The formation of ice particles in the Earth's atmosphere strongly affects the properties of clouds and their impact on climate. Despite the importance of ice formation in determining the properties of clouds, the Intergovernmental Panel on Climate Change (IPCC, 2007) was unable to assess the impact of atmospheric ice formation in their most recent report because our basic knowledge is insufficient. Part of the problem is the paucity of quantitative information on the ability of various atmospheric aerosol species to initiate ice formation. Here we review and assess the existing quantitative knowledge of ice nucleation by particles immersed within supercooled water droplets. We introduce aerosol species which have been identified in the past as potentially important ice nuclei and address their ice-nucleating ability when immersed in a supercooled droplet. We focus on mineral dusts, biological species (pollen, bacteria, fungal spores and plankton), carbonaceous combustion products and volcanic ash. In order to make a quantitative comparison we first introduce several ways of describing ice nucleation and then summarise the existing information according to the time-independent (singular) approximation. Using this approximation in combination with typical atmospheric loadings, we estimate the importance of ice nucleation by different aerosol types. According to these estimates we find that ice nucleation below about -15 °C is dominated by soot and mineral dusts. Above this temperature the only materials known to nucleate ice are biological, with quantitative data for other materials absent from the literature. We conclude with a summary of the challenges our community faces.

Figure 1a. In experiments with similar dust surface areas, the temperature at which 50% of droplets were frozen was 250.5 K for K-feldspar, followed by 247 K for Na/Ca-feldspar, 242.5 K for quartz, and below 237.5 K for the clay minerals and calcite. These results suggest that it is the minerals of the feldspar group, in particular K-feldspar, that make mineral dust an effective immersion mode IN in the atmosphere. This data contrasts with the prevailing view 1,2 that clay minerals are the most important component of atmospheric mineral dust for ice nucleation.Droplet freezing temperatures are dependent on experimental parameters such as droplet volume and mineral surface area and are therefore of limited value 2 . In order to normalise the efficiency with which a material nucleates ice we determine the nucleation sites per unit surface area 2,11,14 (n s ; Figure 1b; see supplementary online material). This method of quantifying ice nucleation efficiency neglects the role of time dependence in nucleation, on the basis that IN particle-to-particle variability is more important than the time dependence of nucleation 2,11,14,15 . Our derived n s values for 9 -19 μm size droplets are shown in Figure 1b.This data shows that the feldspar minerals, in particular K-feldspar, are the most efficient mineral dust IN per unit surface area.In airborne dusts the abundance of clay minerals tends to be greater than the feldspars, hence it is not clear which minerals dominate ice nucleation in the atmosphere. The n s values presented in Figure 1b were combined with the average mineralogical composition of atmospheric dust to estimate the temperature-dependent IN concentration (shown in Figure 2). We have assumed that all particles are spherical in order to estimate their surface area and have made two limiting calculations, one assuming that dust particles are internally mixed (i.e. each particle contains all eight minerals) and the other assuming they are externally mixed (each particle is composed of an individual mineral). The mixing state of atmospheric dust is poorly constrained but atmospheric dust falls between these two limiting cases 16 .Despite only accounting for 3% of atmospheric dust by mass, K-feldspar dominates the number of IN above 248 K in both the internally and externally mixed cases. One potential caveat to this conclusion is that clay mineral particles may have a smaller particle size than feldspar or quartz 13 , and therefore may have a greater surface area per unit mass which would increase the concentration of clay IN. However, even if the surface area of the clays was 100 times higher (likely an overestimate 7 ), the feldspars remain the dominant ice nucleating minerals (Supplementary Figure 4). contains the most K-feldspar (20 wt%). In general, the more feldspar a sample contains the higher the freezing temperature. We hypothesise that that the feldspar component controlled the nucleation of ice in these experiments, highlighting the need to characterise sample mineralogy in such work.The mineralog...

Abstract. Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques.Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain Published by Copernicus Publications on behalf of the European Geosciences Union. N. Hiranuma et al.: A comparison of 17 IN measurement techniquesIN data as a function of particle concentration, temperature (T ), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto drydispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, n s , to develop a representative n s (T ) spectrum that spans a wide temperature range (−37 • C < T < −11 • C) and covers 9 orders of magnitude in n s .In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 • C in terms of temperature, by 3 orders of magnitude with respect to n s . In addition, we show evidence that the immersion freezing efficiency expressed in n s of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the n s parameterization solely as a function of temperature. We also characterized the n s (T ) spectra and identified a section with a steep slope between −20 and −27 • C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below −27 • C. While the agreement between different instruments was reasonable below ∼ −27 • C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance,...

Most studies of the role of biological entities as atmospheric ice-nucleating particles have focused on relatively rare supermicron particles such as bacterial cells, fungal spores and pollen grains. However, it is not clear that there are sufficient numbers of these particles in the atmosphere to strongly influence clouds. Here we show that the ice-nucleating activity of a fungus from the ubiquitous genus Fusarium is related to the presence of nanometre-scale particles which are far more numerous, and therefore potentially far more important for cloud glaciation than whole intact spores or hyphae. In addition, we quantify the ice-nucleating activity of nano-ice nucleating particles (nano-INPs) washed off pollen and also show that nano-INPs are present in a soil sample. Based on these results, we suggest that there is a reservoir of biological nano-INPs present in the environment which may, for example, become aerosolised in association with fertile soil dust particles.