We measured optical properties and ionic composition of the aerosol at Neumayer Station from 2004 through 2006 by an integrating nephelometer and chemical analysis of daily aerosol samples, respectively. From this unique data set, we discussed the seasonality of optical parameters along with the chemical composition of the aerosol. Austral summer (November through March) was characterized by mean particle number concentrations of 472 ± 260 cm^-3 compared to 168 ± 160 cm^-3 during winter (April through October), mean scattering Ångström exponents of 1.5 ± 0.6 compared to 1.2 ± 0.5 during winter, and mean hemispheric backscattering ratios at 700 nm of 0.21 ± 0.13 compared to 0.17 ± 0.08 during winter. In contrast, light scattering coefficients (σ <inf>sp</inf>) showed a broad maximum during winter (4.8 ± 5.3 Mm^-1 for σ<inf>sp</inf>(550)). The mean single scattering albedo was 0.99 ± <inf>0.02</inf>^0.01 at 550 nm. We further derived mass scattering and mass backscattering efficiencies for biogenic sulfate aerosol (BSA) at 450 nm, 550 nm, and 700 nm for relative humidities between 5% and 11%. At 550 nm, the scattering efficiency for biogenic sulfate aerosol α <inf>BSA</inf>^S(550) was 8.9 ± 0.7 m² g^-1 with a corresponding backscattering efficiency α <inf>BSA</inf>^bs(550) of 1.0 ± 0.08 m² g^-1. From the seasonality of the aerosol composition, we inferred a dominant contribution of sulfate aerosol regarding radiative forcing in the lower troposphere from December through January, while the impact of sea salt aerosol prevailed for the rest of the year at Neumayer. Copyright 2008 by the American Geophysical Union.