Auflistung nach Autor:in "Choularton, Tom"

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Aerosol partitioning between the interstitial and the condensed phase in mixed‐phase clouds
(Wiley, 13.12.2007) Verheggen, Bart; Cozic, Julie; Weingartner, Ernest; Bower, Keith; Mertes, Stephan; Connolly, Paul; Gallagher, Martin; Flynn, Michael; Choularton, Tom; Baltensperger, Urs
The partitioning of aerosol particles between the cloud and the interstitial phase (i.e., unactivated aerosol) has been investigated during several Cloud and Aerosol Characterization Experiments (CLACE‐3, CLACE‐3½ and CLACE‐4) conducted in winter and summer 2004 and winter 2005 at the high alpine research station Jungfraujoch (3580 m altitude, Switzerland). Ambient air was sampled using different inlets in order to determine the activated fraction of aerosol particles, FN, defined as the fraction of the total aerosol number concentration (with particle diameter dp > 100 nm) that has been incorporated into cloud particles. The liquid and ice water content of mixed‐phase clouds were characterized by analyzing multiple cloud probes. The dependence of the activated fraction on several environmental factors is discussed on the basis of more than 900 h of in‐cloud observations and parameterizations for key variables are given. FN is found to increase with increasing liquid water content and to decrease with increasing particle number concentration in liquid clouds. FN also decreases with increasing cloud ice mass fraction and with decreasing temperature from 0 to −25°C. The Wegener‐Bergeron‐Findeisen process probably contributed to this trend, since the presence of ice crystals causes liquid droplets to evaporate, thus releasing the formerly activated particles back into the interstitial phase. Ice nucleation could also have prevented additional cloud condensation nuclei from activating. The observed activation behavior has significant implications for our understanding of the indirect effect of aerosols on climate.
01A - Beitrag in wissenschaftlicher Zeitschrift
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
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