2017, Nichman, Leonid, Järvinen, Emma, Dorsey, James, Connolly, Paul, Duplissy, Jonathan, Fuchs, Claudia, Ignatius, Karoliina, Sengupta, Kamalika, Stratmann, Frank, Möhler, Ottmar, Schnaiter, Martin, Gallagher, Martin

Optical probes are frequently used for the detection of microphysical cloud particle properties such as liquid and ice phase, size and morphology. These properties can eventually influence the angular light scattering properties of cirrus clouds as well as the growth and accretion mechanisms of single cloud particles. In this study we compare four commonly used optical probes to examine their response to small cloud particles of different phase and asphericity. Cloud simulation experiments were conducted at the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at European Organisation for Nuclear Research (CERN). The chamber was operated in a series of multi-step adiabatic expansions to produce growth and sublimation of ice particles at super- and subsaturated ice conditions and for initial temperatures of -30, -40 and -50°C. The experiments were performed for ice cloud formation via homogeneous ice nucleation. We report the optical observations of small ice particles in deep convection and in situ cirrus simulations. Ice crystal asphericity deduced from measurements of spatially resolved single particle light scattering patterns by the Particle Phase Discriminator mark 2 (PPD-2K, Karlsruhe edition) were compared with Cloud and Aerosol Spectrometer with Polarisation (CASPOL) measurements and image roundness captured by the 3View Cloud Particle Imager (3V-CPI). Averaged path light scattering properties of the simulated ice clouds were measured using the Scattering Intensity Measurements for the Optical detectioN of icE (SIMONE) and single particle scattering properties were measured by the CASPOL. We show the ambiguity of several optical measurements in ice fraction determination of homogeneously frozen ice in the case where sublimating quasi-spherical ice particles are present. Moreover, most of the instruments have difficulties of producing reliable ice fraction if small aspherical ice particles are present, and all of the instruments cannot separate perfectly spherical ice particles from supercooled droplets. Correlation analysis of bulk averaged path depolarisation measurements and single particle measurements of these clouds showed higher R2 values at high concentrations and small diameters, but these results require further confirmation. We find that none of these instruments were able to determine unambiguously the phase of the small particles. These results have implications for the interpretation of atmospheric measurements and parametrisations for modelling, particularly for low particle number concentration clouds.

2019, Ehrlich, André, Wendisch, Manfred, Lüpkes, Christof, Buschmann, Matthias, Bozem, Heiko, Chechin, Dmitri, Clemen, Hans-Christian, Dupuy, Régis, Eppers, Olliver, Hartmann, Jörg, Herber, Andreas, Jäkel, Evelyn, Järvinen, Emma, Jourdan, Olivier, Kästner, Udo, Kliesch, Leif-Leonard, Köllner, Franziska, Mech, Mario, Mertes, Stephan, Neuber, Roland, Ruiz-Donoso, Elena, Schnaiter, Martin, Schneide, Johannes, Stapf, Johannes, Zanatta, Marco

The Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign was carried out north-west of Svalbard (Norway) between 23 May and 6 June 2017. The objective of ACLOUD was to study Arctic boundary layer and mid-level clouds and their role in Arctic amplification. Two research aircraft (Polar 5 and 6) jointly performed 22 research flights over the transition zone between open ocean and closed sea ice. Both aircraft were equipped with identical instrumentation for measurements of basic meteorological parameters, as well as for turbulent and radiative energy fluxes. In addition, on Polar 5 active and passive remote sensing instruments were installed, while Polar 6 operated in situ instruments to characterize cloud and aerosol particles as well as trace gases. A detailed overview of the specifications, data processing, and data quality is provided here. It is shown that the scientific analysis of the ACLOUD data benefits from the coordinated operation of both aircraft. By combining the cloud remote sensing techniques operated on Polar 5, the synergy of multi-instrument cloud retrieval is illustrated. The remote sensing methods were validated using truly collocated in situ and remote sensing observations. The data of identical instruments operated on both aircraft were merged to extend the spatial coverage of mean atmospheric quantities and turbulent and radiative flux measurement. Therefore, the data set of the ACLOUD campaign provides comprehensive in situ and remote sensing observations characterizing the cloudy Arctic atmosphere. All processed, calibrated, and validated data are published in the World Data Center PANGAEA as instrument-separated data subsets (Ehrlich et al., 2019b, https://doi.org/10.1594/PANGAEA.902603).

