Surface UV radiation is affected by the presence of atmospheric SO2, which has strong absorption bands at UV wavelengths (Fig. 6.3). Sources of atmospheric SO2 are both manmade and natural. Measurements of SO2 are important for tracking and assessing impacts of emissions from pollution sources, such as coal burning power plants. Measurements are also important for tracking and quantifying naturally occurring SO2 emitted by volcanoes. Pollution sources typically result in a few DU of column SO2 unless observations are made near a source. Column SO2 in the vicinity of a volcanic eruption can be more significant (Kerr et al., 1982; Krueger et al., 1995). Ground-based measurements of atmospheric SO2 using the Brewer instrument have played an important role in the development and validation of satellite-based SO2 measurements (Schaefer et al., 1997) used primarily for detecting and tracking volcanic emissions.
The SO2 absorption spectrum at UV wavelengths has significantly more structure than that of ozone as shown in Fig. 6.3. The differences in structure allow the measurement of SO2 in addition to ozone. The presence of SO2 in the atmosphere can add a false signal to total ozone measured with the Dobson instrument (Kerr et al., 1985). However, the operational total ozone algorithm for the Brewer instrument (Eq. (6.3)) is insensitive to SO2. The Brewer algorithm for measuring SO2 uses the peak absorption feature at 306.3 nm (Fig. 6.3) and is described in detail by Kerr et al. (1985). Column SO2 is usually measured using the direct sun method since it the most accurate method and values are reported to the WOUDC
when available. SO2 can be seen on zenith sky measurements if values are significantly large, particularly during the passage of debris from a volcanic eruption (e.g., Kerr et al., 1982).
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