At the Zeppelin Station on Svalbard, Stockholm University, Department of Environmental Science and Analytical Chemistry (ACES) measures trends in atmospheric carbon dioxide in background atmosphere (Table 4, #1.6, Table 5, ##3–4). In collaboration with NOAA/CMDL in Boulder, USA, air is regularly sampled in flasks for analysis of CO2, CH4, CO, 13CO2, H2, N2O, SF6, and 18O in CO2. At the top of the micrometeorological tower (102 m) at Norunda north of Uppsala, carbon dioxide and methane concentrations are also measured (Fig. 2, Table 5, #5). Other sites for CO2 measurements are the flux sites described below. Air samples are taken at 10 sites in northern Sweden for analysis of SO2, NO2, and surface-near ozone (Fig. 2, Table 4, #1.2) in the air- and precipitation chemistry network. At the Zeppelin Station on Svalbard, Stockholm University, Department of Environmental Science and Analytical Chemistry (ACES) also measures the amount and composition of aerosols in the background atmosphere. Measurements include particle concentration and size distribution, light absorption and scattering, and cloud residual properties (cloud residuals are the particles which took part in cloud droplet or ice crystal formation)