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

Paola Formenti,Mattheus Hanghome,Andreas Namwoonde,Stephen Broccardo,Willy Maenhaut,Danitza Klopper,Roelof Burger,Karine Desboeufs,Wolfgang Junkermann,Guillaume Siour,Cécile Gaimoz,Anaïs Feron,Stuart John Piketh,Mathieu Cazaunau,Nicola Walton,Samuel Mafwila,Edosa Omoregie

doi:10.5194/acp-18-17003-2018

Abstract

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. 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.

Highlights

Aerosol particles of natural and anthropogenic origin affect the Earth’s climate and modulate the greenhouse effect of long-lived gases (Boucher et al, 2013)
This paper presents the first results of the mass concentrations of light-absorbing carbonaceous aerosols on the Atlantic coast of Namibia from 3 years of observations at the Henties Bay Aerosol Observatory (HBAO; 22◦ S, 14◦05 E) long-term ground-based surface station
This paper presents the first long-term time series of equivalent black carbon concentrations in the marine boundary layer on the south-east Atlantic coast offshore southern Africa

Summary

Introduction

Aerosol particles of natural and anthropogenic origin affect the Earth’s climate and modulate the greenhouse effect of long-lived gases (Boucher et al, 2013). The extent of this modulation depends on their nature, in particular on their chemical composition and size distribution determining their interactions with radiation and clouds. Light-absorbing aerosols, such as black carbon (BC) from fossil fuel combustion and biomass burning, can reduce the amount of outgoing radiation at the top of atmosphere (TOA), adding to the greenhouse effect (Haywood and Shine, 1995; Jacobson, 2001; Chung and Seinfeld, 2002; Bond and Bergstrom, 2006; Koch and Del Genio, 2010; Bond et al, 2013). By entrainment into clouds, BC-containing aerosols could cause the cloud to evaporate and rise (Hansen et al, 1997) and reduce the cloud mean drop size diameters, increase droplet concentrations and reflectivity (Seinfeld and Pandis, 1997)

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