T. L. Malkin

The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds

The amount of ice present in mixed-phase clouds, which contain both supercooled liquid water droplets and ice particles, affects cloud extent, lifetime, particle size and radiative properties. The freezing of cloud droplets can be catalysed by the presence of aerosol particles known as ice nuclei. One of the most important ice nuclei is thought to be mineral dust aerosol from arid regions. It is generally assumed that clay minerals, which contribute approximately two-thirds of the dust mass, dominate ice nucleation by mineral dust, and many experimental studies have therefore focused on these materials. Here we use an established droplet-freezing technique to show that feldspar minerals dominate ice nucleation by mineral dusts under mixed-phase cloud conditions, despite feldspar being a minor component of dust emitted from arid regions. We also find that clay minerals are relatively unimportant ice nuclei. Our results from a global aerosol model study suggest that feldspar ice nuclei are globally distributed and that feldspar particles may account for a large proportion of the ice nuclei in Earth’s atmosphere that contribute to freezing at temperatures below about −15 °C.

Structure of ice crystallized from supercooled water

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. At ambient conditions, ice is considered to exist in two crystalline forms: stable hexagonal ice and metastable cubic ice. Using X-ray diffraction data and Monte Carlo simulations, we show that ice that crystallizes homogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I. Stacking disorder and stacking faults have been reported earlier for metastable ice I, but only for ice crystallizing in mesopores and in samples recrystallized from high-pressure ice phases rather than in water droplets. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder. These findings highlight the need to reevaluate the physical and thermodynamic properties of this metastable ice as a function of the nature and extent of stacking disorder using well-characterized samples.

An iron(II) complex exhibiting five anhydrous phases, two of which interconvert by spin-crossover with wide hysteresis

[FeL2][BF4]2·2H2O (L = 2,6-di{5-methylpyrazol-3-yl}pyridine) adopts a 1 : 1 high : low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1AA) or by thermal dehydration (phase 1BB). Upon cooling in vacuo, the latter undergoes a thermal spin-state transition centred near T1/2 = 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1BB, initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1AA, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.

Stacking disorder in ice I

Traditionally, ice I was considered to exist in two well-defined crystalline forms at ambient pressure: stable hexagonal ice (ice Ih) and metastable cubic ice (ice Ic). However, it is becoming increasingly evident that what has been called cubic ice in the past does not have a structure consistent with the cubic crystal system. Instead, it is a stacking-disordered material containing cubic sequences interlaced with hexagonal sequences, which is termed stacking-disordered ice (ice Isd). In this article, we summarise previous work on ice with stacking disorder including ice that was called cubic ice in the past. We also present new experimental data which shows that ice which crystallises after heterogeneous nucleation in water droplets containing solid inclusions also contains stacking disorder even at freezing temperatures of around -15 °C. This supports the results from molecular simulations, that the structure of ice that crystallises initially from supercooled water is always stacking-disordered and that this metastable ice can transform to the stable hexagonal phase subject to the kinetics of recrystallization. We also show that stacking disorder in ice which forms from water droplets is quantitatively distinct from ice made via other routes. The emerging picture of ice I is that of a very complex material which frequently contains stacking disorder and this stacking disorder can vary in complexity depending on the route of formation and thermal history.

Ground and Airborne U.K. Measurements of Nitryl Chloride: An Investigation of the Role of Cl Atom Oxidation at Weybourne Atmospheric Observatory

Nitryl Chloride (ClNO2) measurements from the Weybourne Atmospheric Observatory (WAO) are reported from March and April 2013 using a quadruple chemical ionisation mass spectrometer (CIMS) with the I- ionisation scheme. WAO is a rural coastal site with generally low NOx concentrations, a type of location poorly studied for ClNO2 production. Concentrations of ClNO2 exceeded that of the limit of detection (0.8 ppt) on each night of the campaign, as did concentrations of N2O5, which was also measured simultaneously with the Cambridge Broadband Cavity Enhanced Absorption Spectrometer (BBCEAS). A peak concentration of 65 ppt of ClNO2 is reported here. Vertical profiles of ClNO2 from early- to mid-morning flights in close proximity to WAO are also reported, showing elevated concentrations at low altitude. The photolysis of observed ClNO2 and a box model utilising the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. This model utilised numerous VOCs from the second Tropospheric ORganic CHemistry project (TORCH 2) in 2004, at the same location and time of year. From this the relative importance of the oxidation of three groups of measured VOCs (alkanes, alkenes and alkynes) by OH radicals, Cl atoms and O3 is compared. Cl atom oxidation was deemed generally insignificant at this time and location for total oxidation due to the much lower concentration of ClNO2 observed, even following the night of greatest ClNO2 production.

Heterogeneous Ice Nucleation by Soufriere Hills Volcanic Ash Immersed in Water Droplets

Fine particles of ash emitted during volcanic eruptions may sporadically influence cloud properties on a regional or global scale as well as influencing the dynamics of volcanic clouds and the subsequent dispersion of volcanic aerosol and gases. It has been shown that volcanic ash can trigger ice nucleation, but ash from relatively few volcanoes has been studied for its ice nucleating ability. In this study we quantify the efficiency with which ash from the Soufriere Hills volcano on Montserrat nucleates ice when immersed in supercooled water droplets. Using an ash sample from the 11th February 2010 eruption, we report ice nucleating efficiencies from 246 to 265 K. This wide range of temperatures was achieved using two separate droplet freezing instruments, one employing nanolitre droplets, the other using microlitre droplets. Soufriere Hills volcanic ash was significantly more efficient than all other ash samples that have been previously examined. At present the reasons for these differences are not understood, but may be related to mineralogy, amorphous content and surface chemistry.

Atmospheric ice nucleation by fertile soil dusts particles: Relative importance of mineral and biological components

Dusts emitted from agricultural soils may represent a significant source of atmospheric particulates at mid-latitudes. Such dusts, which can be aerosolised by anthropogenic agricultural activities, have previously been estimated to be present in the atmosphere at sufficient number densities that they could potentially compete with other known ice nuclei (IN). In contrast to soils from arid regions, such as the Sahara, fertile soils contain a larger fraction of biological material, which can lead to an enhancement in the ice nucleating ability of their associated dusts. However, considerable uncertainties remain regarding the relative efficacy of soil dust particles from fertile soils as IN. Using an experimental methodology designed to increase sensitivity to a wide range of ice nucleation efficiencies, we have characterized the immersion mode ice nucleating activities of sub 11 μm particles extracted from surface soils collected in four locations around England. By using a variety of droplet sizes, from ...