The algorithm for calculating the column ozone of the atmosphere is based on the work of Gao et al. (2001), which uses the direct-beam irradiance for the retrieval. Given spectral measurements of UV-B, the total column ozone Q in Dobson units is calculated as:
Px Rayleigh scattering optical depth at wavelength A;
ma air mass corresponding to the SZA (#) at the time when the measurement is taken;
P observed station atmospheric pressure;
P0 mean sea level atmospheric pressure;
ax ozone absorption coefficient (base e) at wavelength A;
extraterrestrial voltage intercept at zero air mass at wavelength A;
Vi direct normal voltage after cosine and dark current bias corrections at wavelength A;
/u ratio of the actual and the vertical paths of solar radiation through the ozone layer;
R average earth radius (6,371.229 km);
z height of the station above mean sea level;
h height of ozone layer above mean sea level at station location (20 km is assumed);
d solar zenith angle.
Total column ozone is computed using those 3-min average measurements when the irradiance at 311 nm is greater than 0.002,5 Wm-2nm-1 and the condition d > 70° holds. Daily total column ozone is then determined by averaging these 3-min results if there are at least five of such 3-min averages.
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