The Aerosols, Radiation and Clouds in Southern Africa Field Campaign in Namibia: Overview, Illustrative Observations, and Way Forward

Abstract

The Aerosol, Radiation and Clouds in southern Africa (AEROCLO-sA) project investigates the role of aerosols on the regional climate of southern Africa. This is a unique environment where natural and anthropogenic aerosols and a semipermanent and widespread stratocumulus (Sc) cloud deck are found. The project aims to understand the dynamical, chemical, and radiative processes involved in aerosol–cloud–radiation interactions over land and ocean and under various meteorological conditions. The AEROCLO-sA field campaign was conducted in August and September of 2017 over Namibia. An aircraft equipped with active and passive remote sensors and aerosol in situ probes performed a total of 30 research flight hours. In parallel, a ground-based mobile station with state-of-the-art in situ aerosol probes and remote sensing instrumentation was implemented over coastal Namibia, and complemented by ground-based and balloonborne observations of the dynamical, thermodynamical, and physical properties of the lower troposphere. The focus laid on mineral dust emitted from salty pans and ephemeral riverbeds in northern Namibia, the advection of biomass-burning aerosol plumes from Angola subsequently transported over the Atlantic Ocean, and aerosols in the marine boundary layer at the ocean–atmosphere interface. This article presents an overview of the AEROCLO-sA field campaign with results from the airborne and surface measurements. These observations provide new knowledge of the interactions of aerosols and radiation in cloudy and clear skies in connection with the atmospheric dynamics over southern Africa. They will foster new advanced climate simulations and enhance the capability of spaceborne sensors, ultimately allowing a better prediction of future climate and weather in southern Africa.

Light absorption properties of aerosols over Southern West Africa

“…Further, the range of SSA values over SWA was slightly lower than that reported on Ascension Island to the south-west of the DACCIWA region, which underscores that the evolution of SSA occurred long time after emission. While the mechanism responsible for this phenomenon warrants further study, our results support the growing body of evidence that the optical parameters used in regional/global climate modeling studies, especially absorption by biomass burning aerosols, have to be better constrained using these recent observations to determine the direct and semi-direct radiative effects of smoke particles over this region (Mallet et al 2019). In particular and regarding the very high absorbing properties of smoke, specific attention should be dedicated to the semi-direct effect of biomass burning aerosols at the regional scale and its relative contribution to the indirect radiative effect.…”

Overview of aerosol optical properties over southern West Africa from DACCIWA aircraft measurements

“…Second, there is evidence that fresh BC particles become coated with sulfate and organic species as the plume ages in a manner that enhances their light absorption (Lack et al, 2012;Schwarz et al, 2008). Finally, organic particles produced during the combustion phase can be lost during the transport through photobleaching, volatilization and/or cloud-phase reactions (Clarke et al, 2007;Lewis et al, 2008;Forrister et al, 2015), which is consistent with the low SSA and AAE values we observed. Assessing whether these aging processes impact the chemical components and henceforth optical properties of transported biomass burning aerosol would need extensive investigation of aerosol chemical composition that will be carried out in a subsequent paper.…”

“…Currently there are few field measurements of well-aged biomass burning emissions. Our knowledge of biomass burning aerosol primarily comes from laboratory experiments and near-field measurements taken within a few hours of a wildfire (Abel et al, 2003;Yokelson et al, 2009;Adler et al, 2011;Haywood et al, 2003b;Vakkari et al, 2014;Zhong and Jang, 2014;Forrister et al, 2015;Laing et al, 2016;Zuidema et al, 2018). With the exception of the study by Zuidema et al (2018) over the south-eastern Atlantic, it is generally found that the aged biomass burning aerosol particles are less absorbing than freshly emitted aerosols due to a combination of condensation of secondary organic species and an additional increase in size by coagulation.…”

Observation of absorbing aerosols above clouds over the South-East Atlantic Ocean from the geostationary satellite SEVIRI – Part 2: Comparison with MODIS and aircraft measurements from the CLARIFY-2017 field campaign

“…For this reason, the region has been the focus of much work over the past few years. Using aircraft and surface-based instrumentation, 10 large scale field campaigns have been deployed in 2016-2018 (Zuidema et al, 2016), within the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS; , the US DOE LASIC (Layered Atlantic Smoke Interactions with Clouds; Zuidema et al, 2016), the French AEROCLO-sA (AErosol RadiatiOn and CLouds in Southern Africa; Formenti et al, 2019) and the UK CLARIFY-2017 (CLouds and Aerosol Radiative Impacts and Forcing for Year 2017, Haywood et al, 2019) programs. Airborne 15 in situ instruments, active and passive remote sensing instruments, and radiosondes as well as continuous groundbased measurements have been deployed to characterise biomass burning aerosols, clouds and radiation.…”