[1] Ground-based measurements of total aerosol optical depth (AOD), e.g., tropospheric and stratospheric aerosol, have been established at the Koldewey station in Ny-Å lesund, Spitzbergen (Norway, 78.95°N, 11.93°E), since 1991. The basic instrumentation is a multichannel photometer using sunlight. New instruments have been developed to extend the measurement period to polar night. The new instruments are a Sun and Moon photometer (1995) and a star photometer (1996). The instruments and applied methods for aerosol optical depth retrieval for Sun, Moon, and star measurements are briefly discussed. The year-round measurements made it possible to study in detail the interannual and seasonal variations of total AOD in the Arctic. The seasonal variation and the long-term trend of tropospheric aerosol optical depth are discussed, taking into account the stratospheric AOD measured by the Stratospheric Aerosol and Gas Experiment (SAGE II). The lowest tropospheric aerosol optical depth values occur in late summer and fall. Each year, strong Arctic haze events were recorded not only during spring but also in late winter as the first star photometer measurements clearly show. Five-day backward trajectories were used to analyze possible sources for high tropospheric AOD. Elevated tropospheric aerosol optical depth appears for northeasterly, easterly, or westerly winds. Finally, the long-term changes of tropospheric AOD have been assessed. A small positive trend of the tropospheric aerosol optical depth is found for the vicinity of Spitzbergen during the measurement period.

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Short spectral scan times of preferably less than 1 min in the UV region are an important prerequisite for modern spectroradiometers to reflect short-term solar irradiance radiation variations that can occur as a result of fast changes in cloud cover and/or cloud optical depth. Two different types of fast measuring spectroradiometers for solar UV irradiance are compared in a first field campaign: (1) the UV spectroradiometer on filter model basis (UV-SPRAFIMO) and (2) the modified version of the spectroradiometer SPECTRO 320D by Instrument Systems. The UV-SPRAFIMO instrument combines a filter radiometer with five narrowband (FWHMϷ2.0 to 2.7 nm) filters centered at fixed wavelengths in the UV-B and UV-A regions with an advanced neural network-based model. Up to 5 measurements/s can be taken concurrently in the five wavelength channels. After averaging the measurements over preselectable time intervals, the measured irradiances are converted by the neural network model into a full spectrum from 280 to 450 nm at arbitrary wavelength steps (у0.05 nm). The SPECTRO 320D spectroradiometer consists of a grating double monochromator with a cooled photomultiplier tube (PMT) receiver. The instrument version run by Deutscher Wetterdienst (DWD) is thermostatted and equipped with a Schreder type cosine diffuser as the entrance optics. A spectral scan from 290 to 450 nm with a selected 0.2-nm wavelength step takes less than 30 s. The two spectroradiometers are used in a field campaign at Izana (Tenerife Island) at a height of 2440 m above sea level (ASL) to compare measured spectral and integral values of solar irradiance. Results of that comparison and the instruments' characteristics are discussed. This first field comparison shows that due to the fast measurements regime, cloud effects on the measured spectra can be appreciably reduced. The campaign shows an acceptable agreement between the spectra measured by both instruments. It also reveals some issues for further improvements of the instrument design. Some of the improvements, such as a better cosine diffuser for the UV-SPRAFIMO and an output trigger signal used to record start and stop time of the spectral scan of the SPECTRO 320D instrument, are implemented after the field campaign.

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