The Experts below are selected from a list of 267 Experts worldwide ranked by ideXlab platform
Jonathan P Reid - One of the best experts on this subject based on the ideXlab platform.
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simultaneous analysis of the equilibrium hygroscopicity and water transport kinetics of Liquid Aerosol
Analytical Chemistry, 2013Co-Authors: James F Davies, Allen E Haddrell, Andrew M J Rickards, Jonathan P ReidAbstract:We demonstrate that the equilibrium hygroscopic response of an Aerosol droplet and the kinetics of water condensation and evaporation can be retrieved with high accuracy, even close to saturation, through comparative measurements of probe and sample Aerosol droplets. The experimental methodology is described and is based on an electrodynamic balance with a newly designed trapping chamber. Through use of a probe Aerosol, composed of either pure water or a sodium chloride solution of known concentration, the gas-phase relative humidity (RH) can be accurately measured with an uncertainty of typically <0.005. By fast manipulation of the airflows into the chamber, a step-change in RH over a time scale of <0.5 s can be achieved. Using this approach, the kinetics of mass transfer are studied using the comparative procedure, and results are compared to theoretical mass flux predictions. The time-dependent measured mass fluxes for sodium chloride, ammonium sulfate, sorbitol, and galactose are used to calculate dro...
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Spectroscopic studies of the size and composition of single Aerosol droplets
International Reviews in Physical Chemistry, 2007Co-Authors: Jonathan P Reid, Laura Mitchem, Helena Meresman, Rachel SymesAbstract:The characterization of Aerosol properties and processes, non-intrusively and directly, poses a severe analytical challenge. In order to understand the role of Aerosols in often complex environments, it is necessary to probe the particles in situ and without perturbation. Sampling followed by end-of-line analysis can lead to perturbations in particle composition, morphology and size, particularly when analysing Liquid Aerosol droplets containing volatile components. Optical spectroscopy can provide a strategy for the direct assessment of particle size, composition and phase. We review here the application of linear and non-linear Raman spectroscopies in the characterization of Liquid Aerosol droplets. Spontaneous Raman scattering can allow the unambiguous identification of chemical components and the determination of droplet composition. Stimulated Raman spectroscopy can allow the determination of droplet size with nanometre accuracy and can allow the characterization of near-surface composition. When com...
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Controlling and characterizing the coagulation of Liquid Aerosol droplets
Journal of Chemical Physics, 2006Co-Authors: Jariya Buajarern, Laura Mitchem, Andrew D Ward, David Mcgloin, N. Hendrik Nahler, Jonathan P ReidAbstract:We demonstrate that optical tweezers can be used to control and characterize the coagulation and mixing state of Aerosols. Liquid Aerosol droplets of 2-14 mum in diameter are optically trapped and characterized by spontaneous and stimulated Raman scatterings, which together provide a unique signature of droplet size and composition. From the conventional bright field image, the size of the trapped droplet can be estimated and compared with that determined from stimulated Raman scattering, and the motion of the particle within the trapping plane can be recorded. A maximum of four droplets can be manipulated in tandem by forming multiple optical traps through rapid beam steering. The coagulation of two droplets can be studied directly by controlling two droplets. The limiting conditions under which optical forces and capillary forces dominate the Aerosol coagulation event are explored by varying the relative optical trap strengths and characterizing the coagulation of different droplet sizes. Finally, we demonstrate that the coagulation of different Aerosol components can be compared and the mixing state of the final coagulated droplet can be investigated. In particular, we compare the outcome of the coagulation of an aqueous sodium chloride Aerosol droplet with a second aqueous droplet, with an ethanol droplet or with a decane droplet.
