Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions ? Part II: Modelling

International audienceActivation energies ?Gact for the nucleation of nitric acid dihydrate (NAD) in supercooled binary HNO3/H2O solution droplets were calculated from volume-based nucleation rate measurements using the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) aerosol chamber of Forschungszentrum Karlsruhe. The experimental conditions covered temperatures T between 192 K and 197 K, NAD saturation ratios SNAD between 7 and 10, and nitric acid molar fractions of the nucleating sub-micron sized droplets between 0.26 and 0.28. Based on classical nucleation theory, a new parameterisation ?Gact=A×(T lnSNAD)-2+B is fitted to our experimetnal data with A=2.5×106 kcal K2 mol-1 and B=11.2?0.1(T?192) kcal mol-1. A and B were chosen to also achieve good agreement with literature data of ?Gact. The parameter A implies a constant interfacial tension ?sl=51 cal mol-1 cm-2 between the growing NAD germ and the supercooled solution. A slight temperature dependence of the diffusion activation energy is represented by the parameter B. Investigations with a detailed microphysical process model showed that literature formulations of volume-based (Salcedo et al., 2001) and surface-based (Tabazadeh et al., 2002) nucleation rates significantly overestimate NAD formation rates when applied to the conditions of our experiments

Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling

Activation energies &Delta;<i>G</i>_act for the nucleation of nitric acid dihydrate (NAD) in supercooled binary HNO₃/H₂O solution droplets were calculated from volume-based nucleation rate measurements using the AIDA (Aerosol, Interactions, and Dynamics in the Atmosphere) aerosol chamber of Forschungszentrum Karlsruhe. The experimental conditions covered temperatures T between 192 and 197 K, NAD saturation ratios <i>S</i>_NAD between 7 and 10, and nitric acid molar fractions of the nucleating sub-micron sized droplets between 0.26 and 0.28. Based on classical nucleation theory, a new parameterisation for &Delta;<i>G</i>_act=<i>A</i>&times;(<i>T</i> ln <i>S</i>_NAD)^&minus;2+<i>B</i> is fitted to the experimental data with <i>A</i>=2.5&times;10⁶ kcal K² mol^&minus;1 and <i>B</i>=11.2&minus;0.1(T&minus;192) kcal mol^&minus;1. <i>A</i> and <i>B</i> were chosen to also achieve good agreement with literature data of &Delta;<i>G</i>_act. The parameter <i>A</i> implies, for the temperature and composition range of our analysis, a mean interface tension &sigma;_<i>sl</i>=51 cal mol^&minus;1 cm^&minus;2 between the growing NAD germ and the supercooled solution. A slight temperature dependence of the diffusion activation energy is represented by the parameter <i>B</i>. Investigations with a detailed microphysical process model showed that literature formulations of volume-based (Salcedo et al., 2001) and surface-based (Tabazadeh et al., 2002) nucleation rates significantly overestimate NAD formation rates when applied to the conditions of our experiments

New cloud chamber experiments on the heterogeneous ice nucleation ability of oxalic acid in the immersion mode

The heterogeneous ice nucleation ability of oxalic acid in the immersion mode has been investigated by controlled expansion cooling runs with airborne, ternary solution droplets composed of, (i), sodium chloride, oxalic acid, and water (NaCl/OA/H₂O) and, (ii), sulphuric acid, oxalic acid, and water (H₂SO₄/OA/H₂O). Polydisperse aerosol populations with median diameters ranging from 0.5–0.7 μm and varying solute concentrations were prepared. The expansion experiments were conducted in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at initial temperatures of 244 and 235 K. In the ternary NaCl/OA/H₂O system, solid inclusions of oxalic acid, presumably nucleated as oxalic acid dihydrate, were formed by temporarily exposing the ternary solution droplets to a relative humidity below the efflorescence point of NaCl. The matrix of the crystallised NaCl particulates triggered the precipitation of the organic crystals which later remained as solid inclusions in the solution droplets when the relative humidity was subsequently raised above the deliquescence point of NaCl. The embedded oxalic acid crystals reduced the critical ice saturation ratio required for the homogeneous freezing of pure NaCl/H₂O solution droplets at a temperature of around 231 K from 1.38 to about 1.32. Aqueous solution droplets with OA inclusions larger than about 0.27 μm in diameter efficiently nucleated ice by condensation freezing when they were activated to micron-sized cloud droplets at 241 K, i.e., they froze well above the homogeneous freezing temperature of pure water droplets of about 237 K. Our results on the immersion freezing potential of oxalic acid corroborate the findings from a recent study with emulsified aqueous solutions containing crystalline oxalic acid. In those experiments, the crystallisation of oxalic acid diyhdrate was triggered by a preceding homogeneous freezing cycle with the emulsion samples. The expansion cooling cycles with ternary H₂SO₄/OA/H₂O solution droplets were aimed to analyse whether those findings can be transferred to ice nucleation experiments with airborne oxalic acid containing aerosol particles. Under our experimental conditions, the efficiency by which the surface of homogeneously nucleated ice crystals triggered the precipitation of oxalic acid dihydrate was very low, i.e., less than one out of a hundred ice crystals that were formed by homogeneous freezing in a first expansion cooling cycle left behind an ice-active organic crystal that acted as immersion freezing nucleus in a second expansion cooling cycle

Microbiology and atmospheric processes: the role of biological particles in cloud physics

