Atmospheric aerosols originate from either naturally occurring processes or anthropogenic activity. They may influence the climate in two important ways, through direct and indirect effects. Direct effects refer to the scattering and absorption of radiation and their subsequent influence on planetary albedo and the climate system. Indirect effects refer to the increase in available cloud condensation nuclei (CCN) due to an increase in anthropogenic aerosol concentration. This is suspected to increase the cloud droplet number concentration and to decrease the mean diameter of cloud droplets for a constant cloud liquid water content. As a result, the increase in cloud albedo is predicted to influence the Earth's radiation budget. These effects are, however, poorly quantified, and more data are therefore needed for a more accurate modelling of the net effect of climate forcing by anthropogenic changes of the atmospheric composition. The World Meteorological Organization (WMO) has established the Global Atmosphere Watch (GAW) Program in order to ensure long-term measurements in order to detect secular trends and to help closing this gap. Concerning aerosols, the objective of the GAW program is to determine the spatio-temporal distribution of aerosol properties related to climate forcing and air quality up to multi-decadal time scales. Presently, GAW consists of some 20 Global stations, which cover different types of aerosols: Clean and polluted continental, marine, arctic, dust, biomass burning, and free troposphere. An overview will be given on the parameters relevant to the quantification of the aerosol impact on climate, illustrated by examples from a remote site in the Central European Alps.