Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Zamin A. Kanji x
  • All content x
Clear All Modify Search
André Welti, Zamin A. Kanji, F. Lüönd, Olaf Stetzer, and Ulrike Lohmann

Abstract

To identify the temperature and humidity conditions at which different ice nucleation mechanisms are active, the authors conducted experiments on 200-, 400-, and 800-nm size-selected kaolinite particles, exposing them to temperatures between 218 and 258 K and relative humidities with respect to ice (RHi) between 100% and 180%, including the typical conditions for cirrus and mixed-phase-cloud formation. Measurements of the ice active particle fraction as a function of temperature and relative humidity with respect to ice are reported. The authors find enhanced activated fractions when water saturation is reached at mixed-phase-cloud temperatures between 235 and 241 K and a distinct increase in the activated fraction below 235 K at conditions below water saturation. To provide a functional description of the observed ice nucleation mechanisms, the experimental results are analyzed by two different particle-surface models within the framework of classical nucleation theory. Describing the ice nucleation activity of kaolinite particles by assuming deposition nucleation to be the governing mechanism below water saturation was found to be inadequate to represent the experimental data in the whole temperature range investigated. The observed increase in the activated fraction below water saturation and temperatures below 235 K corroborate the assumption that an appreciable amount of adsorbed or capillary condensed water is present on kaolinite particles, which favors ice nucleation.

Full access
Daniel J. Cziczo, Luis Ladino, Yvonne Boose, Zamin A. Kanji, Piotr Kupiszewski, Sara Lance, Stephan Mertes, and Heike Wex

Abstract

It has been known that aerosol particles act as nuclei for ice formation for over a century and a half (see Dufour). Initial attempts to understand the nature of these ice nucleating particles were optical and electron microscope inspection of inclusions at the center of a crystal (see Isono; Kumai). Only within the last few decades has instrumentation to extract ice crystals from clouds and analyze the residual material after sublimation of condensed-phase water been available (see Cziczo and Froyd). Techniques to ascertain the ice nucleating potential of atmospheric aerosols have only been in place for a similar amount of time (see DeMott et al.). In this chapter the history of measurements of ice nucleating particles, both in the field and complementary studies in the laboratory, are reviewed. Remaining uncertainties and artifacts associated with measurements are described and suggestions for future areas of improvement are made.

Full access
Zamin A. Kanji, Luis A. Ladino, Heike Wex, Yvonne Boose, Monika Burkert-Kohn, Daniel J. Cziczo, and Martina Krämer

Abstract

Ice particle formation in tropospheric clouds significantly changes cloud radiative and microphysical properties. Ice nucleation in the troposphere via homogeneous freezing occurs at temperatures lower than −38°C and relative humidity with respect to ice above 140%. In the absence of these conditions, ice formation can proceed via heterogeneous nucleation aided by aerosol particles known as ice nucleating particles (INPs). In this chapter, new developments in identifying the heterogeneous freezing mechanisms, atmospheric relevance, uncertainties, and unknowns about INPs are described. The change in conventional wisdom regarding the requirements of INPs as new studies discover physical and chemical properties of these particles is explained. INP sources and known reasons for their ice nucleating properties are presented. The need for more studies to systematically identify particle properties that facilitate ice nucleation is highlighted. The atmospheric relevance of long-range transport, aerosol aging, and coating studies (in the laboratory) of INPs are also presented. Possible mechanisms for processes that change the ice nucleating potential of INPs and the corresponding challenges in understanding and applying these in models are discussed. How primary ice nucleation affects total ice crystal number concentrations in clouds and the discrepancy between INP concentrations and ice crystal number concentrations are presented. Finally, limitations of parameterizing INPs and of models in representing known and unknown processes related to heterogeneous ice nucleation processes are discussed.

Full access
Yvonne Boose, Zamin A. Kanji, Monika Kohn, Berko Sierau, Assaf Zipori, Ian Crawford, Gary Lloyd, Nicolas Bukowiecki, Erik Herrmann, Piotr Kupiszewski, Martin Steinbacher, and Ulrike Lohmann

Abstract

Ice nucleating particle (INP) concentrations were measured at the High Altitude Research Station Jungfraujoch, Switzerland, 3580 m above mean sea level during the winter months of 2012, 2013, and 2014 with the Portable Ice Nucleation Chamber (PINC). During the measurement periods, the research station was mostly located in the free troposphere, and particle concentrations were low. At temperature T = 241 K, INP concentrations in the deposition regime [relative humidity with respect to water (RHw) = 93%] were, on average, below 1.09 per standard liter of air (stdL−1; normalized to 1013 hPa and 273 K) and 4.7 ± 8.3 stdL−1 in the condensation regime (RHw = 103%) in winter 2014. The deployment of a particle concentrator upstream of PINC decreased the limit of detection (LOD) by a factor of 3 compared to earlier measurements. The authors discuss a potential bias of INP measurements toward higher concentrations if data below the LOD are disregarded and thus recommend reporting subLOD data in future publications. Saharan dust and more local, basaltic dust mixed with marine aerosol were found to constitute the dominant INP type. Bioaerosols were not observed to play a role in ice nucleation during winter because of their low concentration during this period. The INP concentrations at Jungfraujoch are low in comparison to other studies of INP at this temperature. This represents the first study addressing interannual variations of INP concentrations during winter at one location.

Full access
Paul J. DeMott, Ottmar Möhler, Olaf Stetzer, Gabor Vali, Zev Levin, Markus D. Petters, Masataka Murakami, Thomas Leisner, Ulrich Bundke, Holger Klein, Zamin A. Kanji, Richard Cotton, Hazel Jones, Stefan Benz, Maren Brinkmann, Daniel Rzesanke, Harald Saathoff, Mathieu Nicolet, Atsushi Saito, Bjorn Nillius, Heinz Bingemer, Jonathan Abbatt, Karin Ardon, Eli Ganor, Dimitrios G. Georgakopoulos, and Clive Saunders

Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ice nucleation instruments. Through comparison to laboratory cloud simulations, these ice nuclei measurements have provided increased confidence in our ability to quantify primary ice formation by atmospheric aerosols.

Full access