The role of phoretic forces in the identification of particles acting as ice nuclei in mixed phase cloud is discussed. A method used to identify the effective ice nucleating particles is to sample ice crystals, which ...The role of phoretic forces in the identification of particles acting as ice nuclei in mixed phase cloud is discussed. A method used to identify the effective ice nucleating particles is to sample ice crystals, which are afterwards sublimated, and to examine the particles remaining after evaporation. The procedure takes into account only crystal with a maximum diameter of 20 μm, by assuming that small crystals do not scavenge aerosol during growth, and therefore that crystals contain only the effective nucleating particles. This assumption is questionable, however, as experiments have shown that even small ice crystals can scavenge aerosol. Another approach has been to compare the number and elemental composition of residual particles in small ice crystals and of aerosol near the cloud. By considering as example soot and black carbon aerosol, contradictory conclusions on their importance in the processes of ice nucleation have been reported in the literature. We suggest that, in addition to physico-chemical properties of soot/carbon aerosol particles, even the microphysical and environmental parameters involved in the transition of aerosol from gas phase to ice crystals in cloud should be considered. The contribution of phoretic forces should also be considered. After initial growth ice crystals can continue to grow by water vapour diffusion. Laboratory experiments confirm the contribution of diffusiophoresis with Stefan flow in the scavenging by snow crystals up to 3 mm in diameter. The particle scavenging efficiency of snow crystals is related to crystalline shape and depends on air relative humidity and temperature.展开更多
This paper investigated the possibility that aerosol particles are scavenged during the first and fast diffusional growth of small ice crystals. After ice phase formation, riming, scavenging and aggregation may lead t...This paper investigated the possibility that aerosol particles are scavenged during the first and fast diffusional growth of small ice crystals. After ice phase formation, riming, scavenging and aggregation may lead to the collection of additional aerosol particles. Therefore, particles left after ice evaporation in hydrometeors, called ice residuals, may not currently be identical to ice nucleating particles. To overcome this problem, the largest ice crystals are removed during sampling in clouds and only crystals in the initial phase of growth, with diameters lower than 20 - 30 μm, are usually considered. Published papers assume that no aerosol scavenging takes place during the initial phase of growth of small ice crystals. The aim of this paper was to ascertain if this assumption is valid. Experiments were performed in a cold laboratory by considering ice crystals growing in the presence of supercooled droplets. Results showed that crystals can scavenge aerosol even in the first stage of growth. Theoretical considerations show that aerosol scavenging cannot be explained by Brownian diffusion, inertial impaction or interception processes. We suggest that the presence of aerosol in the pristine ice crystals may be due to diffusiophoretic force. During diffusive crystal growth, a flow called Stefan’s flow exists near the hydrometeor surface, driving the nearby aerosol particles towards the surface of the growing hydrometeors.展开更多
A series of measurements of ice-nucleating particles （1NPs） were performed at two sites in Beijing. At the Beijing Meteorological Service （BMS） site, which was an urban site, no INPs were found to be active above ...A series of measurements of ice-nucleating particles （1NPs） were performed at two sites in Beijing. At the Beijing Meteorological Service （BMS） site, which was an urban site, no INPs were found to be active above -15℃. However, at the Yanjiaping （YJP） site, which was a rural site, the concentration of 1NPs active at temperatures above -15℃ was found to be as high as 1.73 g^-1. Two parameterizations were constructed by respectively fitting the data obtained at BMS site and YJP site. The two parameterizations, as well as another parameterization from the literature, were implemented into a parcel model to investigate the effect of INPs active above -15℃ on phase partitioning in mixed-phase clouds. At a vertical velocity of 0.01 m s^-1 , which is typical for stratiform clouds associated with frontal systems, the INPs active above -15℃ nucleate ice crystals at low levels. The growth of these ice crystals remarkably reduces both the maximum liquid water mixing ratio and the altitude where the maximum liquid water mixing ratio is reached. When the vertical velocity of the parcel is increased to 0.1 m s^-l or an even higher value, the evolution of li-quid water mixing, ratio is not controlled by the INPs active above 15℃ but those active below -15℃.展开更多
