Abstract
There is a consensus on the increase in ice nucleating particles (INP) concentration from subsaturated to supersaturated water conditions typically associated with clouds (1 ÷ 2%). However, it is important to evaluate the INP concentration trend when water supersaturation further increases, as supercooled clouds contain pockets of high water vapor supersaturation. Three laboratory dry-generated aerosols, two biological (microcrystalline and fibrous cellulose) and one mineral (Arizona test dust), and a field aerosol, sampled on filters, were investigated. Atmospheric aerosol (PM1 and PM10 fractions) was sampled at Capo Granitola (CG, coastal site in Sicily) and the National Research Council (CNR) research area in Bologna (urban background site). The dynamic filter processing chamber (DFPC) was used to explore the ice nucleation of the sampled aerosol in the deposition and condensation freezing modes. Experiments were performed from water subsaturated conditions (water saturation ratio Sw = 0.94) to Sw = 1.1, at T = −22 °C. At CG we considered separately events with a prevalent contribution of marine aerosol, and those showing a contribution of both marine and continental aerosols. An increase in INP concentration, the aerosol activated fraction (AF) and ice nucleation active surface site density (ns) from water subsaturated conditions to Sw = 1.02 was measured in both laboratory and field campaigns. This increase is due to the transition from deposition nucleation to condensation freezing. The highest increases in AF and ns from Sw = 1.02 to Sw = 1.1 were obtained for urban and mixed aerosol and the lowest for marine aerosol. Samplings performed in Bologna showed a high increase in the average INP concentration from PM1 to PM10. Our results show the importance of performing measurements of ice nucleation efficiency for continental aerosol even at supersaturation values higher than those typically associated with clouds, and also considering the contribution of coarse aerosol particles.
Highlights
Most precipitation in middle and high latitude comes from cold clouds containing ice particles (Heymsfield, 2005; Welti et al, 2009; DeMott et al, 2010).The ice phase in clouds can form through primary processes, homogeneously, or heterogeneously in the presence of aerosol called ice nucleating particles (INP)
Secondary ice formation processes can derive from ice splinter formation during riming, shattering of cloud drops during freezing in free fall, etc. (Hallett and Mossop, 1974; Leisner et al, 2014)
Considering the sampling of PM1 and PM10 at CG (24 April 2016; 00:00 UTC), the ratio between aerosol concentration in the PM1 and coarse particles was much lower than for urban background (UB) aerosol (~75), but in this case there is a negligible increase in INP, i.e. pure soluble coarse particles do not contribute to INP (Fig. 5b)
Summary
Most precipitation in middle and high latitude comes from cold clouds containing ice particles (Heymsfield, 2005; Welti et al, 2009; DeMott et al, 2010). Rogers (1993) performed INP measurements with a CFDC in continental air masses finding that the concentration of ice nuclei was closely related to the ice supersaturation (SS ) for i humidity both below and above water saturation over the temperature range −7 to −20 °C. The International Workshop on Comparing Ice Nucleation Measuring Systems (ICIS 2007) addressed the problem of the increase in INP concentration versus Sw. A spread of four to five INP active fractions between individual instruments at single Sw values above 1.02 was obtained for Saharan dusts (DeMott et al, 2011). For individual instruments an increase in INP concentration was observed by increasing relative humidity (r.h.)
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