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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Aug 2018

Research article | 20 Aug 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Sophie L. Haslett1, Jonathan W. Taylor1, Konrad Deetz2, Bernhard Vogel2, Karmen Babić2, Norbert Kalthoff2, Andreas Wieser2, Cheikh Dione3, Fabienne Lohou3, Joel Brito4, Regis Dupuy4, Alfons Schwarzenboeck4, and Hugh Coe1 Sophie L. Haslett et al.
  • 1School of Earth and Environmental Sciences, University of Manchester, Manchester, UK
  • 2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 3Laboratoire d’Aérologie, Université Paul Sabatier Toulouse III (UPS), Toulouse, France
  • 4Laboratoire de Météorologie Physique, Université Clermont Auvergne, Aubière, France

Abstract. Water in the atmosphere exists as both vapour and liquid water contained in particles. At high humidities, more water vapour condenses onto particles and causes them to swell, sometimes up to several times their original size. This significant change in size and chemical composition is termed hygroscopic growth and alters a particle's optical properties. Even in unsaturated conditions, this can change the aerosol direct effect, for example by increasing the extinction of incoming sunlight. This can have an impact on a region's energy balance and affect visibility. Here, aerosol and relative humidity measurements collected from aircraft and radiosondes during the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) campaign were used to estimate the effect of highly humid layers of air on aerosol optical properties during the monsoon season in southern West Africa. The effects of hygroscopic growth in this region are of particular interest due to the regular occurrence of high humidity and the high levels of pollution in the region. The Zdanovskii, Stokes and Robinson (ZSR) mixing rule is used to estimate the hygroscopic growth of particles under different conditions based on chemical composition. These results are used to estimate the aerosol optical depth (AOD) for 63 relative humidity profiles. A static aerosol profile was assumed. Therefore, these results show the extent of the AOD frequency distribution that can be explained by humidity alone, rather than predicting actual AOD values. The median AOD in the region from these calculations was 0.46, which compares to a median of 0.36 measured by sun photometers. The shape of the AOD frequency distribution was largely comparable to that of the sun photometer measurements, demonstrating that relative humidity is able to account for a large part of the region's AOD variability. Humid layers are found to have the most significant impact on AOD when they reach relative humidities greater than 98%, which can result in a wet AOD up to seven times larger than the dry AOD. Unsaturated humid layers were found to reach these high levels of relative humidity in 37% of observed cases. Aerosol concentrations in southern West Africa are projected to increase substantially in the coming years; results presented here show that the presence of highly humid layers in the region is likely to enhance the consequent effect on AOD significantly.

Sophie L. Haslett et al.
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Sophie L. Haslett et al.
Sophie L. Haslett et al.
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Short summary
As the population in West Africa grows and air pollution increases, it is becoming ever more important to understand the effects of this pollution on the climate and on health. Aerosol particles can grow by absorbing water from the air around them. This paper shows that during the monsoon season, aerosol particles in the region are likely to grow significantly because of the high moisture in the air. This means that climate effects from increasing pollution will be enhanced.
As the population in West Africa grows and air pollution increases, it is becoming ever more...