文摘The role of phoretic forces in the identification of particles acting as ice nuclei in mixed phase cloud is discussed. A method used to identify the effective ice nucleating particles is to sample ice crystals, which are afterwards sublimated, and to examine the particles remaining after evaporation. The procedure takes into account only crystal with a maximum diameter of 20 μm, by assuming that small crystals do not scavenge aerosol during growth, and therefore that crystals contain only the effective nucleating particles. This assumption is questionable, however, as experiments have shown that even small ice crystals can scavenge aerosol. Another approach has been to compare the number and elemental composition of residual particles in small ice crystals and of aerosol near the cloud. By considering as example soot and black carbon aerosol, contradictory conclusions on their importance in the processes of ice nucleation have been reported in the literature. We suggest that, in addition to physico-chemical properties of soot/carbon aerosol particles, even the microphysical and environmental parameters involved in the transition of aerosol from gas phase to ice crystals in cloud should be considered. The contribution of phoretic forces should also be considered. After initial growth ice crystals can continue to grow by water vapour diffusion. Laboratory experiments confirm the contribution of diffusiophoresis with Stefan flow in the scavenging by snow crystals up to 3 mm in diameter. The particle scavenging efficiency of snow crystals is related to crystalline shape and depends on air relative humidity and temperature.
文摘This paper investigated the possibility that aerosol particles are scavenged during the first and fast diffusional growth of small ice crystals. After ice phase formation, riming, scavenging and aggregation may lead to the collection of additional aerosol particles. Therefore, particles left after ice evaporation in hydrometeors, called ice residuals, may not currently be identical to ice nucleating particles. To overcome this problem, the largest ice crystals are removed during sampling in clouds and only crystals in the initial phase of growth, with diameters lower than 20 - 30 μm, are usually considered. Published papers assume that no aerosol scavenging takes place during the initial phase of growth of small ice crystals. The aim of this paper was to ascertain if this assumption is valid. Experiments were performed in a cold laboratory by considering ice crystals growing in the presence of supercooled droplets. Results showed that crystals can scavenge aerosol even in the first stage of growth. Theoretical considerations show that aerosol scavenging cannot be explained by Brownian diffusion, inertial impaction or interception processes. We suggest that the presence of aerosol in the pristine ice crystals may be due to diffusiophoretic force. During diffusive crystal growth, a flow called Stefan’s flow exists near the hydrometeor surface, driving the nearby aerosol particles towards the surface of the growing hydrometeors.
基金Supported by the National Natural Science Foundation of China(41775138 and 41330421)Natural Science Foundation of Beijing(8172023)+1 种基金Beijing Municipal Science and Technology Commission(D171100000717001)Science and Technology Project of Beijing Meteorological Service(BMBKJ201701007)
文摘A series of measurements of ice-nucleating particles （1NPs） were performed at two sites in Beijing. At the Beijing Meteorological Service （BMS） site, which was an urban site, no INPs were found to be active above -15℃. However, at the Yanjiaping （YJP） site, which was a rural site, the concentration of 1NPs active at temperatures above -15℃ was found to be as high as 1.73 g^-1. Two parameterizations were constructed by respectively fitting the data obtained at BMS site and YJP site. The two parameterizations, as well as another parameterization from the literature, were implemented into a parcel model to investigate the effect of INPs active above -15℃ on phase partitioning in mixed-phase clouds. At a vertical velocity of 0.01 m s^-1 , which is typical for stratiform clouds associated with frontal systems, the INPs active above -15℃ nucleate ice crystals at low levels. The growth of these ice crystals remarkably reduces both the maximum liquid water mixing ratio and the altitude where the maximum liquid water mixing ratio is reached. When the vertical velocity of the parcel is increased to 0.1 m s^-l or an even higher value, the evolution of li-quid water mixing, ratio is not controlled by the INPs active above 15℃ but those active below -15℃.
