M. Krämer

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (86)172.21 Total impact

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    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. eISSN 1867-8610 = AMTD
    Atmospheric Measurement Techniques 04/2014; 7:3159-3251. · 3.21 Impact Factor
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    ABSTRACT: accurate measurements of water vapor at the low mixing ratios (< 10 ppm) encountered in the upper troposphere and lower stratosphere (UT/LS) has proven to be a significant analytical challenge evidenced by persistent disagreements between high-precision hygrometers. These disagreements have caused uncertainties in the description of the physical processes controlling dehydration of air in the tropical tropopause layer and entry of water into the stratosphere and have hindered validation of satellite water vapor retrievals. A 2011 airborne intercomparison of a large group of in situ hygrometers onboard the NASA WB-57F high-altitude research aircraft and balloons has provided an excellent opportunity to evaluate progress in the scientific community toward improved measurement agreement. In this work we intercompare the measurements from the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) and discuss the quality of agreement. Differences between values reported by the instruments were reduced in comparison to some prior campaigns but were nonnegligible and on the order of 20% (0.8 ppm). Our analysis suggests that unrecognized errors in the quantification of instrumental background for some or all of the hygrometers are a likely cause. Until these errors are understood, differences at this level will continue to somewhat limit our understanding of cirrus microphysical processes and dehydration in the tropical tropopause layer.
    01/2014; 119(4).
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    ABSTRACT: In January 2010 and December 2011, synopticscale polar stratospheric cloud (PSC) fields were probed during seven flights of the high-altitude research aircraft M- 55 Geophysica within the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interaction) and the ESSenCe (ESSenCe: ESA Sounder Campaign) projects. Particle size distributions in a diameter range between 0.46 and 40 μm were recorded by four different optical in situ instruments. Three of these particle instruments are based on the detection of forward-scattered light by single particles. The fourth instrument is a grayscale optical array imaging probe. Optical particle diameters of up to 35 μm were detected with particle number densities and total particle volumes exceeding previous Arctic measurements. Also, gas-phase and particle-bound NOy was measured, as well as water vapor concentrations.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2014; 14:12071-12120. · 5.51 Impact Factor
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    ABSTRACT: Bei chemischen Analysatoren auf Basis optischer Absorptionsmessungen stellt sich häufig die Problematik, dass die optische Messstelle von der Lichtquelle räumlich getrennt werden muss, um Störeinflüsse auf diese zu minimieren, Platzrestriktionen zu umgehen, Gefahrenbereiche zu meiden und eine Gasanalyse ohne Probennahme zu ermöglichen. Für die hochspezifische Wasserdampfmessung auf dem neuen deutschen Forschungsflugzeug HALO wurde das Konzept der direkt fasergekoppelten White-Zelle auf den simultanen Einsatz zweier optischer Wellenlängen (1.4 µm und 2.6 µm) erweitert. Ziel ist dabei, trotz einer bis zu 1000 km/h schnellen Durchströmung der offenen Messzelle, Gastemperaturen von -70 °C bis +50 °C und einem Außendruck von 100-1000 hPa die absolute Bestimmung des Wasserdampfgehaltes der Luft (1 bis 30000 ppmv) mit hoher Zeitauflösung auf der Außenhaut zu ermöglichen. Hierfür werden beiden Wellenlängen über zwei single mode-Lichtwellenleiter in eine gemeinsame White-Zelle ohne weitere Transferoptiken eingekoppelt. Auf der Tagung werden wir den Aufbau und erste Messdaten zur Beurteilung der optischen Qualität sowie deren Eignung für den Nachweis weitere Spezies wie z.B. CO2.diskutieren.
