Mass spectrometric measurements of relative humidity in contrails

Abstract

While contrail formation requires environments which are at least saturated with respect to ice, a majority of in situ observations in contrails shows values clearly below ice saturation. The reasons for that systematic deviation might be caused e.g. by measurement biases in absolute humidity and temperature or by contrail dynamics. The formation of vortices within a wing span behind the aircraft, their descent and the interaction with the ambient atmosphere may cause a decrease in saturation and spatially very inhomogeneous RHi fields in contrails. In order to improve the instrumental uncertainties, we further developed an accurate and fast method for airborne humidity measurements in the upper troposphere, the Atmospheric Ionization Mass Spectrometer (AIMS) with in-flight calibration. Inside its ion source ambient water vapor is directly ionized and the produced water vapor clusters H3O+(H2O)n are detected with the mass spectrometer as a direct measure of absolute ambient humidity. The AIMS instrument is calibrated in-flight with a catalytic water vapor calibration source. The time resolution of better than 4 Hz allows for humidity measurements on horizontal scales of 50 m. This new method differs significantly from previously used airborne hygrometers. Therefore our observations could provide a complementary piece in the ice-saturation puzzle and new insights to the set of, obviously difficult, humidity measurements in the upper troposphere. Here, we present water vapor measurements in young contrails performed during the CONCERT2011 (CONtrail and Cirrus ExpeRimenT) campaign onboard the DLR research aircraft Falcon. We distinguish between contrail, cirrus and clear sky observations using information on the extinction and the mixing ratio of reactive nitrogen (NOy) detected onboard the Falcon. We find differences in the distributions of relative humidity over ice (RHi) inside and out of contrails in clear-sky and cirrus environments. Further we investigate RHi profiles from different contrail producing passenger aircraft, 2 Boeing 777 and an Airbus 321. In contrast to most previous observations, our measurements generally confirm simulated RHi distributions in aircraft exhaust plumes.