Pollution slightly enhances atmospheric cooling by low-level clouds in tropical West Africa

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 modelling, 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 DLR aircraft Falcon 20 during the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) campaign in June and July 2016. Clouds below 1800 meter 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 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 µm to 50 µm around noon increases by 35 % in the elevated aerosol outflow of coastal cities and conurbations with elevated aerosol loadings from median CDNC of 240 cm^-3 (52 cm^-3 to 501 cm^-3 interquartile range to 324 cm^-3 (60 cm^-3 to 740 cm^-3 interquartile range). Higher CDNC resulted in a 17 % decrease in effective cloud droplet diameter from a median d_eff of 14.8 µm to a d_eff of 12.4 µm in polluted clouds.

Radiative transfer simulations show a non-negligible influence of droplet number concentrations and particle sizes on the net radiative forcing at the top of atmosphere of -16.3 W m^-2 of the polluted with respect to the less polluted clouds and lead to a change in instantaneous heating rates of -18 K day^-1 at top of the clouds at noon. It was found that the net radiative forcing at top of atmosphere accounts for only 2.6 % of the net forcing of the cloud-free reference case. Thus, polluted low-level clouds add only a relatively small contribution on top of the already exerted cooling by low-level clouds in view of a background atmosphere with elevated aerosol loading. 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.