    114. Jahrestagung der Deutsche Gesellschaft für angewandte Optik e. V. (DGaO); 05/2013
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    ABSTRACT: EGU General Assembly 2013, Geophysical Research Abstracts, Vol. 15, EGU2013-9191, 2013
    Euopean Geosciences Uninon General Assembly, 7-12. April 2013; 04/2013
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    ABSTRACT: EGU General Assembly 2013, Geophysical Research Abstracts, Vol. 15, EGU2013-7311-3, 2013
    Euopean Geosciences Uninon General Assembly, 7-12. April 2013; 04/2013
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    ABSTRACT: EGU General Assembly 2013, Geophysical Research Abstracts, Vol. 15, EGU2013-9191, 2013
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    ABSTRACT: We present high-resolution measurements of water vapour, aerosols and clouds in the Arctic stratosphere in January and February 2010 carried out by in-situ instrumentation on balloon-sondes and high-altitude aircraft combined with satellite observations. The measurements provide unparalleled evidence of dehydration and rehydration due to gravitational settling of ice particles. An extreme cooling of the Arctic stratospheric vortex during the second half of January 2010 resulted in a rare synoptic-scale outbreak of ice PSCs (polar stratospheric clouds) detected remotely by the lidar aboard the CALIPSO satellite. The widespread occurrence of ice clouds was followed by sedimentation and consequent sublimation of ice particles, leading to vertical redistribution of water inside the vortex. A sequence of balloon and aircraft soundings with chilled mirror and Lyman-α hygrometers (CFH, FISH, FLASH) and backscatter sondes (COBALD) conducted in January 2010 within the LAPBIAT and RECONCILE campaigns captured various phases of this phenomenon: ice formation, irreversible dehydration and rehydration. Consistent observations of water vapour by these independent measurement techniques show clear signatures of irreversible dehydration of the vortex air by up to 1.6 ppmv in the 20-24 km altitude range and rehydration by up to 0.9 ppmv in a 1 km-thick layer below. Comparison with space-borne Aura MLS water vapour observations allow the spatiotemporal evolution of dehydrated air masses within the Arctic vortex to be derived and upscaled.
    Atmospheric Chemistry and Physics 01/2013; 13(5):14249-14295. · 4.88 Impact Factor
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    ABSTRACT: The MACPEX mission permitted observation of aerosol size distributions in the 4 to 1000 nm diameter range, cloud particles and water vapor in and around clouds in the mid-latitude upper troposphere. The NMASS consists of 5 condensation particle counters (cpcs) operating in parallel. The 5 cpcs have lower detection limits of approximately 4 nm, 8 nm, 16 nm, 32 nm and 50 nm. The FCAS measures the optical size of particles in the 100 nm to 1000 nm range. The data from these instruments are combined to provide size distributions from 4 to 1000 nm. Size distributions that show a local maximum in the smallest size range are evidence for recent new particle formation since the lifetime of particles in this size range is short due to coagulation. Size distributions showing evidence of new particle formation were observed inside and near clouds in the altitude range from 10 to 14 km. The cloud particles in these high clouds are expected to be ice. Care was taken to avoid interpreting shattering of ice on the aerosol inlets as new particles. The size distributions showing new particle formation are contrasted with size distributions that do not show new particle formation in and out of the clouds. Temperature, relative humidity and trace gas abundances in air parcels exhibiting new particle formation are contrasted with those in air parcels not showing new particle formation.
    AGU Annu. Mtg., San Francisco, CA; 12/2012
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    ABSTRACT: Ice clouds are known to be major contributors to radiative forcing in the Earth's atmosphere, yet describing their microphysical properties in climate models remains challenging. Among these properties, the ice water content (IWC) of cirrus clouds is of particular interest both because it is measurable and because it can be directly related to a number of other radiatively important variables such as extinction and effective radius. This study expands upon the work of Schiller et al. (2008), extending a climatology of IWC by combining datasets from several European and US airborne campaigns and ground-based lidar measurements over Jülich, Germany. The relationship between IWC and temperature is further investigated using the new merged dataset and probability distribution functions (PDFs). A PDF-based formulation allows for representation of not only the mean values of IWC, but also the variability of IWC within a temperature band. The IWC-PDFs are found to be bimodal over the whole cirrus temperature range, which might be attributed to different cirrus formation mechanisms such as heterogeneous and homogeneous freezing. The PDFs of IWC are further compared to distributions of cirrus ice crystal number and mass mean radius, which show that the general relationship between IWC and temperature appears to be influenced much more by particle number than by particle size.