2016, Järvinen, Emma, Ignatius, Karoliina, Nichman, Leonid, Kristensen, Thomas B., Fuchs, Claudia, Hoyle, Christopher R., Höppel, Niko, Corbin, Joel C., Craven, Jill, Duplissy, Jonathan, Ehrhart, Sebastian, El Haddad, Imad, Frege, Carla, Gordon, Hamish, Jokinen, Tuija, Kallinger, Peter, Kirkby, Jasper, Kiselev, Alexei, Naumann, Karl-Heinz, Petäjä, Tuukka, Pinterich, Tamara, Prevot, Andre S.H., Saathoff, Harald, Schiebel, Thea, Sengupta, Kamalika, Simon, Mario, Slowik, Jay G., Tröstl, Jasmin, Virtanen, Annele, Vochezer, Paul, Vogt, Steffen, Wagner, Andrea C., Wagner, Robert, Williamson, Christina, Winkler, Paul M., Yan, Chao, Baltensperger, Urs, Donahue, Neil M., Flagan, Rick C., Gallagher, Martin, Hansel, Armin, Kulmala, Markku, Stratmann, Frank, Worsnop, Douglas R., Möhler, Ottmar, Leisner, Thomas, Schnaiter, Martin

Under certain conditions, secondary organic aerosol (SOA) particles can exist in the atmosphere in an amorphous solid or semi-solid state. To determine their relevance to processes such as ice nucleation or chemistry occurring within particles requires knowledge of the temperature and relative humidity (RH) range for SOA to exist in these states. In the Cosmics Leaving Outdoor Droplets (CLOUD) experiment at The European Organisation for Nuclear Research (CERN), we deployed a new in situ optical method to detect the viscous state of α-pinene SOA particles and measured their transition from the amorphous highly viscous state to states of lower viscosity. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical particles at relative humidities near the deliquescence point. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to a spherical shape was observed as the RH was increased to between 35 % at −10 °C and 80 % at −38 °C, confirming previous calculations of the viscosity-transition conditions. Consequently, α-pinene SOA particles exist in a viscous state over a wide range of ambient conditions, including the cirrus region of the free troposphere. This has implications for the physical, chemical, and ice-nucleation properties of SOA and SOA-coated particles in the atmosphere.

2018, Andreae, Meinrat O., Afchine, Armin, Albrecht, Rachel, Holanda, Bruna Amorim, Artaxo, Paulo, Barbosa, Henrique M. J., Borrmann, Stephan, Cecchini, Micael A., Costa, Anja, Dollner, Maximilian, Fütterer, Daniel, Järvinen, Emma, Jurkat, Tina, Klimach, Thomas, Konemann, Tobias, Knote, Christoph, Krämer, Martina, Krisna, Trismono, Machado, Luiz A. T., Mertes, Stephan, Minikin, Andreas, Pöhlker, Christopher, Pöhlker, Mira L., Pöschl, Ulrich, Rosenfeld, Daniel, Sauer, Daniel, Schlager, Hans, Schnaiter, Martin, Schneider, Johannes, Schulz, Christiane, Spanu, Antonio, Sperling, Vinicius B., Voigt, Christiane, Walser, Adrian, Wang, Jian, Weinzierl, Bernadett, Wendisch, Manfred, Ziereis, Helmut

Airborne observations over the Amazon Basin showed high aerosol particle concentrations in the upper troposphere (UT) between 8 and 15ĝ€km altitude, with number densities (normalized to standard temperature and pressure) often exceeding those in the planetary boundary layer (PBL) by 1 or 2 orders of magnitude. The measurements were made during the German–Brazilian cooperative aircraft campaign ACRIDICON–CHUVA, where ACRIDICON stands for Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems and CHUVA is the acronym for Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (global precipitation measurement), on the German High Altitude and Long Range Research Aircraft (HALO). The campaign took place in September–October 2014, with the objective of studying tropical deep convective clouds over the Amazon rainforest and their interactions with atmospheric trace gases, aerosol particles, and atmospheric radiation.

Aerosol enhancements were observed consistently on all flights during which the UT was probed, using several aerosol metrics, including condensation nuclei (CN) and cloud condensation nuclei (CCN) number concentrations and chemical species mass concentrations. The UT particles differed sharply in their chemical composition and size distribution from those in the PBL, ruling out convective transport of combustion-derived particles from the boundary layer (BL) as a source. The air in the immediate outflow of deep convective clouds was depleted of aerosol particles, whereas strongly enhanced number concentrations of small particles (< 90ĝ€nm diameter) were found in UT regions that had experienced outflow from deep convection in the preceding 5–72ĝ€h. We also found elevated concentrations of larger (> 90ĝ€nm) particles in the UT, which consisted mostly of organic matter and nitrate and were very effective CCN.

Our findings suggest a conceptual model, where production of new aerosol particles takes place in the continental UT from biogenic volatile organic material brought up by deep convection and converted to condensable species in the UT. Subsequently, downward mixing and transport of upper tropospheric aerosol can be a source of particles to the PBL, where they increase in size by the condensation of biogenic volatile organic compound (BVOC) oxidation products. This may be an important source of aerosol particles for the Amazonian PBL, where aerosol nucleation and new particle formation have not been observed. We propose that this may have been the dominant process supplying secondary aerosol particles in the pristine atmosphere, making clouds the dominant control of both removal and production of atmospheric particles.

