Aerosol Optical Depth

Atmospheric aerosols play an important role in scattering and absorbing radiation as it passes through the atmosphere. The intensity and angular distribution of surface radiation are affected by the aerosols' optical depth (r in Eq. (6.2)), vertical distribution, and absorption and scattering characteristics. Aerosol optical depth (r) is comprised of two components: absorbing and scattering (i.e., r = rabs + rscat). Aerosols typically reduce surface UV irradiance by a few percent and can reduce surface irradiance by more than 50% in some cases.

There are a number of sources of atmospheric aerosols. Natural sources include volcanoes, dessert dust, and smoke from forest fires. Manmade sources include pollution from industrial activity (smog). The measurement of aerosols is important for several scientific applications ranging from local to global issues. Local issues include the prediction of smog alerts in the vicinity of cities and tracking of smoke from forest fires. Regional issues include transport of pollution across borders. Global issues include the impact of widespread manmade aerosols on our changing climate and the impact of volcanic aerosols on the ozone layer.

The Brewer instrument is capable of determining r using measurements of direct solar radiation provided the extraterrestrial response of the instrument is accurately known and appropriate corrections are applied. Measurements must be made at several wavelengths in order to quantify and correct for the absorption of absorbing atmospheric gases such as ozone, SO2, and NO2.

Extraterrestrial values (log (Io 2) terms in Eq. (6.2)) are determined by making measurements of log (Ij) (in Eq. (6.2)) at a clean site (e.g., MLO) to determine the values of log (Ij) + fix m + axO3 ^ from many measurements made throughout a day. A plot of these values against sec (SZA) yields a straight line with intercept log (Iol). Intercept values made at several wavelengths averaged over a period of several days are used as the extraterrestrial reference for the particular instrument. Once the extraterrestrial values are known, it is possible to determine tx from measurements made at a field site using Eq. (6.2).

Corrections that must be applied to the measurements include dark count, dead time, instrument temperature, neutral density filter, internal polarization, and stray light (if required). These corrections must be applied both to the calibration measurements as well as to the field site measurements. Further details of the methods of instrument calibration and the method for measuring r are discussed in Kerr (2002) and Arola and Koskela (2004). Results of optical depth measurements using the Brewer instrument have been reported by many researchers (Bais, 1997; Kerr, 1997; Carvalho and Henriques, 2000; Jaroslawski and Krzyscin, 2000; Jaroslawski and Krzyscin, 2005; Meleti and Cappellani, 2000; Grobner et al., 2001; Kirchhoff et al., 2001; Kerr, 2002; Cheymol and De Backer, 2003; Grobner and Melita, 2004; Kazadzis et al., 2007).

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