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controlling and characterizing the coagulation of Liquid Aerosol droplets
Journal of Chemical Physics, 2006Co-Authors: Jariya Buajarern, Laura Mitchem, Andrew D Ward, Hendrik N Nahler, David Mcgloin, Jonathan P ReidAbstract:We demonstrate that optical tweezers can be used to control and characterize the coagulation and mixing state of Aerosols. Liquid Aerosol droplets of 2–14μm in diameter are optically trapped and characterized by spontaneous and stimulated Raman scatterings, which together provide a unique signature of droplet size and composition. From the conventional bright field image, the size of the trapped droplet can be estimated and compared with that determined from stimulated Raman scattering, and the motion of the particle within the trapping plane can be recorded. A maximum of four droplets can be manipulated in tandem by forming multiple optical traps through rapid beam steering. The coagulation of two droplets can be studied directly by controlling two droplets. The limiting conditions under which optical forces and capillary forces dominate the Aerosol coagulation event are explored by varying the relative optical trap strengths and characterizing the coagulation of different droplet sizes. Finally, we demo...
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spectroscopy of growing and evaporating water droplets exploring the variation in equilibrium droplet size with relative humidity
Journal of Physical Chemistry A, 2006Co-Authors: Laura Mitchem, Jariya Buajarern, Andrew D Ward, Rebecca J Hopkins, Richard J J Gilham, Roy L Johnston, Jonathan P ReidAbstract:We demonstrate that the thermodynamic properties of a single Liquid Aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2−6 μm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence ...
Bernd Kärcher - One of the best experts on this subject based on the ideXlab platform.
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Cirrus Clouds and Their Response to Anthropogenic Activities
Current Climate Change Reports, 2017Co-Authors: Bernd KärcherAbstract:Cirrus clouds are ubiquitous, long-lived, high-level ice clouds that exert a considerable global radiative effect on the climate system. This review assesses recent observational and modeling evidence of how anthropogenic activities might affect cirrus. Changes in physical properties and chemical composition of Liquid Aerosol particles will unlikely affect cirrus significantly, but anthropogenic influences may occur through changes in heterogeneous ice nuclei. Two main uncertain factors contribute to the current inability to constrain background cirrus formation: small-scale variability in dynamical forcings that drive ice nucleation parameterizations and the ability of airborne particles to act as efficient heterogeneous ice nuclei. These uncertainties keep us from drawing robust conclusions about anthropogenic influences on cirrus. Microphysical and macrophysical representation of cirrus in global models must first be advanced before we can predict changes in climate with fewer uncertainties. Some climate intervention studies suggest a potential cooling effect of deliberately perturbed cirrus, but at the risk of modifying precipitation inadvertently.
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Formation of nitric acid/water ice particles in cirrus clouds
Geophysical Research Letters, 2006Co-Authors: Bernd Kärcher, Christian VoigtAbstract:Nitric acid (HNO3) in cirrus ice crystals has been measured in the last decade during airborne field campaigns at latitudes 53 degrees S-68 degrees N. The HNO3 content in ice crystals, expressed in terms of HNO3/H2O molar ratio, and the fraction of HNO3 in ice derived from those measurements exhibit a clear upward trend with decreasing temperature. The observations are explained by a novel model describing dissolution of HNO3 in Liquid Aerosol particles serving as freezing nuclei and subsequent trapping of HNO3 during ice crystal growth. The efficiency of trapping increases with decreasing temperature. Efficient trapping occurs via diffusional burial of the ambient HNO3 below about 203 K, because of long residence times of HNO3 molecules at the ice surface. This opens the possibility for HNO3-induced modifications of processes affecting ice crystal growth. At warmer temperatures, molecular processes in the ice surface layer cause an increasingly rapid escape of adsorbed HNO3 into the gas phase and render trapping less efficient despite faster ice growth rates.
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Properties of particles in the tropopause region
AIP Conference Proceedings, 2001Co-Authors: Bernd Kärcher, Susan SolomonAbstract:Liquid Aerosol particles and ice crystals in subvisible cirrus clouds in the tropopause region are investigated in terms of microphysical, chemical, and optical properties. These particle properties are examined by means of a thermodynamic equilibrium model and are related to satellite extinction measurements.
