A sun-photometer designed for aerosol studies was built and it was installed permanently at San Luis City, Argentina. This instrument measures the attenuation of direct solar light at wavelengths were molecular absorption are minimal, 380 nm, 500 nm, 770 nm, and 1060 nm. It makes continuous measurements mounted on a solar tracker and data are filed every minute. An extra channel at 940 nm was introduced for water measurements. Results in San Luis confirm that aerosols normally present in that place are dust-like particles with a very shallow extinction spectrum. Such aerosols can not be represented by the standard form of the angstrom's formula where (alpha) equals 1.2 is normally assumed. This fact must be taken into account when radiative-transfer models are used, specially in the UV region. During two short periods of time, the meter was operated at different places: Las Cuevas, at 3200 m height at Los Andes mountains, and Pampa Amarilla, the site for a future cosmic ray observatory in Argentina. Typical results for these geographical regions corresponded to clean atmospheres with small quantity of large particles. The water channel was calibrated by simultaneous measurements with a Microtops II, a commercial sun-photometer; in the near future the radiometer will be contrasted against water-radiosonde measurements.