The optical thickness of the atmosphere, &tgr;, was deduced from measurements of narrowband direct solar UV-B radiation. A measurement campaign was organized 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 (280--320 nm). The three observation sites were chosen to cover a wide range of measurement conditions; located near the tropical Atlantic Ocean (Natal, 5.8 ¿S, 35.2 ¿W), on the Andes mountains (La Paz, 16.5 ¿S, 68.1 ¿W), and in the biomass burning area of central Brazil (Campo Grande, 19.2 ¿S, 54.3 ¿W). The UV-B measurements were made with a Brewer spectrophotometer at each site. Since the instrument measures atmospheric ozone and SO2 simultaneously, it is possible, from the total atmospheric optical thickness &tgr;, to deduce the aerosol optical thickness &tgr;aerosol. The results have been combined in different ways to compare with satellite data, showing good performance. Time variations as short as about 10 min can be seen. 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 &tgr; in relatively short time intervals, for example, for one case, from 3.5 to 7.0 in about 30 min. Absolute values for &tgr; at Natal and La Paz were near 2.0 and at Campo Grande, between 2.5 and 3.0, but with occasional highs of about 4.5. For &tgr;aerosol, Natal and La Paz had values between 0.0 and 0.4, whereas Campo Grande had most values near 0.4, with occasional highs near 1.0, 1.2, and 2.2. ¿ 2001 American Geophysical Union