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On the composition and optical extinction of particles in the tropopause region
Journal of Geophysical Research, 1999Co-Authors: Bernd Kärcher, Susan SolomonAbstract:Liquid Aerosol particles and ice crystals in subvisible cirrus clouds in the tropopause region are characterized in terms of size distributions, chemical composition, and optical extinction. These particle properties are studied by means of simple models and are related to satellite extinction measurements, particularly for midlatitudes. Sulfuric acid Aerosols can take up nitric acid near the ice frost point, just before ice nucleation. Aerosols in the tropopause region may show a larger spread of extinction and extinction ratios at different wavelengths than background stratospheric Aerosols. The high surface areas and low extinction ratios of subvisible cirrus deduced from satellite observations are unlikely to be due purely to Aerosols, except for high sulfate loadings. It is shown that mixtures of Liquid Aerosols and ice particles can more readily explain these data with only small cloud fractions along the line of sight of the optical sensors. The efficiency of heterogeneous chlorine activation in Aerosol/cloud mixtures, the availability of water vapor, sulfate, and nitrate, and the effects of temperature, ammonium, organics, ice nuclei and aircraft emissions on the properties of particles in the tropopause region are explored.
Laura Mitchem - One of the best experts on this subject based on the ideXlab platform.
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Spectroscopic studies of the size and composition of single Aerosol droplets
International Reviews in Physical Chemistry, 2007Co-Authors: Jonathan P Reid, Laura Mitchem, Helena Meresman, Rachel SymesAbstract:The characterization of Aerosol properties and processes, non-intrusively and directly, poses a severe analytical challenge. In order to understand the role of Aerosols in often complex environments, it is necessary to probe the particles in situ and without perturbation. Sampling followed by end-of-line analysis can lead to perturbations in particle composition, morphology and size, particularly when analysing Liquid Aerosol droplets containing volatile components. Optical spectroscopy can provide a strategy for the direct assessment of particle size, composition and phase. We review here the application of linear and non-linear Raman spectroscopies in the characterization of Liquid Aerosol droplets. Spontaneous Raman scattering can allow the unambiguous identification of chemical components and the determination of droplet composition. Stimulated Raman spectroscopy can allow the determination of droplet size with nanometre accuracy and can allow the characterization of near-surface composition. When com...
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Controlling and characterizing the coagulation of Liquid Aerosol droplets
Journal of Chemical Physics, 2006Co-Authors: Jariya Buajarern, Laura Mitchem, Andrew D Ward, David Mcgloin, N. Hendrik Nahler, Jonathan P ReidAbstract:We demonstrate that optical tweezers can be used to control and characterize the coagulation and mixing state of Aerosols. Liquid Aerosol droplets of 2-14 mum in diameter are optically trapped and characterized by spontaneous and stimulated Raman scatterings, which together provide a unique signature of droplet size and composition. From the conventional bright field image, the size of the trapped droplet can be estimated and compared with that determined from stimulated Raman scattering, and the motion of the particle within the trapping plane can be recorded. A maximum of four droplets can be manipulated in tandem by forming multiple optical traps through rapid beam steering. The coagulation of two droplets can be studied directly by controlling two droplets. The limiting conditions under which optical forces and capillary forces dominate the Aerosol coagulation event are explored by varying the relative optical trap strengths and characterizing the coagulation of different droplet sizes. Finally, we demonstrate that the coagulation of different Aerosol components can be compared and the mixing state of the final coagulated droplet can be investigated. In particular, we compare the outcome of the coagulation of an aqueous sodium chloride Aerosol droplet with a second aqueous droplet, with an ethanol droplet or with a decane droplet.
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controlling and characterizing the coagulation of Liquid Aerosol droplets
Journal of Chemical Physics, 2006Co-Authors: Jariya Buajarern, Laura Mitchem, Andrew D Ward, Hendrik N Nahler, David Mcgloin, Jonathan P ReidAbstract:We demonstrate that optical tweezers can be used to control and characterize the coagulation and mixing state of Aerosols. Liquid Aerosol droplets of 2–14μm in diameter are optically trapped and characterized by spontaneous and stimulated Raman scatterings, which together provide a unique signature of droplet size and composition. From the conventional bright field image, the size of the trapped droplet can be estimated and compared with that determined from stimulated Raman scattering, and the motion of the particle within the trapping plane can be recorded. A maximum of four droplets can be manipulated in tandem by forming multiple optical traps through rapid beam steering. The coagulation of two droplets can be studied directly by controlling two droplets. The limiting conditions under which optical forces and capillary forces dominate the Aerosol coagulation event are explored by varying the relative optical trap strengths and characterizing the coagulation of different droplet sizes. Finally, we demo...
