<>Method The nucleated ice fraction, Nfraction, the ratio of the number of ice particles activated to become ice to the initial seeding dust particle concentration requires accurate measurements of Nice. Several methods are available using a variety of instrumentation in the AIDA chamber to determine Nice. For this study we have used two approaches. The first method uses direct measurements of Nice from the Small Ice Detector (SID) probe, e.g. Hirst et al., (2001), which measures particle concentration and asphericity thus discriminating between water and ice, in the size range (0.5<Dp<30 mm). For ice particles that grow outside the accurate range of SID, (10<Dp<1000 mm) concentrations were determined using a combination of measurements from a Cloud Particle Imager (CPI) and theoretical models. Corrections were developed for the CPI sample volume and for mis-sizing for ice crystals as a function of habit for sizes < 40 mm based on laboratory and field studies, Connolly et al. (2006). Theoretical considerations were required to calculate ice number concentrations at the point of primary nucleation as, quite often, ice particles formed from seeds or droplets that were smaller than the low size range of the CPI. Firstly, a time-series of ice and liquid (where droplets were used) number concentration (also segregated with size) was calculated according to Connolly (2005). This data was then used with the measurements of chamber temperature, relative humidity and pressure to retrieve a number concentration. The growth rate equation for ice particles documented by Chen and Lamb (1994) was used in order to retrieve the true ice concentration at the time of nucleation by integrating backwards in time starting with the size distribution measured by the CPI. Growth trajectories were obtained for size-concentration categories of particles and these were used to infer number concentrations. The point of nucleation and concentrations agreed well with other probes including SID. <>
Figure 3 Figure 4 Figures 1-3 show examples of how the fraction of activated ice crystals depended on the variation of relative humidity with respect to ice (RHice) for three different dust types, Saharan (SD1 and SD2) and Asian Dust (AD1) at a given temperature, T. The experiments consistently showed a bimodal behaviour in Nfraction with two distinct activation modes whose relative magnitudes were a function of T and RHice. Figure 4 shows a surface plot of Nice as a function of RHice and T. Note that through either an increase in temperature or RHice, the second mode can become active. At the critical -38oC level (not shown) homogenous nucleation then became the dominant nucleation mode.
The two modes appeared to behave very differently with one activating below water saturation and the other at water saturation. The high RH mode nucleated on average between five and ten times the fraction of the low RH mode for SD. This behaviour is not accounted for in numerical cloud models.
The magnitude of the second activation mode was found to be similar for both SD and AD dusts, highlighting that parameterisation of ice nucleation on desert dust may be simplified in some cases. Interestingly, the active fraction of AD was observed to increase dramatically when water saturation was approached, up to eight-fold in going from typically 5% sub-saturated with respect to liquid water to just above water saturation. In another experiment, Arizona test dusts (ATD) were examined and they were found to be significantly more active with a high fraction of dust particles being activated as ice nuclei at considerably lower saturation ratios than for AD. These results may have implications for explaining some observations of high ice number concentrations in clouds; however, further work is needed in this area to confirm this hypothesis.
Non-linear regression models were applied to the data to provide a functional description of the bimodal behaviour and were of the form
The results will be described in greater detail and the consequences for its inclusion in cloud resolving models discussed.