Abstract The interaction between air pollutants and solar radiation is discussed in terms of three examples: measurements of the direct solar beam to determine atmospheric turbidity; the role of solar radiation in initiating photochemical smog processes; and urban-rural differences of incident solar energy. Turbidity measurements from several non-urban stations, particularly in the south-central and south-eastern U.S., show a slight increase during the past 10–15 yr, with most of the change occurring during summer. Measurements at certain urban stations indicate a decreasing turbidity trend. Coincident measurements of incident solar radiation at urban and rural sites are presented for St. Louis and Los Angeles. The urban-rural differences of incident total and ultraviolet (UV) solar energy ranged from near zero to more than 50 per cent. The solar depletion was much larger over Los Angeles than St. Louis. The impact of solar radiation on photochemical smog over Los Angeles is shown be discussing the sensitivity of ozone concentrations to variations of UV radiation. The results of a numerical photochemical diffusion model indicate that the seasonal decrease of UV energy from July to November has a substantial effect on ozone levels, especially over the western part of the Los Angeles Basin.