Solar Ultraviolet Attenuation during the Australian (Red Dawn) Dust Event of 23 September 2009

Abstract

Abstract The large dust event of 23 September 2009 is recognized as the most significant on record in Australia since the 1940s. At its peak, reported hourly average particulate concentrations exceeded 15,000 μg m−3, significantly exceeding the Australian National Air Quality Standard of 50 μg m−3 and daily September averages of 20–25 μg m−3. Measurements of the spectral surface ultraviolet irradiance monitored continuously before, after, and during the event of 23 September 2009 are presented. These measurements highlight the significance of extreme dust load on the ultraviolet irradiance, and exceed previously published UV attenuations measured in Saharan and Chinese dust events. Measured spectral attenuations of up to 99.5% in the ultraviolet B and 97.6% in the ultraviolet A were associated with the arrival of low-altitude cumuliform cloud and the dust plume ahead of an approaching cold front on the morning of 23 September 2009. The data presented cover the period of maximum attenuation and the afternoon, which had no cloud cover. This research characterizes for the first time the influence of a frontal desert dust storm on the surface ultraviolet irradiance in the naturally high ambient ultraviolet climate of subtropical Australia.