Abstract. Clouds composed of both ice particles and supercooled liquid water droplets exist at temperatures above ∼236 K. These mixed phase clouds, which strongly impact climate, are very sensitive to the presence of solid particles that can catalyse freezing. In this paper we describe experiments to determine the conditions at which the clay mineral kaolinite nucleates ice when immersed within water droplets. These are the first immersion mode experiments in which the ice nucleating ability of kaolinite has been determined as a function of clay surface area, cooling rate and also at constant temperatures. Water droplets containing a known amount of clay mineral were supported on a hydrophobic surface and cooled at rates of between 0.8 and 10 K min −1 or held at constant sub-zero temperatures. The time and temperature at which individual 10-50 µm diameter droplets froze were determined by optical microscopy. For a cooling rate of 10 K min −1 , the median nucleation temperature of 10-40 µm diameter droplets increased from close to the homogeneous nucleation limit (236 K) to 240.8 ± 0.6 K as the concentration of kaolinite in the droplets was increased from 0.005 wt% to 1 wt%. This data shows that the probability of freezing scales with surface area of the kaolinite inclusions. We also show that at a constant temperature the number of liquid droplets decreases exponentially as they freeze over time. The constant cooling rate experiments are consistent with the stochastic, singular and modified singular descriptions of heterogeneous nucleation; however, freezing during cooling and at constant temperature can be reconciled best with the stochastic approach. We report temperature dependent nucleation rate coefficients (nucleation events per unit Correspondence to: B. J. Murray (b.j.murray@leeds.ac.uk) time per unit area) for kaolinite and present a general parameterisation for immersion nucleation which may be suitable for cloud modelling once nucleation by other important ice nucleating species is quantified in the future.

Rates of homogeneous nucleation of ice in micrometre-sized water droplets are reported. Measurements were made using a new system in which droplets were supported on a hydrophobic substrate and their phase was monitored using optical microscopy as they were cooled at a controlled rate. Our nucleation rates are in agreement, given the quoted uncertainties, with the most recent literature data. However, the level of uncertainty in the rate of homogeneous freezing remains unacceptable given the importance of homogeneous nucleation to cloud formation in the Earth's atmosphere. We go on to use the most recent thermodynamic data for cubic ice (the metastable phase thought to nucleate from supercooled water) to estimate the interfacial energy of the cubic ice-supercooled water interface. We estimate a value of 20.8 +/- 1.2 mJ m(-2) in the temperature range 234.9-236.7 K.

Phase correction of Fourier Transform – Ion Cyclotron Resonance (FT-ICR) mass spectrometry data allows the spectra to be presented in absorption mode. Absorption mode spectra offer superior mass resolving power (up to a factor of 2), mass accuracy, and sensitivity over the conventional magnitude mode. Hitherto, the use of absorption mode in FT-ICR mass spectrometry has required either specially adapted instrumentation or a manually intensive process of phase correction or has ignored the potentially significant effects of image charge and the associated frequency shifts. Here we present an algorithm that allows spectra recorded on un-adapted FT-ICR mass spectrometers to be phase corrected, their baseline deviations removed, and then an absorption mode plot presented in an automated manner which requires little user interaction.

Abstract. Nanoparticles of iron oxide (crystalline and amorphous), silicon oxide and magnesium oxide were investigated for their propensity to nucleate ice over the temperature range 180-250 K, using the AIDA chamber in Karlsruhe, Germany.All samples were observed to initiate ice formation via the deposition mode at threshold ice super-saturations (RHi thresh ) ranging from 105% to 140% for temperatures below 220 K. Approximately 10% of amorphous Fe 2 O 3 particles (modal diameter = 30 nm) generated in situ from a photochemical aerosol reactor, led to ice nucleation at RHi thresh = 140% at an initial chamber temperature of 182 K. Quantitative analysis using a singular hypothesis treatment provided a fitted function [n s (190K) = 10 (3.33×s ice )+8.16 ] for the variation in ice-active surface site density (n s :m −2 ) with ice saturation (s ice ) for Fe 2 O 3 nanoparticles. This was implemented in an aerosol-cloud model to determine a predicted deposition (mass accommodation) coefficient for water vapour on ice of 0.1 at temperatures appropriate for the upper atmosphere. Classical nucleation theory was used to determine representative contact angles (θ) for the different particle compositions. For the in situ generated Fe 2 O 3 particles, a slight inverse temperature dependence was observed with θ = 10.5 • at 182 K, decreasing to 9.0 • at 200 K (compared with 10.2 • and 11.4 • respectively for the SiO 2 and MgO particle samples at the higher temperature).These observations indicate that such refractory nanoparticles are relatively efficient materials for the nucleation of ice under the conditions studied in the chamber which correspond to cirrus cloud formation in the upper troposphere. The results also show that Fe 2 O 3 particles do not act as Correspondence to: J. M. C. Plane (j.m.c.plane@leeds.ac.uk) ice nuclei under conditions pertinent for tropospheric mixed phase clouds, which necessarily form above ∼233 K. At the lower temperatures (<150 K) where noctilucent clouds form during summer months in the high latitude mesosphere, higher contact angles would be expected, which may reduce the effectiveness of these particles as ice nuclei in this part of the atmosphere.