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

Circular depolarization ratios of single water droplets and finite ice circular cylinders: a modeling study

Computations of the phase matrix elements for single water droplets and ice crystals in fixed orientations are presented to determine if circular depolarization &amp;delta;&lt;sub&gt;C&lt;/sub&gt; is more accurate than linear depolarization for phase discrimination. T-matrix simulations were performed to calculate right-handed and left-handed circular depolarization ratios &amp;delta;&lt;sub&gt;+C&lt;/sub&gt;, respectively &amp;delta;&lt;sub&gt;&amp;minus;C&lt;/sub&gt; and to compare them with linear ones. Ice crystals are assumed to have a circular cylindrical shape where their surface-equivalent diameters range up to 5 μm. The circular depolarization ratios of ice particles were generally higher than linear depolarization and depended mostly on the particle orientation as well as their sizes. The fraction of non-detectable ice crystals (&amp;delta;&lt;0.05) was smaller considering a circular polarized light source, reaching 4.5%. However, water droplets also depolarized light circularly for scattering angles smaller than 179° and size parameters smaller than 6 at side- and backscattering regions. Differentiation between ice crystals and water droplets might be difficult for experiments performed at backscattering angles which deviate from 180° unlike LIDAR applications. Instruments exploiting the difference in the &lt;I&gt;P&lt;/I&gt;&lt;sub&gt;44&lt;/sub&gt;/&lt;I&gt;P&lt;/I&gt;&lt;sub&gt;11&lt;/sub&gt; ratio at a scattering angle around 115° are significantly constrained in distinguishing between water and ice because small droplets with size parameters between 5 and 10 do cause very high circular depolarizations at this angle. If the absence of the liquid phase is confirmed, the use of circular depolarization in single particle detection is more sensitive and less affected by particle orientation

Contact freezing: a review of experimental studies

This manuscript compiles both theoretical and experimental information on contact freezing with the aim to better understand this potentially important but still not well quantified heterogeneous freezing mode. There is no complete theory that describes contact freezing and how the energy barrier has to be overcome to nucleate an ice crystal by contact freezing. Experiments on contact freezing conducted using the cold plate technique indicate that it can initiate ice formation at warmer temperatures than immersion freezing. Additionally, a qualitative difference in the freezing temperatures between contact and immersion freezing has been found using different instrumentation and different ice nuclei. There is a lack of data on collision rates in most of the reported data, which inhibits a quantitative calculation of the freezing efficiencies. Thus, new or modified instrumentation to study contact nucleation in the laboratory and in the field are needed to identify the conditions at which contact nucleation could occur in the atmosphere. Important questions concerning contact freezing and its potential role for ice cloud formation and climate are also summarized

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

Laboratory studies of immersion and deposition mode ice nucleation of ozone aged mineral dust particles

Ice nucleation in the atmosphere is central to the understanding the microphysical properties of mixed-phase and cirrus clouds. Ambient conditions such as temperature (T) and relative humidity (RH), as well as aerosol properties such as chemical composition and mixing state play an important role in predicting ice formation in the troposphere. Previous field studies have reported the absence of sulfate and organic compounds on mineral dust ice crystal residuals sampled at mountain top stations or aircraft based measurements despite the long-range transport mineral dust is subjected to. We present laboratory studies of ice nucleation for immersion and deposition mode on ozone aged mineral dust particles for 233 T ns) are reported and observed to increase as a function of decreasing temperature. We present first results that demonstrate enhancement of the ice nucleation ability of aged mineral dust particles in both the deposition and immersion mode due to ageing. We also present the first results to show a suppression of heterogeneous ice nucleation activity without the condensation of a coating of (in)organic material. In immersion mode, low ozone exposed Ka particles showed enhanced ice activity requiring a median freezing temperature of 1.5 K warmer than that of untreated Ka, whereas high ozone exposed ATD particles showed suppressed ice nucleation requiring a median freezing temperature of 3 K colder than that of untreated ATD. In deposition mode, low exposure Ka had ice active fractions of an order of magnitude higher than untreated Ka, whereas high ozone exposed ATD had ice active fractions up to a factor of 4 lower than untreated ATD. From our results, we derive and present parameterizations in terms of ns(T) that can be used in models to predict ice nuclei concentrations based on available aerosol surface area

Time dependence of immersion freezing: an experimental study on size selected kaolinite particles

The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down to supercooled temperatures, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets increased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freezing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher temperatures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a distribution of contact angles is able to reproduce the ice nucleation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typical mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of −1 K temperature shift for an increase in times scale by one order of magnitude. This suggests that temperature is more important than time

Effect of photochemical ageing on the ice nucleation properties of diesel and wood burning particles

A measurement campaign (IMBALANCE) conducted in 2009 was aimed at characterizing the physical and chemical properties of freshly emitted and photochemically aged combustion particles emitted from a log wood burner and diesel vehicles: a EURO3 Opel Astra with a diesel oxidation catalyst (DOC) but no particle filter and a EURO2 Volkswagen Transporter TDI Syncro without emission aftertreatment. Ice nucleation experiments in the deposition and condensation freezing modes were conducted with the Portable Ice Nucleation Chamber (PINC) at three nominal temperatures, −30 °C, −35 °C and −40 °C. Freshly emitted diesel particles showed ice formation only at −40 °C in the deposition mode at 137% relative humidity with respect to ice (RH&lt;sub&gt;i&lt;/sub&gt;) and 92% relative humidity with respect to water (RH&lt;sub&gt;w&lt;/sub&gt;), and photochemical ageing did not play a role in modifying their ice nucleation behaviour. Only one diesel experiment where α-pinene was added for the ageing process, showed an ice nucleation enhancement at −35 °C. Wood burning particles also act as ice nuclei (IN) at −40 °C in the deposition mode at the same conditions as for diesel particles and photochemical ageing also did not alter the ice formation properties of the wood burning particles. Unlike diesel particles, wood burning particles form ice via condensation freezing at −35 °C whereas no ice nucleation was observed at −30 °C. Photochemical ageing did not affect the ice nucleation ability of the diesel and wood burning particles at the three different temperatures investigated but a broader range of temperatures below −40 °C need to be investigated in order to draw an overall conclusion on the effect of photochemical ageing on deposition/condensation ice nucleation across the entire temperature range relevant to cold clouds

Experimental investigation of homogeneous freezing of sulphuric acid particles in the aerosol chamber AIDA

The homogeneous freezing of supercooled H₂SO₄/H₂O solution droplets was investigated in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe. 24 freezing experiments were performed at temperatures between 189 and 235 K with aerosol particles in the diameter range 0.05 to 1 µm. Individual experiments started at homogeneous temperatures and ice saturation ratios between 0.9 and 0.95. Cloud cooling rates up to -2.8 K min^-1 were simulated dynamically in the chamber by expansion cooling using a mechanical pump. Depending on the cooling rate and starting temperature, freezing threshold relative humidities were exceeded after expansion time periods between about 1 and 10 min. The onset of ice formation was measured with three independent methods showing good agreement among each other. Ice saturation ratios measured at the onset of ice formation increased from about 1.4 at 231 K&nbsp; to about 1.75 at 189 K. The experimental data set including thermodynamic parameters as well as physical and chemical aerosol analysis provides a good basis for microphysical model applications