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The effect of organic coating on the heterogeneous ice nucleation (IN) efficiency of dust particles was investigated at simulated cirrus cloud conditions in the AIDA cloud chamber of Forschungszentrum Karlsruhe. Arizona test dust (ATD) and the clay mineral illite were used as surrogates for atmospheric dust aerosols. The dry dust samples were dispersed into a 3.7 m 3 aerosol vessel and either directly transferred into the 84 m 3 cloud simulation chamber or coated before with the semi-volatile products from the reaction of α-pinene with ozone in order to mimic the coating of atmospheric dust particles with secondary organic aerosol (SOA) substances. The ice-active fraction was measured in AIDA expansion cooling experiments as a function of the relative humidity with respect to ice, RHi, in the temperature range from 205 to 210 K. Almost all uncoated dust particles with diameters between 0.1 and 1.0 μm acted as efficient deposition mode ice nuclei at RHi between 105 and 120%. This high ice nucleation efficiency was markedly suppressed by coating with SOA. About 20% of the ATD particles coated with a SOA mass fraction of 17 wt% were ice-active at RHi between 115 and 130%, and only 10% of the illite particles coated with an SOA mass fraction of 41 wt% were ice-active at RHi between 160 and 170%. Only a minor fraction of pure SOA particles were ice-active at RHi between 150 and 190%. Strong IN activation of SOA particles was observed only at RHi above 200%, which is clearly above water saturation at the given temperature. The IN suppression and the shift of the heterogeneous IN onset to higher RHi seem to depend on the coating thickness or the fractional surface coverage of the mineral particles. The results indicate that the heterogeneous ice nucleation potential of atmospheric mineral particles may also be suppressed if they are coated with secondary organics.

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.

We report the first application of a vertical-cavity surfaceemitting laser (VCSEL) for calibration- and sampling-free, high-speed, in situ H2O concentration measurements in IC engines using direct TDLAS (tunable diode laser absorption spectroscopy). Measurements were performed in a single-cylinder research engine operated under motored conditions with a time resolution down to 100 μs (i.e., 1.2 crank angle degrees at 2000 rpm). Signal-to-noise ratios (1σ) up to 29 were achieved, corresponding to a H2O precision of 0.046 vol.% H2O or 39 ppm · m. The modulation frequency dependence of the performance was investigated at different engine operating points in order to quantify the advantages of VCSEL against DFB lasers.