For a bimodal size distribution of ambient aerosol, an upper limit in particle size can be chosen for the fine aerosol fraction so that the extinction coefficient for light scattering and absorption is directly proportional to the fine mass concentration, with no dependence on the coarse mass concentration. With the size parameters of the modes taken to be 'grand average continental' values of Whitby, the upper size limit is 3.4 micrometers (aerodynamic diameter). Consequently, the extinction coefficient could be inferred from a measurement of the fine mass concentration only, even in arid regions where coarse aerosol concentrations usually far exceed the fine.