Auflistung nach Autor:in "Crawford, Ian"

Gerade angezeigt 1 - 3 von 3
  • Vorschaubild nicht verfügbar
    Publikation
    Influence of particle chemical composition on the phase of cold clouds at a high‐alpine site in Switzerland
    (Wiley, 23.09.2009) Targino, Admir Créso; Coe, Hugh; Cozic, Julie; Crosier, Jonathan; Crawford, Ian; Bower, Keith; Flynn, Michael; Gallagher, Martin; Allan, James; Verheggen, Bart; Weingartner, Ernest; Baltensperger, Urs; Choularton, Tom [in: Journal of Geophysical Research: Atmospheres]
    This paper studies the influence of particle chemical composition on the phase of cold clouds observed during two intensive measurement periods of the Cloud and Aerosol Characterization Experiments conducted at the Jungfraujoch site (Switzerland). Cloud droplets and particles were sampled simultaneously using a suite of optical, chemical, and microphysical instruments connected downstream of a total inlet and an interstitial inlet. Sulphate and organic matter were the most abundant semivolatile species observed in the particulate phase during both campaigns. Periods of relatively large loadings of organic and inorganic species were also accompanied by enhancement of light‐absorbing aerosol concentrations. The cloud phase exhibited sharp transitions, alternating between highly glaciated and liquid phases over a few seconds within the same cloud event. It was also observed that conditions of elevated pollution were accompanied by an increase in occurrence of glaciated periods. The 24‐hour cloud event investigated on the 8 March 2004 was in the mixed phase for approximately 260 minutes, in the glaciated phase for approximately 64 minutes and in the liquid phase for the remainder of the time. On the 23 March 2004, another 24‐hour cloud event was captured in which the number of minutes as mixed‐phase and glaciated cloud were 196 and 31, respectively. The loadings of BC as well as organic and inorganic species were larger during the first period. The investigation was extended for the whole data set, and a statistical analysis was performed across the chemical data measured off the total inlet. The amount of organic and inorganic material found in liquid and glaciated clouds was statistically different, with organic and inorganic material as well as BC being enriched in glaciated conditions. The case studies and the statistical analysis together suggest an influence of the particle chemical composition on the cloud phase, which may be important in perturbing cloud microphysics in polluted regions.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild
    Publikation
    Single particle characterization of black carbon aerosols at a tropospheric alpine site in Switzerland
    (Copernicus, 09.08.2010) Liu, D.; Flynn, Michael; Gysel, Martin; Targino, Admir Créso; Crawford, Ian; Bower, Keith; Choularton, Thomas; Jurányi, Zsófia; Steinbacher, Martin; Hüglin, Christoph; Curtius, Joachim; Kampus, M.; Petzold, Andreas; Weingartner, Ernest; Baltensperger, Urs; Coe, Hugh [in: Atmospheric Chemistry and Physics]
    The refractory black carbon (rBC) mass, size distribution (190–720 nm) and mixing state in sub-micron aerosols were characterized from late February to March 2007 using a single particle incandescence method at the high alpine research station Jungfraujoch (JFJ), Switzerland (46.33° N, 7.59° E, 3580 m a.s.l.). JFJ is a ground based location, which is at times exposed to continental free tropospheric air. A median mass absorption coefficient (MAC) of 10.2±3.2 m2 g−1 at λ=630 nm was derived by comparing single particle incandescence measurements of black carbon mass with continuous measurements of absorption coefficient. This value is comparable with other estimates at this location. The aerosols measured at the site were mostly well mixed and aged during transportation via the free troposphere. Pollutant sources were traced by air mass back trajectories, trace gases concentrations and the mass loading of rBC. In southeasterly wind directions, mixed or convective weather types provided the potential to vent polluted boundary layer air from the southern Alpine area and industrial northern Italy, delivering enhanced rBC mass loading and CN concentrations to the JFJ. The aerosol loadings at this site were also significantly influenced by precipitation, which led to the removal of rBC from the atmosphere. Precipitation events were shown to remove about 65% of the rBC mass from the free tropospheric background reducing the mean loading from 13±5 ng m−3 to 6±2 ng m−3(corrected to standard temperature and pressure). Overall, 40±15% of the observed rBC particles within the detectable size range were mixed with large amounts of non-refractory materials present as a thick coating. The growth of particle size into the accumulation mode was positively linked with the degree of rBC mixing, suggesting the important role of condensable materials in increasing particle size and leading to enhanced internal mixing of these materials with rBC. It is the first time that BC mass, size distribution and mixing state are reported in the free troposphere over Europe. These ground based measurements also provide the first temporal study of rBC in the European free troposphere quantitatively measured by single particle methods. At the present time there is only limited information of BC and its mixing state in the free troposphere, especially above Europe. The results reported in this paper provide an important constraint on modelled representation of BC.
    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