ECN publication
The impact of aerosols on regional climate
Weijers, E.P.; Brink, H.M. ten; Boers, R.; Timmermans, R.; Schaap, M.; Meijgaard, E. van
Published by: Publication date:
ECN Environment & Energy Engineering 19-3-2012
ECN report number: Document type:
ECN-O--12-007 Other
Number of pages: Full text:
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The magnitude of the regional first Aerosol Indirect forcing Effect (AIE) was addressed. This is the increase in reflectivity of clouds due to manmade aerosol. Two approaches were followed: 1) modelling using generic knowledge on the interaction of aerosol and cloud, and 2) an experimental study on the importance of a specific regional aerosol, viz. ammonium nitrate. 1) The Chemical Transport Model LOTOS-EUROS provided the aerosol field of anthropogenic sulphate and natural sea salt for Europe. Four different parameterisations were used to convert these concentration fields into fields of the number of Cloud Condensation Nuclei, which in turn was used to determine the cloud droplet effective radius. Different parameterisations gave rise to differences of up to 5 W m-2 in estimates for the AIE for Europe. An aerosol-cloud module was developed for the regional climate model RACMO2. The module was tested in an offline radiation module with a comprehensive set of measurements on cloud and aerosol parameters and surface radiative fluxes obtained at the meteo-tower of Cabauw. 2) The importance of the regional aerosol in cloud formation was studied. This was done in a large cloud chamber that allowed the use of a novel aerosol detection instrument developed in the project. It was found that the compound ammonium nitrate plays a major role in the cloud formation. Its origins are traffic and manure from agricultural origin. It has become the most abundant regional aerosol type and appears to be as active as the better known sulphate aerosol. This forms the basis of the parameterisation of its cloud forming properties.

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