure what is referred to as global solar radiation;
i.e., direct plus diffuse (scattered) solar radiation incidence on a
horizontal surface. The sensing element of the typical pyranometer is
protected by a clear glass dome which both protects the sensing element, and
functions as a filter preventing entry of energy outside the solar spectrum
(i.e., long-wave radiation). The glass domes used on typical pyranometers
are transparent to wavelengths in the range of 0.28 to 2.8 µm. Filters can
be used instead of the clear glass dome to measure radiation in different
spectral intervals; e.g., ultraviolet radiation.
WMO
specifications for several classes of pyranometers are given in Table 2-1 [9].
First class and secondary standard pyranometers typically employ a
thermopile for the sensing element. The thermopile consists of a series of
thermojunction pairs, an optically black primary junction, and an optically
white reference junction (in some pyranometers, the reference thermojunction
is embedded in the body of the instrument). The temperature difference
between the primary and reference junctions which results when the
pyranometer is operating generates an electrical potential proportional to
the solar radiation.
Second class pyranometers typically employ photo-cells
for the sensing element. Though less costly than other types of
pyranometers, the spectral response of the photovoltaic pyranometer is
limited to the visible spectrum.
First
class or second class pyranometers should normally be used for measuring
global solar radiation, depending on the application. If the solar radiation
data are to be used in procedures for estimating tability (Section 6.4) then second class (photovoltaic) pyranometers
are acceptable. For most other applications, first class or secondary
standard pyranometers should be used. Applications requiring ultraviolet
(UV) radiation data should not employ photovoltaic measurements as these
instruments are not sensitive to UV radiation.