    Atmospheric Chemistry and Physics 11/2012; 12(11):29443-29474. · 4.88 Impact Factor
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    P. Spichtinger, M. Krämer
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    ABSTRACT: The occurrence of high, persistent ice supersaturation inside and outside cold cirrus in the tropical tropopause layer (TTL) remains an enigma that is intensely debated as the "ice supersaturation puzzle". However, it was recently confirmed that observed supersaturations are consistent with very low ice crystal concentrations, which is incompatible with the idea that homogeneous freezing is the major method of ice formation in the TTL. Thus, the tropical tropopause "ice supersaturation puzzle" has become an "ice nucleation puzzle". To explain the low ice crystal concentrations, a number of mainly heterogeneous freezing methods have been proposed. Here, we reproduce in situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamic conditions in the TTL, namely the superposition of slow large-scale updraughts with high-frequency short waves. From the simulations, it follows that the full range of observed ice crystal concentrations can be explained when the model results of the scenarios are mixed for both heterogeneous/homogeneous and pure homogeneous ice formation occurring in very slow (<1 cm s-1) and faster (>1 cm s-1) large-scale updraughts. This statistical analysis shows that about 80% of TTL cirrus can be explained by "classical" homogeneous ice nucleation, while the remaining 20% stem from heterogeneous and homogeneous freezing occurring within the same environment. The mechanism limiting ice crystal production via homogeneous freezing in an environment full of gravity waves is the shortness of the gravity waves, which stalls freezing events before a higher ice crystal concentration can be formed.
    Atmospheric Chemistry and Physics 10/2012; 12(10):28109-28153. · 4.88 Impact Factor
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    ABSTRACT: FISH, a Lyman Alpha Fluorescence Hygrometer used on aircraft and balloons since 1990, participated in recent field and laboratory comparison experiments including the airborne campaign MACPEX out of Houston, USA, with the high altitude aircraft WB-57 and the extensive laboratory intercomparison campaign AQUAVIT at the AIDA chamber. The aim was to investigate the reasons for discrepancies of water measurements between different hygrometers in recent field experiments, which exceeded the stated uncertainties of these advanced hygrometers. Recent studies have shown that even 0.5 ppmv of uncertainty in those measurements matter in terms of radiative forcing and microphysics. However, the absolute discrepancies during campaigns in the 2010ies was up to 1 ppmv. During AquaVIT the discrepancies below 10 ppmv have shown the same tendency with lower magnitude. The relative discrepancies between the different hygrometers during MACPEX was the same as for previous experiments. Nevertheless, the absolute discrepancies may vary between individual instruments for different campaigns. In total they were in the range of 0.8 ppmv.
    04/2012;
  • Bulletin of the American Meteorological Society 02/2012; 93(2):Bulletin of the American Meteorological Society. · 11.57 Impact Factor
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    ABSTRACT: The frequency of occurrence of cirrus clouds and contrails, their life time, ice crystal size spectra and thus their radiative properties depend strongly on the ambient distribution of the relative humidity with respect to ice (RHice). Ice clouds do not form below a certain supersaturation and both cirrus and contrails need at least saturation conditions to persist over a longer period. Under subsaturated conditions, cirrus and contrails should dissipate. During the mid-latitude aircraft experiment CONCERT 2008 (CONtrail and Cirrus ExpeRimenT), RHice and ice crystals were measured in cirrus and contrails. Here, we present results from 2.3/1.7 h of observation in cirrus/contrails during 6 flights. Thin and subvisible cirrus with contrails mbedded therein have been detected frequently in a subsaturated environment. Nevertheless, ice crystals up to radii of 50 μm and larger, but with low number densities were often observed inside the contrails as well as in the cirrus. Analysis of the meteorological situation indicates that the crystals in the contrails were entrained from the thin/subvisible cirrus clouds, which emerged in frontal systems with low updrafts. From model simulations of cirrus evaporation times it follows that such thin/subvisible cirrus can exist for time periods of a couple of hours and longer in a subsaturated environment and thus may represent a considerable part of the cirrus coverage.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 05/2011; 11(2011):5853-5865. · 5.51 Impact Factor
