Ice nuclei properties within a Saharan dust event at the Jungfraujoch in the Swiss Alps

Abstract

Abstract.The new portable ice nucleation chamber (PINC) developed by the Institute for Atmospheric and Climate Sciences of ETH Zurich was operated during two measurement campaigns at the high alpine research station Jungfraujoch situated at 3580 m a.s.l, in March and June 2009. During this time of the year, a high probability of Saharan dust events (SDE) at the Jungfraujoch has been observed. We used an impactor with a cutoff size of 1 µm aerodynamic diameter and operated the system at −31 • C and relative humidities of 127 % and 91 % with respect to ice and water, respectively. Investigation of the ambient number concentration of ice nuclei (IN) in the deposition nucleation mode and during a SDE in the free troposphere is reported. The results discussed in this paper are the first continuous IN measurements over a period of several days at the Jungfraujoch. The average IN concentration found during the campaign in March was 8 particles per liter whereas during the campaign in June, the average number concentration was higher up to 14 particles per liter. Two SDEs were detected on 15 and 16 June 2009. Our measurements show that the SDEs had IN number concentration up to several hundreds per liter. We found the best correlation between the number concentration of the larger particle fraction measured by an optical particle counter and the IN number concentration during a Saharan dust event. This correlation factor is higher for particles larger than 0.5 µm meaning that a higher concentration of larger particles induced higher IN number concentration. No correlation could be found between the black carbon mass concentration and the variations in IN number concentration.

The origins of ice crystals measured in mixed-phase clouds at the high-alpine site Jungfraujoch

“…When using D10, predicted IN values were found to be significantly fewer (sometimes by 3 orders of magnitude) than the concentrations of ice measured at JFJ. Measurements made in previous campaigns of IN activity at this site were also unable to explain the high concentrations of ice (Chou et al, 2011). A powerful secondary ice production mechanism, the Hallett-Mossop (H-M) process (Hallett and Mossop, 1974) is known to operate over the temperature range −3 • C < T < −8 • C, the so-called H-M zone.…”

“…Sampling of aerosol at JFJ and testing for IN active particles to determine an IN concentration has also shown significant seasonal variability linked to changes in aerosol source. Chou et al (2011) used a portable ice nucleation chamber (PINC) to make the first continuous measurements of IN over a period of days at JFJ. They found a mean IN concentration of 8 L −1 during a winter measurement period and 14 L −1 during the summer.…”

A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

“…At temperatures below ∼ −38 • C, the washed dust component may have enhanced water condensation below water saturation, and a formed liquid layer presumably may have stabilized the subcritical ice embryo entrapped inside the liquid. The authors proposed this capillary condensation process as a part of condensation freezing or homogeneous nucleation based on the previous observation (Christenson, 2013) and the theoretical framework (Marcolli, 2014). Above ∼ −38 • C, on the other hand, heterogeneous nucleation might have been suppressed because the liquid layer derived from the deliquescence of soluble impurities from individual particles may have diminished accessibility of water vapor to active sites (e.g., localized surface features such as cracks and edges), originally proposed by Koehler et al (2010), preventing the ice embryo formation.…”

“…Typically, these large ice crystals can be detected and counted by an optical particle counter (OPC) downstream of the instrument while the chamber temperature and humidity conditions are continuously recorded. Since its first appearance in the 1980s with horizontal parallel plates (Hussain and Saunders, 1984;Tomlinson and Fukuta, 1985), several new designs and operational principles have been introduced (e.g., vertically oriented cylinders; Rogers et al, 1988, horizontally oriented parallel plates; Kanji and Abbatt, 2009, vertically oriented parallel plates; Stetzer et al, 2008;Chou et al, 2011;Friedman et al, 2011). An alternative configuration is the continuous flow mixing chamber (e.g., Fast Ice Nucleus Chamber or FINCH; Bundke et al, 2008).…”

“…Previous laboratory experiments have taken diverse approaches in an attempt to mimic ice nucleation and freezing processes. These heterogeneous ice formation processes include deposition nucleation, immersion freezing, condensation freezing, and contact freezing (Vali, 1985), inside-out contact freezing (i.e., freezing of an immersed INP in contact with the droplet surface from the inside; Durant and Shaw, 2005;Fornea et al, 2009) and surface condensation freezing (i.e., freezing of supercooled water or residual aqueous solution trapped on particle surfaces, e.g., by the inverse Kelvin effect; Christenson, 2013;Hiranuma et al, 2014a;Marcolli, 2014;Welti et al, 2014;Wex et al, 2014). Without INPs, pure cloud water droplets or solution within particles can be supercooled to below −37 • C before freezing (Koop et al, 2000;Murray et al, 2010;Rosenfeld and Woodley, 2000).…”