Search results for "Supersaturation"

Nitric Acid Trihydrate (NAT) formation at low NAT supersaturation in Polar Stratospheric Clouds (PSCs)

2005

International audience; A PSC was detected on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (~10-4cm-3) of small nitric acid (HNO3) containing particles (dTNAT, these NAT particles have the potential to grow further and to remove HNO3 from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures just slightly below TNAT (T>TNAT-3.1K). This unique measurement of PSC formation at extremely low NAT saturation ratios (SNAT?10) constrains current NAT nucleation theories. We suggest, that the NAT particles have formed heterogeneo…

In-situ observations and modeling of small nitric acid-containing ice crystals

2007

Measurements in nascent ice forming regions are very rare and help understand cirrus cloud formation and the interactions of trace gases with ice crystals. A tenuous cirrus cloud has been probed with in-situ and remote sensing instruments onboard the high altitude research aircraft Geophysica M55 in the tropical upper troposphere. Besides microphysical and optical particle properties, water (H<sub>2</sub>O) and reactive nitrogen species (NO<sub>y</sub>) have been measured. In slightly ice supersaturated air between 14.2 and 14.9 km altitude, an unusually low ice water content of 0.031 mg m<sup>−3</sup> and small ice crystals with mean radii of 5…

Environmental scanning electron microscopy (ESEM) as a new technique to determine the ice nucleation capability of individual atmospheric aerosol par…

2007

Abstract Heterogeneous ice nucleation on synthetic silver iodide, natural kaolinite and montmorillonite particles via condensation, freezing and deposition modes was studied by environmental scanning electron microscopy (ESEM) in the temperature range of 250–270 K. By increasing the H2O pressure in the sample chamber at constant temperature, ice formation can be studied in situ and can be related to the chemical composition of the particles that can be determined simultaneously. For silver iodide and kaolinite, supersaturation values of first ice formation are in good agreement (1–2% absolute) with diffusion chamber experiments. For both substances, threshold temperatures for the condensati…

Molecular dynamics simulation of the surface tension of aqueous sodium chloride: from dilute to highly supersaturated solutions and molten salt

2018

Sodium chloride (NaCl) is one of the key components of atmospheric aerosols. The surface tension of aqueous NaCl solution (σNaCl,sol) and its concentration dependence are essential to determine the equilibrium water vapor pressure of aqueous NaCl droplets. Supersaturated NaCl solution droplets are observed in laboratory experiments and under atmospheric conditions, but the experimental data for σNaCl,sol are mostly limited up to subsaturated solutions. In this study, the surface tension of aqueous NaCl is investigated by molecular dynamics (MD) simulations and the pressure tensor method from dilute to highly supersaturated solutions. We show that the linear approximation of concentration de…

Ice nucleation properties of the most abundant mineral dust phases

2008

[1] The ice nucleation properties of the nine most abundant minerals occurring in desert aerosols (quartz, albite, microcline, kaolinite, montmorillonite, illite, calcite, gypsum, and hematite) were investigated by environmental scanning electron microscopy (ESEM). In this instrument, the pure minerals are exposed to water vapor at variable pressures and temperatures. The crystallization of ice on the mineral particles is observed by secondary electron imaging, and the supersaturation for an activated particle fraction of 1–3% is determined as function of temperature. In all experiments, condensation of water prior to ice formation was not observed within detectable limits, even at water su…

Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation

2010

Abstract. During the FROST-2 (FReezing Of duST) measurement campaign conducted at the Leipzig Aerosol Cloud Interaction Simulator (LACIS), we investigated changes in the ice nucleation properties of 300 nm Arizona Test Dust mineral particles following thermochemical processing by varying amounts and combinations of exposure to sulphuric acid vapour, ammonia gas, water vapour, and heat. The processed particles' heterogeneous ice nucleation properties were determined in both the water subsaturated and supersaturated humidity regimes at −30 °C and −25 °C using Colorado State University's continuous flow diffusion chamber. The amount of sulphuric acid coating material was estimated by an aeroso…

Aerosol physicochemical effects on CCN activation simulated with the chemistry-climate model EMAC

2017

Abstract This study uses the EMAC atmospheric chemistry-climate model to simulate cloud properties with a prognostic cloud droplet nucleation scheme. We present modeled global distributions of cloud condensation nuclei (CCN) number concentrations and CCN activation rates, together with the effective hygroscopicity parameter κ, to describe the aerosol chemical composition effect on CCN activation. Large particles can easily activate into cloud droplets, even at low κ values due to the dominant size effect in cloud droplet formation. Small particles are less efficiently activated as CCN, and are more sensitive to aerosol composition and supersaturation. Since the dominant fraction of small pa…

2018

Abstract. The growth of small cloud droplets and ice crystals is dominated by the diffusion of water vapor. Usually, Maxwell's approach to growth for isolated particles is used in describing this process. However, recent investigations show that local interactions between particles can change diffusion properties of cloud particles. In this study we develop an approach for including these local interactions into a bulk model approach. For this purpose, a simplified framework of local interaction is proposed and governing equations are derived from this setup. The new model is tested against direct simulations and incorporated into a parcel model framework. Using the parcel model, possible i…

The ice nucleating ability of pollen

2001

Abstract Laboratory experiments are described where the water uptake by a variety of pollen was studied quantitatively, followed by the investigation of the ice nucleating ability of four kinds of pollen in the deposition and the condensation freezing modes. The diameters of the pollen selected for the freezing experiments were between 25 and 70 μm. The freezing experiments in the deposition mode including also pollen resuspended from decayed leaves, and crushed pollen grains were carried out at different temperatures down to −33 °C combined with various supersaturations with respect to ice up to 35%. The condensation freezing experiments were carried out at temperatures down to −18 °C at s…

A laboratory study of the uptake of HNO3 and HCl vapor by snow crystals and ice spheres at temperatures between 0 and −40°C

1995

Abstract A laboratory experiment is described during which the uptake of HNO3 and HCl vapor by dendritic snow crystals and by single crystalline and polycrystalline small ice spheres was studied at ppbv and ppmv gas levels and at temperatures between 0 and −40°C. In one experimental investigation the vapor was allowed to be adsorbed onto the surface of the ice particles. During another experimental investigation the ice particles were allowed to grow from water vapor on fine fibers in the presence of the HNO3 and HCl vapor. The results of our experiments show that under both conditions significant amounts of HNO3 and HCl became scavenged by the ice particles. Scavenging by adsorption was ma…