This thesis discusses properties of ambient aerosols in the Netherlands which are controlling the magnitude of the local aerosol radiative forcing. Anthropogenic aerosols influence climate by changing the radiative transfer through the atmosphere via two effects, one is direct and a second is indirect. Due to the scattering of solar light on aerosol particles the Earth surface receives less radiation and thus cools, which is called the direct aerosol effect.The indirect effect includes processes by which aerosols influence the radiation balance indirectly - via clouds. The indirect effect includes mechanisms by which anthropogenic aerosol particles enhance the reflectivity and prolong the life time of marine stratoform clouds by increasing their droplet number concentration. Anthropogenic aerosols were demonstrated to have a considerable cooling effect on climate, comparable in magnitude to that of the greenhouse gases, but opposite in sign. However, calculation of the aerosol radiative forcing is much more complex than that for the green house gases. This results in high uncertainties in the estimates of the effects of anthropogenic aerosols on climate.Both types of the aerosol radiative forcing and their magnitude, as estimated by the Intergovernmental Panel on Climate Change (IPCC), are discussed in Chapter 1. The aerosol radiative forcing is a regional phenomenon because of the limited life time of aerosols in the atmosphere. Thus, the aerosol effects should be assessed locally.In Chapter 2 chemical composition of local ambient aerosols in the size range relevant to the radiative forcing is discussed. It is shown that ammonium nitrate, which is omitted by the IPCC, is a major contributor to the direct, and perhaps to the indirect forcing. The contribution of this compound to the local direct forcing is equal to that of anthropogenic sulfate aerosols. Measurements of ammonium nitrate content in submicrometer aerosol are subject to artifacts due to the volatility of this compound. Automated techniques which ...