Three years of measurements of light-absorbing aerosols over coastal Namibia: seasonality, origin, and transport

Paola Formenti (1) , Stuart John Piketh (2) , Andreas Namwoonde (3) , Danitza Klopper (2) , Roelof Burger , Mathieu Cazaunau (4) , Anaïs Féron (5) , Cécile Gaimoz (5) , Stephen Broccardo , Nicola Walton , Karine Desboeufs (1) , Guillaume Siour (1) , Mattheus Hanghome , Samuel Mafwila , Edosa Omoregie , Wolfgang Junkermann (6) , Willy Maenhaut (7)

1 LISA (UMR_7583) - Laboratoire Interuniversitaire des Systèmes Atmosphériques
2 NWU - North-West University [Potchefstroom]
3 NUST - Namibia University of Science and Technology
4 ICARE - Institut de Combustion, Aérothermique, Réactivité et Environnement
5 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
6 IMK - Institute for Meteorology and Climate Research
7 UGENT - Universiteit Gent = Ghent University

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. The average of daily concentrations of equivalent black carbon (eBC) over the whole sampling period is 53 (±55) ng m −3. Peak values above 200 ng m −3 and up to 800 ng m −3 occur seasonally from May to August, ahead of the dry season peak of biomass burning in southern Africa (August to October). Analysis of 3-day air mass back-trajectories show that air masses from the South Atlantic Ocean south of Henties Bay are generally cleaner than air having originated over the ocean north of Henties Bay, influenced by the outflow of the major biomass burning plume, and from the continent, where wildfires occur. Additional episodic peak concentrations, even for oceanic transport, indicate that pollution from distant sources in South Africa and maritime traffic along the Atlantic ship tracks could be important. While we expect the direct radiative effect to be negligible, the indirect effect on the microphysical properties of the stratocumulus clouds and the deposition to the ocean could be significant and deserve further investigation, specifically ahead of the dry season.

Domaines: Océan, Atmosphère

Paola Formenti, Stuart John Piketh, Andreas Namwoonde, Danitza Klopper, Roelof Burger, et al.. Three years of measurements of light-absorbing aerosols over coastal Namibia: seasonality, origin, and transport. Atmospheric Chemistry and Physics, 2018, 18 (23), pp.17003-17016. ⟨10.5194/acp-18-17003-2018⟩. ⟨hal-02352581⟩