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How swelling of aerosols is dimming the atmosphere in Africa

Southern West Africa (SWA) is significantly affected by air pollution (aerosols and trace gases) from local sources but also from long-range transport. The moist regime in the monsoon layer alters the aerosol properties leading to remarkable changes in the aerosol optical depth.

Applying the regional modeling system COSMO-ART the impact of the high aerosol burden on clouds and atmospheric dynamics in SWA was analyzed. The aerosol impacts on radiation were quantified This research was realized in the framework of DACCIWA.

In the beginning we were astonished by the modeled aerosol optical depths up to 10 and more” says Konrad Deetz who performed the work as part of his PhD. “The further data analysis revealed that the AOD increase is related to the water uptake of the aerosols.

Fig. 1: Analysis of the aerosol liquid water content (ALWC) at 500 m above ground level over land on 3 July 2016, 6 UTC. (a) Map of the total ALWC (shown are the countries Ivory Coast, Ghana, Togo and Benin, from west to east) and (b) pie chart of the ALWC contribution from the individual aerosol modes (%) to the total ALWC in (a). AIT/S – Aitken mode, ACC/S – accumulation mode, CSEASA/B/C – sea salt.

The higher the relative humidity, the more water can be absorbed by the aerosol. The question can arise whether we talk about cloud droplets with an aerosol core or aerosol particles with a water shell. Although a mean aerosol growth factor of sea salt up to 5 is modeled, the particles are still below 1 micron in diameter, which is significantly smaller than a typical cloud droplet (10 microns).

In my thesis I have analyzed the aerosol impact on the atmospheric dynamics over SWA in detail. However, the aerosol liquid water content aspect could only be handled as a side issue. Now it is time to set the focus on this open question” says Konrad Deetz.

To assess the ALWC impact on the radiative transfer, a sensitivity study was conducted with a reference simulation and an additional simulation that neglects the ALWC in the radiative transfer calculations. First results reveal a mean increase in downward shortwave radiation at the surface of about  40 W m-2 over Ivory Coast when neglecting the ALWC. It is expected that the effects are much stronger in the downwind areas of the large coastal cities. A publication dealing with that topic is already submitted.

[Working group: Aerosols, Trace Gases and Climate Processes]