Abstract. Upper tropospheric observations outside and inside of cirrus clouds indicate water vapour mixing ratios sometimes exceeding water saturation. Relative humidities over ice (RH ice ) of up to and more than 200% have been reported from aircraft and balloon measurements in recent years.From these observations a lively discussion continues on whether there is a lack of understanding of ice cloud microphysics or whether the water measurements are tainted with large uncertainties or flaws.Here, RH ice in clear air and in ice clouds is investigated. Strict quality-checked aircraft in situ observations of RH ice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183-240 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super-or subsaturations inside of cirrus are frequently observed at low temperatures (<205 K) in our field data set. To explain persistent RH ice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis -using conventional microphysics -of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus can possibly be explained by unexpected, frequent very low ice crystal numbers that could scarcely be caused by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers.Correspondence to: M. Krämer (m.kraemer@fz-juelich.de) Thus, our lack of understanding of the high supersaturations, with implications for the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.

Abstract. The AquaVIT-1 intercomparison of atmospheric water vapor measurement techniques was conducted at the aerosol and cloud simulation chamber AIDA (Aerosol Interaction and Dynamics in the Atmosphere) at the Karlsruhe Institute of Technology, Germany, in October 2007. The overall objective was to intercompare state-of-the-art and prototype atmospheric hygrometers with each other and with independent humidity standards under controlled conditions. This activity was conducted as a blind intercomparison with coordination by selected referees. The effort was motivated by persistent discrepancies found in atmospheric measurements involving multiple instruments operating on research aircraft and balloon platforms, particularly in the upper troposphere and lower stratosphere, where water vapor reaches Published by Copernicus Publications on behalf of the European Geosciences Union. D. W. Fahey et al.: The AquaVIT-1 water vapor intercomparisonits lowest atmospheric values (less than 10 ppm). With the AIDA chamber volume of 84 m 3 , multiple instruments analyzed air with a common water vapor mixing ratio, by extracting air into instrument flow systems, by locating instruments inside the chamber, or by sampling the chamber volume optically. The intercomparison was successfully conducted over 10 days during which pressure, temperature, and mixing ratio were systematically varied (50 to 500 hPa, 185 to 243 K, and 0.3 to 152 ppm). In the absence of an accepted reference instrument, the absolute accuracy of the instruments was not established. To evaluate the intercomparison, the reference value was taken to be the ensemble mean of a core subset of the measurements. For these core instruments, the agreement between 10 and 150 ppm of water vapor is considered good with variation about the reference value of about ±10 % (±1σ ). In the region of most interest between 1 and 10 ppm, the core subset agreement is fair with variation about the reference value of ±20 % (±1σ ). The upper limit of precision was also derived for each instrument from the reported data. The implication for atmospheric measurements is that the substantially larger differences observed during in-flight intercomparisons stem from other factors associated with the moving platforms or the non-laboratory environment. The success of AquaVIT-1 provides a template for future intercomparison efforts with water vapor or other species that are focused on improving the analytical quality of atmospheric measurements on moving platforms.

show clear evidence for dehydration inside the polar vortex. At 30-50 hPa, total hydrogen is reduced by approximately 0.5 ppmv. This phenomenon is apparent in all five in situ balloon observations of this period; therefore it is probable that dehydration occurred over extended regions and a long period of this winter which was characterized by a wellconfined vortex and low stratospheric temperatures. At altitudes below 50 hPa, where dehydration was strongest in previous Arctic observations and in the austral spring, total hydrogen values (2ÁCH 4 + H 2 O + H 2 ) were similar to those found in Arctic profiles from other years where there was no dehydration and to those found at midlatitudes. In some of the dehydrated air masses, small solid particles were found whose crystallization might be connected to the earlier formation of ice particles. Back trajectory calculations for the January observations indicate that the probed air masses had experienced temperatures below the ice frost point in a synoptic-scale cold region several days before the observations. Most likely, the air was dehydrated there. In addition, temperatures in these air masses dropped below ice saturation several hours prior to the observations in the lee of the Scandinavian mountain ridge. For the March measurements, no ice saturation was apparent in the recent history of the air masses, again indicating that dehydration in the Arctic winter 1999/2000 was not a local phenomenon.

Abstract. The AquaVIT-1 Intercomparison of Atmospheric Water Vapor Measurement Techniques was conducted at the aerosol and cloud simulation chamber AIDA at the Karlsruhe Institute of Technology, Germany, in October 2007. The overall objective was to intercompare state-of-the-art and prototype atmospheric hygrometers with each other and with independent humidity standards under controlled conditions. This activity was conducted as a blind intercomparison with coordination by selected referees. The effort was motivated by persistent discrepancies found in atmospheric measurements involving multiple instruments operating on research aircraft and balloon platforms, particularly in the upper troposphere and lower stratosphere where water vapor reaches its lowest atmospheric values (less than 10 ppm). With the AIDA chamber volume of 84 m3, multiple instruments analyzed air with a common water vapor mixing ratio, either by extracting air into instrument flow systems, locating instruments inside the chamber, or sampling the chamber volume optically. The intercomparison was successfully conducted over 10 days during which pressure, temperature, and mixing ratio were systematically varied (50 to 500 hPa, 185 to 243 K, and 0.3 to 152 ppm). In the absence of an accepted reference instrument, the reference value was taken to be the ensemble mean of a core subset of the measurements. For these core instruments, the agreement between 10 and 150 ppm of water vapor is considered good with variation about the reference value of about ±10% (±1σ). In the region of most interest between 1 and 10 ppm, the core subset agreement is fair with variation about the reference value of ±20% (±1σ). The upper limit of precision was also derived for each instrument from the reported data. These results indicate that the core instruments, in general, have intrinsic skill to determine unknown water vapor mixing ratios with an accuracy of at least ±20%. The implication for atmospheric measurements is that the substantially larger differences observed during in-flight intercomparisons stem from other factors associated with the moving platforms or the non-laboratory environment. The success of AquaVIT-1 provides a template for future intercomparison efforts with water vapor or other species that are focused on improving the analytical quality of atmospheric measurements on moving platforms.

Different whole air samplers were flown on large balloons launched from Kiruna (67.9°N; 21.1°E) in the period from 30 November 1991 until 20 March 1992. Thirteen vertical profiles of the N2O mixing ratio were obtained from the analyses of the stratospheric air samples collected at altitudes between about 10 km and 31 km. The series of profile observations illustrates the temporal variation of the vertical structure of the Arctic polar vortex over Northern Scandinavia during EASOE. Already by mid December the N2O mixing ratios observed at the 600 K isentropic level (about 24–25 km) were as low as 20 ppbv, due to the effect of diabatic subsidence. In December subsidence rates of about 100–180 m/day were observed. Dynamic wave activity at the edge of the vortex led to significant sideways erosion at lower altitudes. However, inside the vortex, at altitudes above 600 K, N2O mixing ratios of less than 10 ppbv were observed until the end of March.