The Arctic is an ideal region for analyzing satellite measurements. Most of the instruments suitable for the characterization of aerosols and trace gases in the troposphere are carried on polar orbiting satellites and their tracks are most densely packed at the poles. Therefore, overpasses over a fixed location are much more frequent close to the poles than at lower latitudes (see Figure 9, for an example), providing excellent opportunities for comparisons with ground-based or aircraft measurements in the Arctic . The Arctic is also special because of the high albedo of ice and snow surfaces, which can enhance signals in DOAS-type retrievals of trace gas columns, but also makes detection of light scattering by aerosols more difficult.

Meteorological satellite data. Detection of cloud, water vapour and surface features at high temporal resolution (< 1 hour) in the mid-latitudes at all longitudes is made possible by six geostationary satellites positioned around the equator. However, north of the Arctic Circle the geostationary images are distorted by the curvature of the Earth’s surface. Therefore, at high latitudes polar orbiting weather satellites must be relied upon to provide images of clouds and surface features at high temporal resolution. While each of the four NOAA Polar-orbiting Operational Environmental Satellites (POES) passes over the Arctic region once every 102 minutes, their inclined orbit, coupled with the rotation of the Earth means the portion of the Arctic viewed by a particular satellite changes with each orbit. The inconsistency of the viewable region of the Arctic makes it difficult to track meteorological features associated with air pollution transport. For POLARCAT, merged visible, IR, and water vapour products using images from the Terra and Aqua satellites, all four NOAA/POES satellites and geostationary satellites will be created.

Aerosol and trace gas satellite measurements. A wide range of satellite data on the composition of both the troposphere and the stratosphere will be available for POLARCAT. Table 2 has been compiled from information given by people involved in the POLARCAT consortium.