Pollution slightly enhances atmospheric cooling by low-level clouds in tropical West Africa
Authors: Hahn Valerian , Meerkötter Ralf , Voigt Christiane , Gisinger Sonja , Sauer Daniel , Catoire Valéry , Dreiling Volker , Coe Hugh , Flamant Cyrille , Kaufmann Stefan , Kleine Jonas , Knippertz Peter , Moser Manuel , Rosenberg Philip , Schlager Hans , Schwarzenboeck Alfons , Taylor Jonathan 
Published: August 2023

Abstract. Reflection of solar radiation by tropical low-level clouds has an important cooling effect on climate and leads to decreases in surface
temperatures. Still, the effect of pollution on ubiquitous tropical continental low-level clouds and the investigation of the related impact on
atmospheric cooling rates are poorly constrained by in situ observations and modeling, in particular during the West African summer monsoon
season. Here, we present comprehensive in situ measurements of microphysical properties of low-level clouds over tropical West Africa, measured with
the Deutsches Zentrum für Luft- und Raumfahrt (DLR) aircraft Falcon 20 during the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) campaign in June and July
2016. Clouds below 1800 m altitude, identified as boundary layer clouds, were classified according to their carbon monoxide (CO)
pollution level into pristine and less polluted clouds (CO < 135 ppbv) and polluted low-level clouds
(CO > 155 ppbv) as confirmed by the linear CO to accumulation aerosol number concentration correlation. Whereas slightly
enhanced aerosol background levels from biomass burning were measured across the entire area, clouds with substantially enhanced aerosol levels were
measured in the outflow of major coastal cities, as well as over rural conurbations in the hinterlands. Here we investigate the impact of pollution
on cloud droplet number concentration and size during the West African monsoon season. Our results show that the cloud droplet number concentration
(CDNC) measured in the size range from 3 to 50 µm around noon increases by 26 % in the elevated aerosol outflow of coastal cities
and conurbations with elevated aerosol loadings from median CDNC of 240 cm−3 (52 to 501 cm−3 interquartile range) to
324 cm−3 (60 to 740 cm−3 interquartile range). Higher CDNC resulted in a 17 % decrease in effective cloud droplet diameter
from a median deff of 14.8 µm to a deff of 12.4 µm in polluted clouds. Radiative transfer simulations show a non-negligible influence of higher droplet number concentrations and smaller particle sizes on the
diurnally averaged (noon) net radiative forcing at the top of atmosphere of −3.9 W m−2 (−16.3 W m−2) of polluted with
respect to less polluted clouds and lead to a change in instantaneous heating rates of −22.8 K d−1 (−17.7 K d−1) at the
top of clouds. Thus, the atmospheric cooling by low-level clouds increases only slightly in the polluted case due to the already elevated background
aerosol concentrations. Additionally, the occurrence of mid- and high-level cloud layers atop buffer this effect further, so that the net radiative
forcing and instantaneous heating rate of low-level clouds turn out to be less sensitive towards projected future increases in anthropogenic
pollution in West Africa.