The Global Atmosphere Watch Programme (GAW) at the Jungfraujoch High-Alpine Research Station (3580 m asl), Switzerland

Welcome to the GAW Homepage of the Laboratory for Atmospheric Chemistry at the Paul Scherrer Institute

Introduction to Climate Forcing by Aerosols

Aerosols may influence the atmosphere in two important ways, through direct and indirect effects (see Schwartz, 1998). 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 for a constant cloud liquid water content (LWC). As a result, the increase in cloud albedo is predicted to influence the Earth’s radiation budget. Cloud lifetimes and precipitation frequencies are also thought to be affected. Despite the uncertainty, it is believed that in regions with high anthropogenic aerosol concentrations, aerosol forcing may be of the same magnitude, but opposite in sign to the effect of all greenhouse gases.

The GAW Programme of the WMO

The GAW Programme has been established by the WMO (World Meteorological Organisation) to provide measurements, scientific assessments, and other information on changes in the global chemical composition and related physical characteristics of the atmosphere. 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 20 Global stations, which cover different types of aerosols: Clean and polluted continental, marine, arctic, dust, biomass burning, and free troposphere (see Figure 1). Most of these sites are located in pristine areas. Therefore, the Scientific Advisory Group (SAG) for Aerosols and Aerosol Optical Depth recently made additional recommendations that would provide better coverage of the polluted continental aerosol type. In addition, there are about 300 Regional stations where measurements are conducted closer to the source areas. In this way, Global stations may then serve as standards for Regional stations.

Figure 1. GAW Baseline Stations of the WMO Programme

Priorities have been set for measurements at the Global and Regional stations. Regional stations will be expected to measure optical depth, mass concentration in two size fractions, major chemical components in two size fractions, and the scattering coefficient. At Global stations, a larger number of measurements are envisaged, with optical depth, mass concentration in two size fractions, major chemical components in two size fractions, the scattering and hemispheric backscattering coefficient at various wavelengths, the absorption coefficient, aerosol number concentration, cloud condensation nuclei (CCN) at 0.5% supersaturation, as well as diffuse, global and direct solar radiation, and additional parameters to be measured intermittently. Data are managed by the World Data Center for Aerosols in Ispra (Italy), and it is planned to establish a World Calibration Center to ensure global data comparability. With the global coverage of the sites and the large number of aerosol parameters measured, the GAW aerosol program is believed to be an excellent candidate for ground truthing of satellite data.

The Central European Baseline Station

The European GAW Baseline station comprises the Zugspitze/Hohenpeissenberg (2962 m, Germany), the Jungfraujoch  (3454 m, Switzerland) and Sonnblick (3106 m, Austria) high-alpine stations (Figure 2). The National Weather Institutes ofall three countries have agreed to combine measurements under the DACH (Germany, Austria, Switzerland) agreement .  In order to obtain as extensive a database as possible, each station will focus on a particular physical/chemical aspect of climatically important aerosol parameters. Further information can be found on the EMPA website. A report of the first integration phase is due to be published early in 2000.

Figure 2. High-Alpine Stations which form the central European GAW Baseline Station under the DACH Agreement.