Auflistung nach Autor:in "Bower, Keith N."

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  • Vorschaubild
    Publikation
    Chemical composition of free tropospheric aerosol for PM1 and coarse mode at the high alpine site Jungfraujoch
    (Copernicus, 31.01.2008) Cozic, Julie; Verheggen, Bart; Weingartner, Ernest; Crosier, Jonathan; Bower, Keith N.; Flynn, Michael; Coe, Hugh; Henning, Silvia; Steinbacher, Martin; Henne, Stephan; Collaud Coen, Martine; Petzold, Andreas; Baltensperger, Urs [in: Atmospheric Chemistry and Physics]
    The chemical composition of submicron (fine mode) and supermicron (coarse mode) aerosol particles has been investigated at the Jungfraujoch high alpine research station (3580 m a.s.l., Switzerland) as part of the GAW aerosol monitoring program since 1999. A clear seasonality was observed for all major components throughout the period with low concentrations in winter (predominantly free tropospheric aerosol) and higher concentrations in summer (enhanced vertical transport of boundary layer pollutants). In addition, mass closure was attempted during intensive campaigns in March 2004, February–March 2005 and August 2005. Ionic, carbonaceous and non-refractory components of the aerosol were quantified as well as the PM1 and coarse mode total aerosol mass concentrations. A relatively low conversion factor of 1.8 for organic carbon (OC) to particulate organic matter (OM) was found in winter (February–March 2005). Organics, sulfate, ammonium, and nitrate were the major components of the fine aerosol fraction that were identified, while calcium and nitrate were the only two measured components contributing to the coarse mode. The aerosol mass concentrations for fine and coarse mode aerosol measured during the intensive campaigns were not typical of the long-term seasonality due largely to dynamical differences. Average fine and coarse mode concentrations during the intensive field campaigns were 1.7 μg m−3 and 2.4 μg m−3 in winter and 2.5 μg m−3 and 2.0 μg m−3 in summer, respectively. The mass balance of aerosols showed higher contributions of calcium and nitrate in the coarse mode during Saharan dust events (SDE) than without SDE.
    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
  • Vorschaubild
    Publikation
    Scavenging of black carbon in mixed phase clouds at the high alpine site Jungfraujoch
    (Copernicus, 11.04.2007) Cozic, Julie; Verheggen, Bart; Mertes, Stephan; Connolly, Paul; Bower, Keith N.; Petzold, Andreas; Baltensperger, Urs; Weingartner, Ernest [in: Atmospheric Chemistry and Physics]
    The scavenging of black carbon (BC) in liquid and mixed phase clouds was investigated during intensive experiments in winter 2004, summer 2004 and winter 2005 at the high alpine research station Jungfraujoch (3580 m a.s.l., Switzerland). Aerosol residuals were sampled behind two well characterized inlets; a total inlet which collected cloud particles (droplets and ice particles) as well as interstitial (unactivated) aerosol particles; an interstitial inlet which collected only interstitial aerosol particles. BC concentrations were measured behind each of these inlets along with the submicrometer aerosol number size distribution, from which a volume concentration was derived. These measurements were complemented by in-situ measurements of cloud microphysical parameters. BC was found to be scavenged into the condensed phase to the same extent as the bulk aerosol, which suggests that BC was covered with soluble material through aging processes, rendering it more hygroscopic. The scavenged fraction of BC (FScav,BC), defined as the fraction of BC that is incorporated into cloud droplets and ice crystals, decreases with increasing cloud ice mass fraction (IMF) from FScav,BC=60% in liquid phase clouds to FScav,BC~5–10% in mixed-phase clouds with IMF>0.2. This can be explained by the evaporation of liquid droplets in the presence of ice crystals (Wegener-Bergeron-Findeisen process), releasing BC containing cloud condensation nuclei back into the interstitial phase. In liquid clouds, the scavenged BC fraction is found to decrease with decreasing cloud liquid water content. The scavenged BC fraction is also found to decrease with increasing BC mass concentration since there is an increased competition for the available water vapour.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild nicht verfügbar
    Publikation
    The influence of small aerosol particles on the properties of water and ice clouds
    (Royal Society of Chemistry, 09.08.2008) Choularton, Thomas W.; Bower, Keith N.; Weingartner, Ernest; Crawford, Ian; Coe, Hugh; Gallagher, Martin W.; Flynn, Michael; Crosier, Jonathan; Connolly, Paul; Targino, Admir Créso; Alfarra, M. Rami; Baltensperger, Urs; Sjögren, Staffan; Verheggen, Bart; Cozic, Julie; Gysel, Martin [in: Faraday Discussions]
    In this paper, results are presented of the influence of small organic- and soot-containing particles on the formation of water and ice clouds. There is strong evidence that these particles have grown from nano particle seeds produced by the combustion of oil products. Two series of field experiments are selected to represent the observations made. The first is the CLoud-Aerosol Characterisation Experiment (CLACE) series of experiments performed at a high Alpine site (Jungfraujoch), where cloud was in contact with the ground and the measuring station. Both water and ice clouds were examined at different times of the year. The second series of experiments is the CLOud Processing of regional Air Pollution advecting over land and sea (CLOPAP) series, where ageing pollution aerosol from UK cities was observed, from an airborne platform, to interact with warm stratocumulus cloud in a cloud-capped atmospheric boundary layer. Combining the results it is shown that aged pollution aerosol consists of an internal mixture of organics, sulfate, nitrate and ammonium, the organic component is dominated by highly oxidized secondary material. The relative contributions and absolute loadings of the components vary with location and season. However, these aerosols act as Cloud Condensation Nuclei (CCN) and much of the organic material, along with the other species, is incorporated into cloud droplets. In ice and mixed phase cloud, it is observed that very sharp transitions (extending over just a few metres) are present between highly glaciated regions and regions consisting of supercooled water. This is a unique finding; however, aircraft observations in cumulus suggest that this kind of structure may be found in these cloud types too. It is suggested that this sharp transition is caused by ice nucleation initiated by oxidised organic aerosol coated with sulfate in more polluted regions of cloud, sometimes enhanced by secondary ice particle production in these regions.
    01A - Beitrag in wissenschaftlicher Zeitschrift