M. Niemand

A Particle-Surface-Area-Based Parameterization of Immersion Freezing on Desert Dust Particles

AbstractIn climate and weather models, the quantitative description of aerosol and cloud processes relies on simplified assumptions. This contributes major uncertainties to the prediction of global and regional climate change. Therefore, models need good parameterizations for heterogeneous ice nucleation by atmospheric aerosols. Here the authors present a new parameterization of immersion freezing on desert dust particles derived from a large number of experiments carried out at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber facility. The parameterization is valid in the temperature range between −12° and −36°C at or above water saturation and can be used in atmospheric models that include information about the dust surface area. The new parameterization was applied to calculate distribution maps of ice nuclei during a Saharan dust event based on model results from the regional-scale model Consortium for Small-Scale Modelling–Aerosols and Reactive Trace Gases (COSMO-ART). The ...

A New Ice Nucleation Active Site Parameterization for Desert Dust and Soot

AbstractBased on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach.The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechan...

Heterogeneous Ice Nucleation Properties of Natural Desert DustParticles Coated with a Surrogate of Secondary Organic Aerosol

Ice nucleation abilities of surface collected mineral dust particles from the Sahara (SD) and Asia (AD) are investigated for the 15 temperature (T) range 253 – 233 K and for supersaturated relative humidity (RH) conditions in the immersion-freezing regime. The dust particles were also coated with a proxy of secondary organic aerosol (SOA) from the dark ozonolysis of α-pinene to better understand the influence of atmospheric coatings on the immersion freezing ability of mineral dust particles. The measurements are conducted on poly-disperse particles in the size range 0.01 – 3 μm with three different ice nucleation chambers. Two of the chambers follow the continuous flow diffusion chamber (CFDC) principle (Portable Ice Nucleation 20 Chamber, PINC) and the Colorado State University CFDC (CSU-CFDC), whereas the third was the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) cloud expansion chamber. From observed activated fractions (AF) and ice nucleation active site (INAS) densities, it is concluded within experimental uncertainties that there is no significant difference between the ice nucleation ability of the particular SD and AD samples examined. A small bias towards higher INAS densities for uncoated versus SOA coated dusts is found but this is well within the 1σ (66 % prediction bands) region of the average fit to 25 the data, which captures 75 % of the INAS densities observed in this study. Furthermore, no systematic differences are observed between SOA coated and uncoated dusts in both SD and AD cases, regardless of coating thickness (3 – 60 nm). The results suggest that any differences observed are within the uncertainty of the measurements or differences in cloud chamber parameters such as size fraction of particles sampled, and residence time, as well as assumptions in using INAS densities to compare poly-disperse aerosol measurements which may show variable composition with particle size. Coatings with similar 30 properties to that of the SOA in this work and with coating thickness up to 60 nm are not expected to impede or enhance the immersion mode ice nucleation ability of mineral dust particles.

Parameterizations of ice formation derived from AIDA cloud simulation experiments

Since 2003, the AIDA cloud chamber has been used for comprehensive series of ice nucleation experiments with a variety of different aerosols and in wide ranges of temperature, relative humidity and cooling rate. Ice nucleation onset and ice formation rates have been obtained as a function of aerosol parameters, ice supersaturation, temperature and cooling rate for homogeneous freezing of water droplets and solution particles, immersion freezing at and below water saturation, and deposition ice nucleation between ice and water saturation. The AIDA team has started a consistent and comprehensive re-analysis of the 10 year data set to provide a new set of parameters for formulating the ice formation in atmospheric models as function of aerosol properties, temperature and humidity. Here we present basic concepts and some selected results.