Quantification of the Direct Aerosol Effect Using Ground Based Radiative Flux Measurements at Cabauw (The Netherlands)

Aerosols affect the Earth's climate profoundly by changing the amount of sunlight at the surface due to both, the direct and indirect aerosol effects. Mishchenko et al. (2007) showed that the aerosol optical thickness (AOT) decreased on a global scale since the beginning of the 1990s, even though regional aerosol column densities vary considerably due to both, natural events and especially anthropogenic activities. The decreasing global AOT will likely increase the surface solar radiative heat flux which eventually reduces the counterbalance to greenhouse gas warming and potentially contribute to the warming trend of the last decade. Using surface solar radiative flux measurements and AOTs derived from sun-photometer measurements at Cabauw we show the magnitude of the aerosol effect for cloud free conditions. This so-called direct aerosol effect is quantified by subtracting modelled radiative fluxes for cloud-free and aerosol-free conditions from radiative fluxes as measured at the BSRN station Cabauw (The Netherlands). To exclude measurements where clouds are visible above the horizon we applied several (longwave and shortwave) cloud detection algorithms and examined sky images taken by an 2-π hemispheric camera. The mean direct aerosol effect was in 2006 about -30 W/m2. During the same period the direct aerosol effect accounted for a reduction of the surface solar radiative flux on average by about 120 W/m2 per unit AOT. The poster presentation will contain an uncertainty analysis of the direct aerosol effect and an airmass trajectory analysis to determine the aerosol origin and some information about its composition. From these results we will derive an aerosol climatology for The Netherlands, which is useful for validation purposes of the regional climate model RACMO.