Auflistung nach Autor:in "Walter, Saskia"

Gerade angezeigt 1 - 2 von 2
  • Vorschaubild nicht verfügbar
    Publikation
    Black carbon enrichment in atmospheric ice particle residuals observed in lower tropospheric mixed phase clouds
    (Wiley, 15.08.2008) Cozic, Julie; Mertes, Stephan; Verheggen, Bart; Cziczo, Daniel James; Gallavardin, Stephane J.; Walter, Saskia; Baltensperger, Urs; Weingartner, Ernest [in: Journal of Geophysical Research: Atmospheres]
    The enrichment of black carbon (BC) mass in residuals of small ice crystals was investigated during intensive experiments in winter 2004 and 2005 at the high alpine research station Jungfraujoch (3580 m asl, Switzerland). Two inlets were used to sample the bulk aerosol (residuals of cloud droplets and ice crystals and nonactivated aerosol particles) and the residual particles of small ice crystals (diameter 5–20 μm). An enrichment of the BC mass fraction in the ice particle residuals was observed by investigating the measured BC mass concentration as a fraction of the bulk (submicrometer) aerosol mass concentration sampled by the two inlets. On the average, the BC mass fraction was 5% for the bulk aerosol but 27% for the ice particle residuals. The observed enrichment of BC in ice particle residuals suggests that some BC‐containing particles may preferentially act as ice nuclei, with important implications for the indirect aerosol effect via glaciation of clouds if these particles represent a significant fraction of the number of ice crystals nucleated.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild nicht verfügbar
    Publikation
    Counterflow virtual impactor based collection of small ice particles in mixed-phase clouds for the physico-chemical characterization of tropospheric ice nuclei. sampler description and first case study
    (Taylor & Francis, 07.09.2007) Mertes, Stephan; Verheggen, Bart; Walter, Saskia; Connolly, Paul; Ebert, Martin; Schneider, Johannes; Bower, Keith N.; Cozic, Julie; Weinbruch, Stephan; Baltensperger, Urs; Weingartner, Ernest [in: Aerosol Science and Technology]
    A ground-based sampling system named Ice-CVI is introduced that is able to extract small ice particles with sizes between 5 and 20 μ m out of mixed-phase clouds. The instrument is based on a counterflow virtual impactor (CVI) removing interstitial particles and is supplemented by additional modules that pre-segregate other constituents of mixed-phase clouds. Ice particles of 20 μ m and smaller are expected to grow only by water vapor diffusion and there is a negligible probability that they scavenge aerosol particles by impaction and riming. Thus, their residuals which are released by the Ice-CVI can be interpreted as the original ice nuclei (IN). In a first field test within the Cloud and Aerosol Characterization Experiment (CLACE-3) at the high alpine research station Jungfraujoch, the collection behavior of the single components and the complete system was evaluated under atmospheric sampling conditions. By comparing parameters measured by the Ice-CVI with corresponding results obtained from other inlets or with in-situ instrumentation it is verified that the small ice particles are representatively collected whereas all other mixed phase cloud constituents are effectively suppressed. In a case study it is observed that super-micrometer particles preferentially serve as IN although in absolute terms the IN concentration is dominated by sub-micrometer particles. Mineral dust (Si), non-volatile organic matter and black carbon could be identified as IN components by means of different chemical analyses. The latter suggests an anthropogenic influence on the heterogeneous ice nucleation in supercooled, tropospheric clouds.
    01A - Beitrag in wissenschaftlicher Zeitschrift