De La Salle University - Manila

LIDAR

SUNPHOTOMETER

Principle of Sunphotometry

The principle of sunphotometry uses the quantity of fundamental importance in atmospheric studies is the optical depth (thickness), which describes how much light passes through a material (Figure 1). This means that relatively high values (~0.5 and above at 500nm) of AOD denote a more polluted the atmosphere. This is expressed in the Beer-Lambert-Bouguer Attenuation Law or Bouguer's Law [Haze-SPAN, 1997]:

where V(l) is the intensity of solar radiation, Vo(l) is the calibration constant , t(l) is the total optical depth, and m(q) is the relative optical airmass and R is the Earth-Sun distance in astronomical units (AU).

Figure 1. Principle of Sunphotometry

The total optical depth of the atmosphere is usually regarded to consist of several components (Rayleigh, Ozone, Water vapor and Aerosol and some minor gase) depending on the spectral response of the detectors used. The Rayleigh (molecular) scattering is due to the scattering of light off of the molecules in the air. This gives us the blue sky. Ozone, on the other hand, absorbs part of the solar radiation (10-20%) in the UV and visible portions of the solar spectrum [Van Heuklon, 1978]. The water vapor exhibits most intense and widest absorption band among the atmospheric gases because it severely absorbs incoming radiation and long-wave energy emitted by the earth [Jiecai et al., 1997]. Aerosols are microscopic liquid or solid particles, such as smoke, dust, fog, or smog, that then to remain dispersed in a gas, usually in air, rather than settle [Hardin and Kahn].