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spectroscopy of growing and evaporating water droplets exploring the variation in equilibrium droplet size with relative humidity
Journal of Physical Chemistry A, 2006Co-Authors: Laura Mitchem, Jariya Buajarern, Andrew D Ward, Rebecca J Hopkins, Richard J J Gilham, Roy L Johnston, Jonathan P ReidAbstract:We demonstrate that the thermodynamic properties of a single Liquid Aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2−6 μm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence ...
Chihwa Wang - One of the best experts on this subject based on the ideXlab platform.
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experimental and computational studies of Liquid Aerosol evaporation
Journal of Aerosol Science, 2008Co-Authors: Chihwa WangAbstract:The phenomenon of Liquid Aerosol evaporation was investigated both experimentally and computationally in this project. A vibrating orifice Aerosol generator (VOAG) was utilised to generate a monodispersed stream of Liquid droplets of a desired initial size. Three types of Liquids, ethanol, hexane and water, of distinct volatilities were used. The droplets were allowed to travel through air at ambient temperature and pressure. The change in diameter of the droplets as evaporation occurs was measured dynamically using a variety of experimental techniques. Global sizing velocimetry (GSV) was applied to visualize the motion and change in diameter of droplets in real time so as to select an optimal set of operating conditions under which coalescence of droplets did not occur. A phase Doppler interferometer (PDI) was used to measure the Sauter mean diameter and axial velocity of droplets at various distances from the nozzle of the VOAG. The size and velocity profiles obtained were compared with calculations carried out using computational fluid dynamics (CFD) coupled with two different theoretical Liquid evaporation models. It was observed that both Liquid evaporation models produced similar droplets size profiles for ethanol and water, but a significant difference in the predicted sizes was observed for hexane. In general, a good qualitative agreement between the experimental and computational results was obtained. This represents the first report of a study involving direct comparisons of dynamic measurements of the size evolution of evaporating Liquid Aerosols with numerical predictions obtained from CFD calculations.
Susan Solomon - One of the best experts on this subject based on the ideXlab platform.
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Properties of particles in the tropopause region
AIP Conference Proceedings, 2001Co-Authors: Bernd Kärcher, Susan SolomonAbstract:Liquid Aerosol particles and ice crystals in subvisible cirrus clouds in the tropopause region are investigated in terms of microphysical, chemical, and optical properties. These particle properties are examined by means of a thermodynamic equilibrium model and are related to satellite extinction measurements.
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On the composition and optical extinction of particles in the tropopause region
Journal of Geophysical Research, 1999Co-Authors: Bernd Kärcher, Susan SolomonAbstract:Liquid Aerosol particles and ice crystals in subvisible cirrus clouds in the tropopause region are characterized in terms of size distributions, chemical composition, and optical extinction. These particle properties are studied by means of simple models and are related to satellite extinction measurements, particularly for midlatitudes. Sulfuric acid Aerosols can take up nitric acid near the ice frost point, just before ice nucleation. Aerosols in the tropopause region may show a larger spread of extinction and extinction ratios at different wavelengths than background stratospheric Aerosols. The high surface areas and low extinction ratios of subvisible cirrus deduced from satellite observations are unlikely to be due purely to Aerosols, except for high sulfate loadings. It is shown that mixtures of Liquid Aerosols and ice particles can more readily explain these data with only small cloud fractions along the line of sight of the optical sensors. The efficiency of heterogeneous chlorine activation in Aerosol/cloud mixtures, the availability of water vapor, sulfate, and nitrate, and the effects of temperature, ammonium, organics, ice nuclei and aircraft emissions on the properties of particles in the tropopause region are explored.