Three years of measurements of light-absorbing aerosols in the marine air at Henties Bay, Namibia: seasonality, origin, and transport

Abstract. Continuous measurements between July 2012 and December 2015 at the Henties Bay Aerosol Observatory (HBAO; 22° S, 14°05’ E), Namibia, show that, during the austral wintertime, transport of light-absorbing black carbon aerosols occurs at low-level into the marine boundary layer from the South East Atlantic coast. Daily concentrations reach 986 ng m-3 and display a seasonal maximum from May to August, ahead of the dry season peak of biomass burning in southern Africa (August to October). This outflow is due to either anti-cyclonic circulation or along-the-coast streamlines and contributes to the transport of both biomass and fossil fuel burning aerosols. We estimate that the particle number concentration associated with this transport could contribute up to 2000 cm-3 to the cloud droplet number concentration with respect to pristine conditions. Their direct radiative effect is negligible.