Aerosol; Cycle and Influence on the Radiation Balance MEasurement and MOdelling of the reduction of Radiation by Aerosol (MEMORA)

Abstract

De drijvende kracht in de door de mens veroorzaakte klimaatverandering is de verandering in de stralingsbalans. Deze wordt niet alleen beinvloed door de broeikasgassen maar ook door aerosolen (fijne in lucht zwevende deeltjes). De aerosolen worden ook door de mens gemaakt, vooral via fossiele verbranding. Aerosolen kaatsen een deel van het zonlicht terug. Dit leidt tot een verminderde instraling. Het aerosol-effect van door de mens gegenereerde aerosolen werkt dus precies omgekeerd aan het 'normale' broeikaseffect van de gassen die de langgolvige aardse uitstraling tegenhouden. Door verminderde instraling warmt de aarde minder op. De invloed van de antropogene aerosolen is groot: De extra door de mens geproduceerde broeikasgassen geven een verandering in de stralingsbalans van ongeveer 2.5W m-2, volgens IPCC. De geschatte tegengestelde bijdrage van door de mens gemaakte aerosol deeltjes is, als mondiaal gemiddelde -0.8 W m-2, maar met een onzekerheid van minstens een factor twee. De onzekerheid in de verstoring van de stralingsbalans ('forcering') door aerosolen vormt momenteel een van de grootste onzekerheden in de klimaatforcering over de industriele periode.<br>

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.<br>