UV-B optical thickness observations of the atmosphere

The optical thickness of the atmosphere, tau, was deduced from measurements of narrowband direct solar W-B radiation. A measurement campaign was organ ized to obtain the radiation at three different sites, during the month of August 1999, using the same methods and instruments, in order to deduce the atmospheric optical thickness in the spectral UV-B range (280320 nm). The three observation sites were chosen to cover a wide range of measurement co nditions; located near the tropical Atlantic Ocean (Natal, 5.8 degreesS, 35 .2 degreesW), on the Andes mountains (La Pat, 16.5 degreesS, 68.1 degreesW) , and in the biomass burning area of central Brazil (Campo Grande, 19.2 deg reesS, 54.3 degreesW). The UV-B measurements were made with a Brewer spectr ophotometer at each site. Since the instrument measures atmospheric ozone a nd SO2 simultaneously, it is possible, from the total atmospheric optical t hickness tau to deduce the aerosol optical thickness tau (aerosol). The res ults have been combined in different ways to compare with satellite data, s howing good performance. Time variations as short as about 10 min can be se en. On clear days the time variations are relatively small, as expected. On the other hand, for the biomass burning site, for conditions of mixed air masses (the instrument is not looking directly at plumes) one can see very large variations in tau in relatively short time intervals, for example, fo r one case, from 3.5 to 7.0 in about 30 min. Absolute values for tau at Nat al and La Pat were near 2.0 and at Campo Grande, between 2.5 and 3.0, but w ith occasional highs of about 4.5. For tau (aerosol) Natal and La Pat had v alues between 0.0 and 0.4, whereas Campo Grande had most values near 0.4, w ith occasional highs near 1.0, 1.2, and 2.2.