2019, Wendisch, Manfred, Macke, Andreas, Ehrlich, André, Lüpkes, Christof, Mech, Mario, Chechin, Dmitry, Dethloff, Klaus, Velasco, Carola Barrientos, Bozem, Heiko, Brückner, Marlen, Clemen, Hans-Christian, Crewell, Susanne, Donth, Tobias, Dupuy, Regis, Ebell, Kerstin, Egerer, Ulrike, Engelmann, Ronny, Engler, Christa, Eppers, Oliver, Gehrmann, Martin, Gong, Xianda, Gottschalk, Matthias, Gourbeyre, Christophe, Griesche, Hannes, Hartmann, Jörg, Hartmann, Markus, Heinold, Bernd, Herber, Andreas, Herrmann, Hartmut, Heygster, Georg, Hoor, Peter, Jafariserajehlou, Soheila, Jäkel, Evelyn, Järvinen, Emma, Jourdan, Olivier, Kästner, Udo, Kecorius, Simonas, Knudsen, Erlend M., Köllner, Franziska, Kretzschmar, Jan, Lelli, Luca, Leroy, Delphine, Maturilli, Marion, Mei, Linlu, Mertes, Stephan, Mioche, Guillaume, Neuber, Roland, Nicolaus, Marcel, Nomokonova, Tatiana, Notholt, Justus, Palm, Mathias, van Pinxteren, Manuela, Quaas, Johannes, Richter, Philipp, Ruiz-Donoso, Elena, Schäfer, Michael, Schmieder, Katja, Schnaiter, Martin, Schneider, Johannes, Schwarzenböck, Alfons, Seifert, Patric, Shupe, Matthew D., Siebert, Holger, Spreen, Gunnar, Stapf, Johannes, Stratmann, Frank, Vogl, Teresa, Welti, André, Wex, Heike, Wiedensohler, Alfred, Zanatta, Marco, Zeppenfeld, Sebastian

Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC) project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns.

2016, Ignatius, Karoliina, Kristensen, Thomas B., Järvinen, Emma, Nichman, Leonid, Fuchs, Claudia, Gordon, Hamish, Herenz, Paul, Hoyle, Christopher R., Duplissy, Jonathan, Garimella, Sarvesh, Dias, Antonio, Frege, Carla, Höppel, Niko, Tröstl, Jasmin, Wagner, Robert, Yan, Chao, Amorim, Antonio, Baltensperger, Urs, Curtius, Joachim, Donahue, Neil M., Gallagher, Martin W., Kirkby, Jasper, Kulmala, Markku, Möhler, Ottmar, Saathoff, Harald, Schnaiter, Martin, Dulac, François, Tomé, Antonio, Virtanen, Annele, Worsnop, Douglas, Stratmann, Frank

There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate heterogeneous ice nucleation and thus influence cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from −38 to −10 °C at 5–15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between −39.0 and −37.2 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nucleating particle budget.

2016, Nichman, Leonid, Fuchs, Claudia, Järvinen, Emma, Ignatius, Karoliina, Höppel, Niko Florian, Dias, Antonio, Heinritzi, Martin, Simon, Mario, Tröstl, Jasmin, Wagner, Andrea Christine, Wagner, Robert, Williamson, Christina, Yan, Chao, Connolly, Paul James, Dorsey, James Robert, Duplissy, Jonathan, Ehrhart, Sebastian, Frege, Carla, Gordon, Hamish, Hoyle, Christopher Robert, Kristensen, Thomas Bjerring, Steiner, Gerhard, McPherson Donahue, Neil, Flagan, Richard, Gallagher, Martin William, Kirkby, Jasper, Möhler, Ottmar, Saathoff, Harald, Schnaiter, Martin, Stratmann, Frank, Tomé, António

Cloud microphysical processes involving the ice phase in tropospheric clouds are among the major uncertainties in cloud formation, weather, and general circulation models. The detection of aerosol particles, liquid droplets, and ice crystals, especially in the small cloud particle-size range below 50 μm, remains challenging in mixed phase, often unstable environments. The Cloud Aerosol Spectrometer with Polarization (CASPOL) is an airborne instrument that has the ability to detect such small cloud particles and measure the variability in polarization state of their backscattered light. Here we operate the versatile Cosmics Leaving OUtdoor Droplets (CLOUD) chamber facility at the European Organization for Nuclear Research (CERN) to produce controlled mixed phase and other clouds by adiabatic expansions in an ultraclean environment, and use the CASPOL to discriminate between different aerosols, water, and ice particles. In this paper, optical property measurements of mixed-phase clouds and viscous secondary organic aerosol (SOA) are presented. We report observations of significant liquid–viscous SOA particle polarization transitions under dry conditions using CASPOL. Cluster analysis techniques were subsequently used to classify different types of particles according to their polarization ratios during phase transition. A classification map is presented for water droplets, organic aerosol (e.g., SOA and oxalic acid), crystalline substances such as ammonium sulfate, and volcanic ash. Finally, we discuss the benefits and limitations of this classification approach for atmospherically relevant concentrations and mixtures with respect to the CLOUD 8–9 campaigns and its potential contribution to tropical troposphere layer analysis.