Beiping Luo

Water activity as the determinant for homogeneous ice nucleation in aqueous solutions

The unique properties of water in the supercooled (metastable) state are not fully understood. In particular, the effects of solutes and mechanical pressure on the kinetics of the liquid-to-solid phase transition of supercooled water and aqueous solutions to ice have remained unresolved. Here we show from experimental data that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution—that is, the ratio between the water vapour pressures of the solution and of pure water under the same conditions. In addition, we show that the presence of solutes and the application of pressure have a very similar effect on ice nucleation. We present a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressure. Recent observations from clouds containing ice are in good agreement with our theory and our results should help to overcome one of the main weaknesses of numerical models of the atmosphere, the formulation of cloud processes.

An analytic expression for the composition of aqueous HNO3‐H2SO4 stratospheric aerosols including gas phase removal of HNO3

An analytic expression is given for the composition and volume of stratospheric HNO3-H2SO4-H2O aerosols in terms of the temperature and total amounts of H2O, HNO3 and H2SO4, taking into account conservation of HNO3. In contrast to previous parameterisations of equilibrium vapour pressures, the present scheme for calculating the aerosol composition does not require an iteration procedure, thus computation times are significantly reduced, making it ideal for inclusion in large-scale atmospheric models. Calculated compositions are compared with the results of a comprehensive thermodynamic model for all stratospheric conditions and maximum deviations are presented.

Do stratospheric aerosol droplets freeze above the ice frost point

Laboratory experiments are presented which show that liquid stratospheric aerosol droplets under polar winter conditions do not freeze for temperatures higher than the water ice saturation temperature (frost point). Calorimetric measurements of the freezing of supercooled H2SO4/HNO3/H2O bulk solutions with concentrations typical of the polar stratospheric aerosol exhibit very small freezing rates, which excludes the possibility of homogeneous freezing of the droplets for temperatures above the frost point. Even heterogeneous formation of H2SO4 and HNO3 hydrates at these temperatures is a very inefficient process unless the stratosphere offers nuclei better suited for nucleation than those present in the laboratory experiments, which appears to be unlikely. Only ice was found to be a potential nucleus suited for the formation of the hydrates, which could cause the hydrates to freeze at temperatures below the frost point.

Size-dependent stratospheric droplet composition in lee wave temperature fluctuations and their potential role in PSC freezing

Rapid temperature fluctuations are shown to cause liquid H2SO4/HNO3/H2O stratospheric aerosols to depart considerably from thermodynamic equilibrium. While HNO3 uptake by larger droplets is diffusively hindered, small droplets can approach the composition of a pure binary HNO3/H2O solution with up to 52 wt% HNO3, 48 wt% H2O and very small amounts of H2SO4. The stoichiometry of these droplets is close to that of nitric acid trihydrate (NAT) and freezing experiments suggest that this could be a suitable pathway for the formation of frozen polar stratospheric clouds (PSCs) of type-Ia.

vapour pressures of H2SO4/HNO3/HCl/HBr/H2O solutions to low stratospheric temperatures

Vapor pressures of H2O, HNO3, HCl and HBr over supercooled aqueous mixtures with sulfuric acid have been calculated using an activity coefficient model, for 185 K less than T less than 235 K, 0 less than wt% (H2SO4) + wt% (HNO3) less than 70, and assuming HCl and HBr to be minor constituents. Predicted vapor pressures agree well with most laboratory data, and give confidence in the validity of the model. The results are parameterized as simple formulae, which reproduce the model results to within 40% and cover the entire stratospherically relevant range of composition and temperature.

The potential of cirrus clouds for heterogeneous chlorine activation

The ER-2 data from ascents and descents through layers of cirrus clouds are utilized to study the heterogeneous reactions of ClONO 2 with H 2 O, of HOCl and ClONO 2 with HCl, and their potential role for the activation of chlorine in the tropopause regions which could affect ozone there. Lacking measured data for the three chlorine containing molecules their abundances as a function of altitude have been calculated from a 2D model. The aerosol surface data measured by a Forward Scattering Spectrometer Probe (FSSP-300) on the ER-2 were corrected for the expected asphericity of cirrus cloud particles by means of a T-matrix method. The results indicate considerable potential of cirrus clouds for chlorine activation. If ClONO 2 and HCl are present in the tropopause region they are likely to be quickly converted to active chlorine by cirrus clouds.

