A procedure to estimate the aerosol extinction parameters of Angstrom's formula using data acquired with photometric broadband standard filters, the Johnson-Cousins filters, is presented. The method is based on the transformation of the heterochromatic magnitudes to the equivalent mean wavelength monochromatic magnitudes by an iterative approach, correcting subsequently the effects of water-vapour extinction in the longer-wavelength filters. As an example, the procedure has been used in the determination of the extinction parameters at the location of our observatory using the filters B, V, R c and I c on the night of 2008 August 28.