The Interpretation of Baseline Atmospheric Turbidity Measurements at Cape Grim, Tasmania

Abstract

Five years of turbidity data at Cape Grim have been analysed. The turbidity at 500 nm in “clean” maritime airmasses from the South to the West shows a seasonal variation, with a minimum in winter. There is also a variation in turbidity with wind speed. The winter minimum can be explained partially by a minimum in wind strength in that season. On the assumption, based on observations at Cape Grim and at other locations, that the boundary layer turbidity is caused by seasalt haze, an attempt is made to interpret the observed turbidity values and their seasonal changes. Optical extinction coefficients at the surface deduced from the measured values of optical depth are compared with extinction coefficients calculated from Mie theory using particle size distributions measured at Cape Grim. Reasonable agreement is obtained when the growth of salt particles in the high maritime humidity is considered, using both theoretical models and previous experimental results together with the rapid increase in salt concentration with wind speed.