APPLICATION OF THE T-MATRIX METHOD TO THE MEASUREMENT OF ASPHERICAL (ELLIPSOIDAL) PARTICLES WITH FORWARD SCATTERING OPTICAL PARTICLE COUNTERS

The response of the FSSP-300 and MASP optical particle counters with forward scattering geometries has been numerically simulated adopting the T-matrix method in order to assess the instrumental properties when exposed to clouds of aspherical particles (here: rotationally symmetric ellipsoids). The resulting scattering cross sections as function of particle size are compared to Mie theory calculations for spherical particles of the same refractive index, scattering geometry, and light wavelength. Based on these T-matrix calculations the FSSP-300 instrument is found suitable for use in populations of aspherical particles with aspect ratios larger than 0.5, if various caveats are taken into consideration. From the T-matrix method new tables result, which relate the instruments size bin limits to actual particle sizes for a given refractive index. Two data sets of measurements in a cirrus cloud and an aircraft contrail are reduced by means of these new assignment tables in order to demonstrate the applicability of the method to atmospheric clouds. This paper also discusses in detail the limitations inherent in the use of the T-matix method as replacement for the Mie theory calculations commonly adopted for the FSSP-300 type of instruments.

Freezing of polar stratospheric clouds in orographically induced strong warming events

Results from laboratory experiments and microphysical modeling are presented that suggest a potential freezing nucleation mechanism for polar stratospheric cloud (PSC) particles above the water ice frost point (Tice). The mechanism requires very high HNO3 concentrations of about 58 wt% in the droplets, and leads to the freezing of nitric acid dihydrate (NAD) in a highly selective manner in the smallest droplets of an ensemble. In the stratosphere such liquid compositions are predicted to occur in aerosol droplets which are warmed adiabatically with rates of about +150 K/h from below 190 K to 194 K. Such rapid temperature changes have been observed in mountain leewaves that occur frequently in the stratosphere, clearly demonstrating the need for a stratospheric gravity wave climatology.

Freezing of stratospheric aerosol droplets

Theoretical calculations are presented for homogeneous and heterogeneous freezing of sulfuric acid droplets under stratospheric conditions, based on classical nucleation theory. In contrast to previous results it is shown that a prominent candidate for freezing, sulfuric acid tetrahydrate (SAT ≡ H2SO4·4H2O), does not freeze homogeneously. The theoretical results limit the homogeneous freezing rate at 200 K to much less than 1 cm−3s−1, a value that may be estimated from bulk phase laboratory experiments. This suggests that the experimental value is likely to be a measure of heterogeneous, not homogeneous nucleation. Thus, under statospheric conditions, freezing of SAT can only occur in the presence of suitable nuclei; however, even for heterogeneous nucleation experimental results impose strong constraints. Since a nitric acid trihydrate (NAT) embryo probably needs a solid body for nucleation, these results put an important constraint on the theory of NAT formation in polar stratospheric clouds.

Solubility of HOCl in water and aqueous H2SO4 to stratospheric temperatures

Henrys law constants K′H (mol kg−1 atm−1) for the reaction HOCl(g)=HOCl(aq) near room temperature, literature data for the associated enthalpy change, and solubilities of HOCl in aqueous H2SO4 (46 to 60 wt%) at temperatures relevant to the stratosphere (200 K≤T≤230 K) are shown to be thermodynamically consistent. Effective Henrys law constants [H*=mHOCl/pHOCl, in mol kg−1 atm−1] of HOCl in aqueous H2SO4 are given by: ln(H*)=6.4946−mH2SO4(−0.04107+54.56/T)−5862 (1/To−1/T) where T(K) is temperature and To=298.15K. The activity coefficient of HOCl in aqueous H2SO4 has a simple Setchenow-type dependence upon H2SO4 molality.