plume appear bluish due to sunlight scattering by tiny sulfate aerosol

A volcanic gas cloud over the LERZ on May 19, 2018 – In this picture, the sun is illuminating the volcanic gas plume from behind. The plume appears orange in color, as the blue component of the sunlight has been preferentially removed by scattering on tiny sulfate aerosols. This looks a bit like a sunset, but note that the meteorological clouds below the volcanic plume do not show this discoloration.