Isolating and identifying atmospheric ice-nucleating aerosols: a new technique

{ "0" : "Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA" , "2" : "Continuous-flow diffusion chamber" , "3" : "Ice nuclei" , "4" : "Aerosol"}
Atmospheric Research (Impact Factor: 2.84). 05/1998; 46(3-4):263-278. DOI: 10.1016/S0169-8095(97)00068-9


Laboratory studies examined two key aspects of the performance of a continuous-flow diffusion chamber (CFD) instrument that detects ice nuclei (IN) concentrations in air samples: separating IN from non-IN, and collecting IN aerosols to determine chemical composition. In the first study, submicron AgI IN particles were mixed in a sample stream with submicron non-IN salt particles, and the sample stream was processed in the CFD at −19°C and 23% supersaturation with respect to ice. Examination of the residual particles from crystals nucleated in the CFD confirmed that only AgI particles served as IN in the mixed stream. The second study applied this technique to separate and analyze IN and non-IN particles in a natural air sample. Energy-dispersive X-ray analyses (EDS) of the elemental composition of selected particles from the IN and non-IN fractions in ambient air showed chemical differences: Si and Ca were present in both, but S, Fe and K were also detected in the non-IN fraction.

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    • "An inlet impactor upstream of the CFDC ensures that aerosol particles larger than ∼1.5 μm (aerodynamic diameter) are removed prior to entering the instrument (Rogers et al., 2001b), so that large aerosol particles are not erroneously identified as ice. An inertial impactor immediately downstream of the CFDC is used to capture ice crystals on Transmission Electron Microscope (TEM) grids, allowing for subsequent identification of the elemental composition of the particles on which ice forms (Kreidenweis et al., 1998). The CFDC is sensitive in real time to all nucleation modes, except contact freezing, since the residence time is fairly short. "
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    ABSTRACT: This paper presents airborne measurements of ice nuclei (IN) number concentration and elemental composition from the mixed-phase Arctic cloud experiment (M-PACE) in northern Alaska during October 2004. Although the project average IN concentration was low, less than 1 L−1 STP, there was significant spatial and temporal variability, with local maximum concentrations of nearly 60 L−1 STP. Immersion and/or condensation freezing appear to be the dominant freezing mechanisms, whereas mechanisms that occur below water saturation played a smaller role. The dominant particle types identified as IN were metal oxides/dust (39%), carbonaceous particles (35%) and mixtures of metal oxides/dust with either carbonaceous components or salts/sulphates (25%), although there was significant variability in elemental composition. Trajectory analysis suggests both local and remote sources, including biomass burning and volcanic ash. Seasonal variability of IN number concentrations based on this study and data from SHEBA/FIRE-ACE indicates that fall concentrations are depleted relative to spring by about a factor of five. Average IN number concentrations from both studies compare favorably with cloud ice number concentrations of cloud particles larger than 125 μm, for temperatures less than −10 °C. Cloud ice number concentrations also were enhanced in spring, by a factor of ∼2, but only over a limited temperature range.
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    • "Their size, morphology, and composition can be studied with single particle analysis techniques. Tests of the ability of the CFD technique to separate IN from non-IN are described in Kreidenweis et al. (1998). Results using this technique in a field study are reported in Chen et al. (1998). "
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    ABSTRACT: A continuous-flow thermal gradient diffusion chamber was developed for operating in an aircraft and detecting ice nucleating aerosol particles in real time. The chamber volume is the annular space between two vertically oriented concentric cylinders. The surfaces of the chamber are coated with ice and held at different temperatures, thus creating a vapor supersaturation. Upstream of the chamber, all particles in the sample air larger than 2-μm diameter are removed with inertial impactors. The air then flows vertically downward through the chamber, where ice crystals nucleate and grow on active ice nuclei to between Ο3- and 10-μm diameter in 3-10 s of residence time. At the outlet of the chamber, an optical particle counter detects all particles larger than Ο0.8 μm. Those particles larger than 3 μm are assumed to be the newly formed ice crystals and comprise the ice nucleus count. This paper describes the principles of operation, hardware and construction, data system, calibration, operational procedures, and performance. Limitations of the technique are presented, and examples of measurements are shown.
    Preview · Article · May 2001 · Journal of Atmospheric and Oceanic Technology
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    • "Supersaturations with respect to water were varied from positive (up to +10%) to negative (-20%), to examine heterogeneous freezing mechanisms; temperatures were not cold enough to detect homogeneous-freezing nuclei. The IN fraction was subsequently collected with an inertial impactor having a 50% cut size of 3 gm, such that only crystals were collected [Kreidenweis et al., 1997]. "
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    ABSTRACT: A newly developed instrument was deployed on the NASA DC-8 airborne laboratory during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS, Spring 1996) to detect and collect heterogeneous ice nucleating particles (IN) in the upper troposphere and lower stratosphere. The elemental compositions of both ambient particles and IN were determined with single particle analysis using analytical electron microscopy. IN collected during flights on May 4 and 8 had enhanced number fractions of metallic, crustal, and carbonaceous particles, compared with the ambient aerosol population, and were relatively deficient in sulfur-containing particles. IN sampled within aircraft exhaust and contrails had higher number fractions of metallic particles, which includes those rich in Zn, Al, and Ti, than the IN sampled in air that was not immediately affected by aircraft exhaust.
    Preview · Article · May 1998
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