Anthropogenic aerosols reflect solar radiation and therefore act as cooling agents. The perturbation ('forcing') of the radiation balance is so large that, according to estimates by IPCC, it could off-set the longwave radiative forcing by the man-made greenhouse gases in areas like Europe.
IPCC reports the uncertainty in the globally averaged aerosol forcing to be in the order of 3 W M-2. This uncertainty exceeds the magnitude of longwave forcing by the man-made greenhouse gases. Of national interest is the local/regional nature of aerosol forcing, which is due to the limited residence time of aerosols in the atmosphere. The claim by IPCC that sulphate is the only relevant man-made aerosol component is highly questionable for Europe. Whereas the value of the aerosol effect is so uncertain, predictions of the coming trend are even more problematic. This is due to the fact that the most important anthropogenic ingredients of the aerosol are not directly emitted, but made in the atmosphere.
A one-to-one relationship between the emission of the precursors and resulting aerosol seems rather unlikely given a non-linear historical trend. Therefore only a combined approach in which aerosol fields are modelled, and modelled fields, together with the calculated reduction in solar flux, should be tested. This approach should provide the tools to perform, with sufficient confidence, scenario calculation for the aerosol effect.
This report shows some of the attempts to improve the current large uncertainty in forcing of the radiation balance by anthropogenic aerosol (in Europe) by a combined experimental/modelling program. This is achieved by assessment of the major aerosol components involved and there sources, using detailed measurements in The Netherlands as a reference for model input and output.