Aerosols and trace gases over the Denver metropolitan area and in one case to 175 km downwind were measured with an instrumented aircraft. Typical background concentrations measured above the polluted layer were light scattering coefficient (bscat), 0.2¿10-4 m-1; total(CN) particle concentration, 1--5¿103 cm-3; O3, 30 parts per billion (ppb); SO24¿10-4 m-1, with increases in particle population in the accumulation mode. At the same time the NO2 concentration was only about one tenth of the value predicted by the O3 and NO concentrations and solar irradiance; removal of NO2 by dissolution in liquid water is indicated. For refractive indices assumed to be realistic for dry urban aerosols, the absorption-to-back-scatter ratio decreased with increasing age of the cloud. Therefore, freshly formed emission plumes had the potential to warm the atmosphere under sunlight, but after 10 hours or more the plume had much less warming potential; cooling probably occurred over most surfaces.