Seasonal variation of aerosol size distributions in Darwin, Australia

This paper investigates the annual cycle in aerosol size distributions in Darwin, Australia, a coastal site in the Tropical Warm Pool where the major aerosol sources are biomass burning and sea salt. We have used radiometer measurements from the Tropical Western Pacific Atmospheric Radiation Measurement facility for March 2002–June 2003. Constrained linear inversion was used to retrieve aerosol size distributions from the aerosol optical thickness (AOT) measurements. A novel technique combining size distribution shape and aerosol optical properties classified the distributions into three classes: biomass burning, mixed/background and marine/sea salt. The biomass burning aerosol was associated with continental winds and only observed during the dry season. The mixed/background aerosol and marine/sea salt occurred throughout the year. The marine class occurred predominantly in the afternoon and was strongly associated with marine winds; the mixed/background aerosol did not show any strong wind dependence or diurnal cycles.

► Darwin aerosol size distributions were obtained by inverting optical thickness data.
► Distributions were classified using a combination of optical and shape properties.
► Seasonal occurrences and wind direction associations were used to identify sources.
► Sources are biomass burning in the dry season, and mixed and marine aerosol all year.
► Darwin shows diurnal patterns with marine aerosol predominantly in the afternoon.