Continuous measurements of turbidity, skylight, solar aureole, and sky polarization have shown significant changes in optical effects since the spread of the El Chichon eruption volcanic cloud to northern mid-latitudes in late Oct. 1982. These effects persisted during 1983 except for the fading of twilight colors.
Frederic E. Volz, "Volcanic turbidity, skylight scattering functions, sky polarization, and twilights in New England during 1983," Appl. Opt. 23, 2589-2593 (1984)