The effects on light extinction of the particles and trace gases in the plume from a modern coalfired power plant located in Arizona have been investigated. The surface area and volume concentrations of the particles peaked at particle diameter of ~ 0.25, 0.55 and 1.11 μm. Concentrations of particulate sulfate, nitrate, and gaseous nitric acid ranged from 0.21 to 0.95, 0.22 to 16.6 and 0.97 to 5.5 μg m −3, respectively. Nitric acid accounted for 25–85 % of the nitrate in the plume. Gas-to-particle (g-to-p) conversion rates in the plume were generally too low to be detected, probably due to low ambient concentrations of the hydroxyl radical. Neither particulate sulfate nor nitrate peaked within the optically-critical size range within a travel time of ~ 4 h from the stack. The correlation coefficient between the light-scattering coefficient due to particles ( b scat ) and sulfate mass in the plume was appreciably lower (0.51) than that (0.66) between b scat and the total mass concentration of particles in the plume in excess of ambient. Scattering by particles and absorption by NO 2 appeared to contribute about equally to light extinction by the plume. Optical depths of the plume, derived from airborne measurements, averaged 0.054 ± 0.036, compared with telephotometer measurements of 0.023 ± 0.035.