As part of a series of papers on the sources, distribution and potential impact of biological particles in the atmosphere, this paper introduces and summarizes the potential role of biological particles in atmospheric clouds. Biological particles like bacteria or pollen may be active as both cloud condensation nuclei (CCN) and heterogeneous ice nuclei (IN) and thereby can contribute to the initial cloud formation stages and the development of precipitation through giant CCN and IN processes. The paper gives an introduction to aerosol-cloud processes involving CCN and IN in general and provides a short summary of previous laboratory, field and modelling work which investigated the CCN and IN activity of bacterial cells and pollen. Recent measurements of atmospheric ice nuclei with a continuous flow diffusion chamber (CFDC) and of the heterogeneous ice nucleation efficiency of bacterial cells are also briefly discussed. As a main result of this overview paper we conclude that a proper assessment of the impact of biological particles on tropospheric clouds needs new laboratory, field and modelling work on the abundance of biological particles in the atmosphere and their CCN and heterogeneous IN properties

Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber

International audienceSeries of infrared extinction spectra of ice crystals were recorded in the 6000?800 cm-1 wavenumber regime during expansion cooling experiments in the large aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe. Either supercooled sulphuric acid solution droplets or dry mineral dust particles were added as seed aerosols to initiate ice formation after having established ice supersaturated conditions inside the chamber. The various ice nucleation runs were conducted at temperatures between 237 and 195 K, leading to median sizes of the nucleated ice particles of 1?15 µm. The measured infrared spectra were fitted with reference spectra from T-matrix calculations to retrieve the number concentration as well as the number size distribution of the generated ice clouds. The ice particles were modelled as finite circular cylinders with aspect ratios ranging from 0.5 to 3.0. Benefiting from the comprehensive diagnostic tools for the characterisation of ice clouds which are available at the AIDA facility, the infrared retrieval results with regard to the ice particle number concentration could be compared to independent measurements with various optical particle counters. This provided a unique chance to quantitatively assess potential errors or solution ambiguities in the retrieval procedure which mainly originate from the difficulty to find an appropriate shape representation for the aspherical particle habits of the ice crystals. Based on these inter-comparisons, we demonstrate that there is no standard retrieval approach which can be routinely applied to all different experimental scenarios. In particular, the concept to account for the asphericity of the ice crystals, the a priori constraints which might be imposed on the unknown number size distribution of the ice crystals (like employing an analytical distribution function), and the wavenumber range which is included in the fitting algorithm should be carefully adjusted to each single retrieval problem

A new temperature and humidity dependent surface site density approach for deposition ice nucleation

Deposition nucleation experiments with Arizona Test Dust (ATD) as a surrogate for mineral dusts were conducted at the AIDA cloud chamber at temperatures between 220 and 250 K. The influence of the aerosol size distribution and the cooling rate on the ice nucleation efficiencies was investigated. Ice nucleation active surface site (INAS) densities were calculated to quantify the ice nucleation efficiency as a function of temperature, humidity and the aerosol surface area concentration. Additionally, a contact angle parameterization according to classical nucleation theory was fitted to the experimental data in order to relate the ice nucleation efficiencies to contact angle distributions. From this study it can be concluded that the INAS density formulation is a very useful tool to describe the temperature- and humidity-dependent ice nucleation efficiency of ATD particles. Deposition nucleation on ATD particles can be described by a temperature- and relative-humidity-dependent INAS density function ns(T, Sice) with ns(xtherm) = 1.88 &times;105 &middot; exp(0.2659 &middot; xtherm) [m&minus;2] , (1) where the temperature- and saturation-dependent function xtherm is defined as xtherm = &minus;(T&minus;273.2)+(Sice&minus;1) &times;100, (2) with the saturation ratio with respect to ice Sice >1 and within a temperature range between 226 and 250 K. For lower temperatures, xtherm deviates from a linear behavior with temperature and relative humidity over ice. Also, two different approaches for describing the time dependence of deposition nucleation initiated by ATD particles are proposed. Box model estimates suggest that the time-dependent contribution is only relevant for small cooling rates and low number fractions of ice-active particles

Numerical simulations of homogeneous freezing processes in the aerosol chamber AIDA

The homogeneous freezing of supercooled H₂SO₄/H₂O aerosols in an aerosol chamber is investigated with a microphysical box model using the activity parameterization of the nucleation rate by Koop et al. (2000). The simulations are constrained by measurements of pressure, temperature, total water mixing ratio, and the initial aerosol size distribution, described in a companion paper Möhler et al. (2003). Model results are compared to measurements conducted in the temperature range between 194 and 235 K, with cooling rates in the range between 0.5 and 2.6 K min^-1, and at air pressures between 170 and 1000 hPa. The simulations focus on the time history of relative humidity with respect to ice, aerosol size distribution, partitioning of water between gas and particle phase, onset times of freezing, freezing threshold relative humidities, aerosol chemical composition at the onset of freezing, and the number of nucleated ice crystals. The latter four parameters can be inferred from the experiments, the former three aid in interpreting the measurements. Sensitivity studies are carried out to address the relative importance of uncertainties of basic quantities such as temperature, total H₂O mixing ratio, aerosol size spectrum, and deposition coefficient of H₂O molecules on ice. The ability of the numerical simulations to provide detailed explanations of the observations greatly increases confidence in attempts to model this process under real atmospheric conditions, for instance with regard to the formation of cirrus clouds or polar stratospheric ice clouds, provided that accurate temperature and humidity measurements are available