Zamin A. Kanji

Solid Ammonium Sulfate Aerosols as Ice Nuclei: A Pathway for Cirrus Cloud Formation

Laboratory measurements support a cirrus cloud formation pathway involving heterogeneous ice nucleation by solid ammonium sulfate aerosols. Ice formation occurs at low ice-saturation ratios consistent with the formation of continental cirrus and an interhemispheric asymmetry observed for cloud onset. In a climate model, this mechanism provides a widespread source of ice nuclei and leads to fewer but larger ice crystals as compared with a homogeneous freezing scenario. This reduces both the cloud albedo and the longwave heating by cirrus. With the global ammonia budget dominated by agricultural practices, this pathway might further couple anthropogenic activity to the climate system.

Resurgence in Ice Nuclei Measurement Research

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 ic...

Ice formation via deposition nucleation on mineral dust and organics: dependence of onset relative humidity on total particulate surface area

We present ice nucleation results for Arizona test dust, kaolinite, montmorillonite, silica, silica coated with a hydrophobic octyl chain, oxalic acid dihydrate, Gascoyne leonardite (a humic material), and Aldrich humic acid (sodium salt). The focus was on deposition mode nucleation below water saturation at 233 K. Particles were deposited onto a hydrophobic cold stage by atomization of a slurry/solution and exposed to a constant partial pressure of water vapor. By lowering the temperature of the stage, the relative humidity with respect to ice (RHi )w as gradually increased until ice nucleation was observed using digital photography. Different numbers of particles were deposited onto the cold stage by varying the atomization solution concentration and deposition time. For the same total particulate surface area, mineral dust particles nucleated ice at lower supersaturations than all other materials. The most hydrophobic materials, i.e. Gascoyne leonardite and octyl silica, were the least active. For our limit of detection of one ice crystal, the ice onset RHi values were dependent on the total surface area of the particulates, indicating that no unique threshold RHi for ice nucleation prevails.

Ice nucleation onto Arizona test dust at cirrus temperatures: effect of temperature and aerosol size on onset relative humidity.

The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC) was used to study ice formation onto monodisperse Arizona Test Dust (ATD) particles. The onset relative humidity with respect to ice (RH(i)) was measured as a function of temperature in the range 251-223 K for 100 nm ATD particles. It was found that for 0.1% of the particles to freeze, water saturation was required at all temperatures except 223 K where particles activated at RH(i) below water saturation. At this temperature, where deposition mode freezing is occurring, we find that the larger the particle size, the lower the onset RH(i). We also demonstrate that the total number of particles present may influence the onset RH(i) observed. The surface area for ice activation, aerosol size, and temperature must all be considered when reporting onset values of ice formation onto ATD mineral dust particles. In addition, we calculate nucleation rates and contact angles of ice germs with ATD aerosols which indicate that there exists a range of active sites on the surface with different efficiencies for activating ice formation.

Exploring the Mechanisms of Ice Nucleation on Kaolinite: From Deposition Nucleation to Condensation Freezing

AbstractTo 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 nucleat...

Overview of Ice Nucleating Particles

AbstractIce 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 a...

The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC): A Simple Design for Ice Nucleation Studies

A new instrument, the University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC), has been designed to study ice nucleation at low temperatures. Based on previous continuous flow instruments, it is a parallel plate model that minimizes convective instabilities by operating horizontally with the warmer plate on top. A variable position sample injector can account for effects arising from gravitational settling of ice particles that form. The residence time in the chamber can vary between 2.6 to 25 s and ice particle formation is monitored with a two-channel optical particle counter. Observation of homogeneous freezing of 100 nm sulfuric acid aerosols was used to verify the accuracy of the calculated relative humidities (RHs) in the chamber to be ±4%, where we report onset RHs for 0.1% of the particles freezing in the temperature range of 218 to 243 K. We also show that the chamber accurately establishes conditions of water saturation by conducting water uptake studies onto sulfuric acid aerosol at 243 K. The two channel OPC allows for ice and water droplet formation to be distinguished under such conditions. The chamber is a simple, cheap, and small design that can be readily assembled for laboratory studies.

Secondary Ice Production: Current State of the Science and Recommendations for the Future

AbstractMeasured ice crystal concentrations in natural clouds at modest supercooling (temperature ~>−10°C) are often orders of magnitude greater than the number concentration of primary ice nucleating particles. Therefore, it has long been proposed that a secondary ice production process must exist that is able to rapidly enhance the number concentration of the ice population following initial primary ice nucleation events. Secondary ice production is important for the prediction of ice crystal concentration and the subsequent evolution of some types of clouds, but the physical basis of the process is not understood and the production rates are not well constrained. In November 2015 an international workshop was held to discuss the current state of the science and future work to constrain and improve our understanding of secondary ice production processes. Examples and recommendations for in situ observations, remote sensing, laboratory investigations, and modeling approaches are presented.

Measurements of Ice Nucleating Particles and Ice Residuals

AbstractIt 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.

Persistence of orographic mixed‐phase clouds

Mixed-phase clouds (MPCs) consist of ice crystals and supercooled water droplets at temperatures between 0 and approximately −38°C. They are thermodynamically unstable because the saturation vapor pressure over ice is lower than that over supercooled liquid water. Nevertheless, long-lived MPCs are ubiquitous in the Arctic. Here we show that persistent MPCs are also frequently found in orographic terrain, especially in the Swiss Alps, when the updraft velocities are high enough to exceed saturation with respect to liquid water allowing simultaneous growth of supercooled liquid droplets and ice crystals. Their existence is characterized by holographic measurements of cloud particles obtained at the high-altitude research station Jungfraujoch during spring 2012 and winter 2013 and simulations with the regional climate model COSMO (Consortium of Small-Scale Modeling).