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. Water in the atmosphere can exist in the solid, liquid orgas phase. At high humidities, if the aerosol population remains constant,more water vapour will condense onto the particles and cause them to swell,sometimes up to several times their original size. This significant change insize and chemical composition is termed hygroscopic growth and alters aparticle's optical properties. Even in unsaturated conditions, this canchange the aerosol direct effect, for example by increasing the extinction ofincoming sunlight. This can have an impact on a region's energy balance andaffect visibility. Here, aerosol and relative humidity measurements collectedfrom aircraft and radiosondes during the Dynamics–Aerosol–Chemistry–CloudInteractions in West Africa (DACCIWA) campaign were used to estimate theeffect of highly humid layers of air on aerosol optical properties during themonsoon season in southern West Africa. The effects of hygroscopic growth inthis region are of particular interest due to the regular occurrence of highhumidity and the high levels of pollution in the region. The Zdanovskii,Stokes and Robinson (ZSR) mixing rule is used to estimate the hygroscopicgrowth of particles under different conditions based on chemical composition.These results are used to estimate the aerosol optical depth (AOD) at λ=525  nm for 63 relative humidity profiles. The median AOD in theregion from these calculations was 0.36, the same as that measured by sunphotometers at the ground site. The spread in the calculated AODs was lessthan the spread from the sun photometer measurements. In both cases, valuesabove 0.5 were seen predominantly in the mornings and corresponded with highhumidities. Observations of modest variations in aerosol load and compositionare unable to explain the high and variable AODs observed using sunphotometers, which can only be recreated by accounting for the very elevatedand variable relative humidities (RHs) in the boundary layer. Mostimportantly, the highest AODs present in the mornings are not possiblewithout the presence of high RH in excess of 95 %. Humid layers are foundto have the most significant impact on AOD when they reach RH greater than98 %, which can result in a wet AOD more than 1.8 times the dry AOD.Unsaturated humid layers were found to reach these high levels of RH in37 % of observed cases. It can therefore be concluded that the high AODspresent across the region are driven by the high humidities and are thenmoderated by changes in aerosol abundance. Aerosol concentrations in southernWest Africa are projected to increase substantially in the coming years;results presented here show that the presence of highly humid layers in theregion is likely to enhance the consequent effect on AOD significantly.

The radiative impact of out-of-cloud aerosol hygroscopic growth during the summer monsoon in southern West Africa