Ian Crawford

Influence of particle chemical composition on the phase of cold clouds at a high-alpine site in Switzerland

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.

Observations of the Variation in Aerosol and Cloud Microphysics along the 20°S Transect on 13 November 2008 during VOCALS-REx

AbstractObservations are presented of the structure of the marine boundary layer (MBL) in the southeastern Pacific made with the U.K. BAe 146 aircraft on 13 November 2008 as it flew at a variety of altitudes along 20°S between the coast of Chile and a buoy 950 km offshore during the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study (VOCALS) Regional Experiment (REx). The purpose of the study is to determine the variations along the 20°S transect in the clouds and boundary layer on this particular day as compared to the typical structure determined from the composite studies. The aircraft flew in three regions on this day: relatively continuous thick stratocumulus clouds, open cells, and closed cells. Results show three particular features. First, the results of the cloud microphysics are consistent with the typical behavior showing a decrease in aerosol particles by a factor of 3–4, and a decrease in cloud droplet number concentration westward from the coast from about 200 ...

Partitioning of Aerosol Particles in Mixed-phase Clouds at a High Alpine Site

6 German Aerospace Centre, D-82234, Wessling, Germany * Now at Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland Abstract 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) conducted in winter and summer 2004 and winter 2005 at the high alpine research station Jungfraujoch (3,580 m altitude, Switzerland). Ambient air was sampled using different inlets in order to determine the scavenged fraction of aerosol particles, F Scav , and of black carbon, F Scav,BC . They denote the fraction of the aerosol volume concentration and of the black carbon (BC) mass, respectively, that has been incorporated into cloud droplets and ice crystals. They are both found to increase with increasing liquid water content, and to decrease with increasing particle number concentration. The scavenged fraction also decreases with increasing cloud ice mass fraction and with decreasing temperature from 0 to −25°C. This can be explained by the WegenerBergeron-Findeisen process, which describes the effect of a water vapour flux from liquid droplets to ice crystals, thus releasing the formerly activated particles back into the interstitial phase. The presence of ice could also have prevented additional particles from activating. BC was found to be scavenged into the cloud 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. However, BC was found to be enriched in small ice crystals compared to the bulk aerosol, indicating that BC containing particles preferentially act as ice nuclei. If this finding is representative, it would mean that in addition to an indirect effect on liquid cloud