7.2.11 Aerosol optical depth observation

   Aerosol optical depth is observed using an automatic sun-tracking sunphotometer. The measurement method is based on the principle that the sun light is scattered and absorbed by aerosol. Figure 7.2.11.1 shows appearance view of the observation system. The sunphotometer used here is a precision filter radiometer (PFR), which is capable of observing 4 wavelengths (i.e., 368, 412, 500, and 862 nm), selected to meet the recommendation by the WMO. Aerosol optical depth is observed continuously from sunrise to sunset, with hourly and other values calculated using only the observations obtained in the condition of no clouds between the observation system and the sun. At Syowa Station in Antarctica, observations are made using a MS-110 sunphotometer capable of observing 5 wavelengths (i.e., 368, 500, 675, 778, and 862 nm), and hourly mean values are calculated from the observations where the optical air mass is 6 or less and no clouds exist between the observation system and the sun. To determine aerosol optical depth, sunlight is directed through an interference filter to create a spectrum. Then from the output of the sensor, corresponding to the intensity of each wavelength , the optical depth of the air () is calculated using the following formula, which takes into consideration the solar elevation angle and other factors:

      (1) where, is the system constant (mV), is the sunphotometer output voltage (mV), is the optical air mass, and is the correction coefficient based on distance from the earth to the sun.

   Aerosol optical depth () can be obtained by subtracting optical depth due to air molecule scattering and optical depth due to ozone absorption from that of the atmosphere. Optical depth due to air molecule scattering () can be obtained with the following formula:

      (2) where, is the local pressure of the observation site at the time of observation (hPa), is the standard atmospheric pressure of 1013.26 (hPa),and is the wavelength (μm).

   An estimate value of the total ozone typical in the area near the station based on TOMS observation is used for optical depth due to ozone absorption (). The Ångström exponent (), which indicates the relation between the amounts of aerosol with a larger particle diameter and smaller diameter, can be obtained from the wavelength-specific aerosol optical depth. The relationship expressed by the following formula holds roughly between Ångström exponent , wavelength , and aerosol optical depth : (Ångström, 1961). In general, the optical depth of large-diameter aerosols does not decrease even if the wavelength increases. On the other hand, the optical depth of small-diameter aerosols decreases with increase in wavelength. Because of this, the Ångström exponent is inversely proportional to the size of aerosol. At larger Ångström exponents there are a relatively larger amount of smaller particles, and at smaller Ångström exponents there are a relatively larger amount of larger particles. For instance, larger particles such as Kosa (aeolian dust) from China have an optical depth of 0.5 or less (Tanaka et al., 1989; Suzuki et al., 2001; Uchiyama et al., 2005), whereas smaller particles, such as the particles of air pollution or smoke in forest fire, are known to be in the order of 1.5 - 2 (Moulin et al., 1997; Eck et al., 2003).