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    ABSTRACT: In situ measurements of ice crystal size distributions in tropical upper troposphere/lower stratosphere (UT/LS) clouds were performed during the SCOUT-AMMA campaign overWest Africa in August 2006. The cloud properties were measured with a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP) operated aboard the Russian high altitude research aircraft M-55 Geophysica with the mission base in Ouagadougou, Burkina Faso. A total of 117 ice particle size distributions were obtained from the measurements in the vicinity of Mesoscale Convective Systems (MCS). Two to four modal lognormal size distributions were fitted to the average size distributions for different potential temperature bins. The measurements showed proportionately more large ice particles compared to former measurements above maritime regions. With the help of trace gas measurements of NO, NOy, CO2, CO, and O3 and satellite images, clouds in young and aged MCS outflow were identified. These events were observed at altitudes of 11.0 km to 14.2 km corresponding to potential temperature levels of 346K to 356 K. In a young outflow from a developing MCS ice crystal number concentrations of up to (8.3±1.6) cm−3 and rimed ice particles with maximum dimensions exceeding 1.5mm were found. A maximum ice water content of 0.05 gm−3 was observed and an effective radius of about 90 μm. In contrast the aged outflow events were more diluted and showed a maximum number concentration of 0.03 cm−3, an ice water content of 2.3×10−4 gm−3, an effective radius of about 18 μm, while the largest particles had a maximum dimension of 61 μm. Close to the tropopause subvisual cirrus were encountered four times at altitudes of 15 km to 16.4 km. The mean ice particle number concentration of these encounters was 0.01 cm−3 with maximum particle sizes of 130 μm, and the mean ice water content was about 1.4×10−4 gm−3. All known in situ measurements of subvisual tropopause cirrus are compared and an exponential fit on the size distributions is established for modelling purposes. A comparison of aerosol to ice crystal number concentrations, in order to obtain an estimate on how many ice particles may result from activation of the present aerosol, yielded low ratios for the subvisual cirrus cases of roughly one cloud particle per 30 000 aerosol particles, while for the MCS outflow cases this resulted in a high ratio of one cloud particle per 300 aerosol particles.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11(2011):5569-5590. · 5.51 Impact Factor
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    ABSTRACT: An overview is presented of airborne systems for in situ measurements of aerosol particles, clouds and radiation that are currently in use on research aircraft around the world. Description of the technology is at a level sufficient for introducing the basic principles of operation and an extensive list of references for further reading is given. A number of newer instruments that implement emerging technology are described and the review concludes with a description of some of the most important measurement challenges that remain. This overview is a synthesis of material from a reference book that is currently in preparation and that will be published in 2012 by Wiley.
    Atmospheric Research 01/2011; 102(2011):10-29. · 2.20 Impact Factor
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    ABSTRACT: One factor limiting the understanding of the climate impact from contrails and aircraft induced cloud modifications is the accurate determination of their optical depth. To this end, 14 contrails were sampled for 2756 s with instruments onboard the research aircraft Falcon during the CONCERT (CONtrail and Cirrus ExpeRimenT) campaign in November 2008. The young (<10 min old) contrails were produced by 9 commercial aircraft with weights of 47 to 508 t, among them the largest operating passenger aircraft, the Airbus A380. The contrails were observed at temperatures between 214 and 224 K and altitudes between 8.8 and 11.1 km. The measured mean in-contrail relative humidity with respect to ice was 89 ± 12%. Six contrails were observed in cloud free air, the others were embedded in thin cirrus clouds. The observed contrails exhibited a mean ice water content of 2 mg m−3 and had a mean number concentration of 117 cm−3 and effective radius of 2.9 μm assuming asphericle particles with an aspect ratio of 0.5. Probability density functions of the extinction, with a mean (median) of 1.2 (0.7) km−1, and of the optical depth τ, with a mean (median) of 0.27 (0.13), are derived from the in situ measurements and are likely representative for young contrails from the present-day commercial aircraft fleet at observation conditions. Radiative transfer estimates using the in-situ measured contrail optical depth lead to a year-2005 estimate of line-shaped contrail radiative forcing of 15.9 mWm−2 with an uncertainty range of 11.1–47.7 mWm−2.
    Geophysical Research Letters 01/2011; 38(2011-L11806):1-5. · 3.98 Impact Factor
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    ABSTRACT: Lineshaped contrails were detected with the research aircraft Falcon during the CONCERT – CONtrail and Cirrus ExpeRimenT – campaign in October/November 2008. The Falcon was equipped with a set of instruments to measure the particle size distribution, shape, extinction and chemical composition as well as trace gas mixing ratios of sulfur dioxide (SO2), reactive nitrogen and halogen species (NO, NOy, HNO3, HONO, HCl), ozone (O3) and carbon monoxide (CO). During 12 mission flights over Europe, numerous contrails, cirrus clouds and a volcanic aerosol layer were probed at altitudes between 8.5 and 11.6 km and at temperatures above 213 K. 22 contrails from 11 different aircraft were observed near and below ice saturation. The observed NO mixing ratios, ice crystal and soot number densities are compared to a process based contrail model. On 19 November 2008 the contrail from a CRJ-2 aircraft was penetrated in 10.1 km altitude at a temperature of 221 K. The contrail had mean ice crystal number densities of 125 cm−3 with effective radii reff of 2.6 μm. The presence of particles with r>50 μm in the less than 2 min old contrail suggests that natural cirrus crystals were entrained in the contrail. Mean HONO/NO (HONO/NOy) ratios of 0.037 (0.024) and the fuel sulfur conversion efficiency to H2SO4 (S#) of 2.9% observed in the CRJ-2 contrail are in the range of previous measurements in the gaseous aircraft exhaust. On 31 October 2010 aviation NO emissions could have contributed by more than 40% to the regional scale NO levels in the mid-latitude lowest stratosphere. The CONCERT observations help to better quantify the climate impact from contrails and will be used to investigate the chemical processing of trace gases on contrails.
    Atmospheric Chemistry and Physics. 01/2010; 10(2010):9039-9056.
  • 10th Conference on Cloud Physics and Atmospheric Radiation; 01/2010
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    ABSTRACT: Lineshaped contrails were detected with the research aircraft Falcon during the CONCERT – CONtrail and Cirrus ExpeRimenT – campaign in October/November 2008. Thereby the Falcon was equipped with a set of instruments to measure particle properties such as the particle size distribution, shape, extinction, chemical composition as well as trace gas concentrations of sulfur dioxide (SO2), reactive nitrogen and halogen species (NO, NOy, HNO3, HONO, HCl), ozone (O3) and carbon monoxide (CO). During 12 mission flights over Western Europe numerous contrails and cirrus clouds were probed at altitudes between 8.5 and 11.6 km and temperatures above 213 K. 22 contrails from 11 different aircraft were observed near and below ice saturation. The observed NO mixing ratios, ice crystal and soot number densities are compared to a process based contrail model. Further we investigate in detail the contrail from a CRJ-2 aircraft detected on 19 November 2008 in 10.1 km altitude. The contrail with an age of 1 to 2 min had average ice crystal concentrations of 128 cm−3 in the size range 0.4y) ratios of 0.037 (0.024) are in the range of previous measurements in the gaseous exhaust. With HONO/NO ratio we can derive a lower limit of the conversion efficiency (εS) of fuel sulfur into H2SO4 of 2.9%. In addition to individual contrails and aircraft plumes, regional stratospheric NO enhancements were detected in the lowest stratosphere. Simulations show that aviation NO emissions could have contributed by more than 40% to the observed NO levels. Besides contrails, also cirrus clouds and a volcanic aerosol layer were measured during the CONCERT campaign. The observations serve to investigate the chemical processing of trace gases on contrails and and help to better quantify the climate impact from contrails.
    Atmospheric Chemistry and Physics 01/2010; · 5.51 Impact Factor

Publication Stats

630 Citations
172.21 Total Impact Points

Institutions

  • 2000–2014
    • Forschungszentrum Jülich
      • Institute of Energy and Climate Research (IEK)
      Jülich, North Rhine-Westphalia, Germany
  • 2003
    • Universität Bremen
      • Institut für Umweltphysik (IUP)
      Bremen, Bremen, Germany
  • 2002
    • University of Szeged
      • Department of Optics and Quantum Electronics
      Szeged, Csongrad megye, Hungary
  • 1992–2002
    • Johannes Gutenberg-Universität Mainz
      • Institute for Atmospheric Physics
      Mayence, Rheinland-Pfalz, Germany