G. Santachiara - Academia.edu (original) (raw)
Papers by G. Santachiara
Atmospheric Research, 2020
Laboratory generated aerosols and aerosol sampled in a field campaign were studied to assess the ... more Laboratory generated aerosols and aerosol sampled in a field campaign were studied to assess the repeatability of heterogeneous ice activation processes and verify if the ice nuclei particles maintained their nucleating capability. Experiments were performed using a filter technique, i.e. aerosol particles were sampled on blackgridded cellulose nitrate membrane filters and assessed for ice nucleating particles in a dynamic filter-processing chamber at our ISAC-CNR laboratory. The laboratory investigations used Arizona Test Dust (ATD) generated by nebulizing a suspension of ATD. After nebulization, the wet aerosol in the carrier gas was dried and then sampled on a filter. Aerosol field sampling (size range PM1 and PM10) was carried out in the Arctic (Ny-Ålesund, Svalbard) from 12 to 23 April 2019. The activation processes were repeated twice for each filter at temperatures of −22°C and −18°C. For aerosol, ATD runs were performed only in conditions of supersaturation (saturation ratio S w = 1.02), while for the Arctic aerosol, conditions below saturation were also considered (S w = 0.96). The experiments showed that approximately 40% of the ATD particles maintain their initial nucleating efficiency, both at T = −22°C and − 18°C, and at S w = 1.02, i.e. in the condensation freezing mode. The percentage of repeatability was higher for ATD with respect to the Artic-sampled aerosol processed in the condensation mode. This may depend on greater particle morphology variation of the mixed organic-inorganic aerosol due to the hydration and dehydration cycle. Recent experiments on nucleation of ice from water vapor on atmospheric aerosol by Kiselev et al. (2017), Campbell and Christenson (2018), David et al. (2019) have evidenced that active sites are related to local surface defects, such as steps, cracks, cavities, atomic lattice distortions caused by impurities, and crystallographic dislocations, leading to a better structure matching between the ice and the particle surface. In addition, the influence of surface charge was evidenced by Abdelmonem et al. (2017) and Marcolli et al. (2016). Holden et al. (2019) have shown that the activity of active sites derives from a combination of nanoscale topography and surface chemistry, e.g. the
Il Nuovo Cimento C
SO 2 oxidation by oxygen in monodisperse water droplets was studied in a cylindrical chamber, wit... more SO 2 oxidation by oxygen in monodisperse water droplets was studied in a cylindrical chamber, without and in the presence of ammonia. The range of SO 2 concentration was from about 10 22 to 5 ppmv, while the NH 3 input concentration was kept constant at about 4 .7 3 10 22 ppmv. The contact time between gases and droplets was 210 s. The experimental results were compared with the theoretical values predicted by the kinetics of Larson et al. (Atmos. Environ., 12 (1978) 1597) and McKay (Atmos. Environ., 5 (1971) 7). Much higher sulphate concentrations were obtained in experiments run in the presence of NH 3 , as opposed to those without NH 3. The experimental results agree with the values predicted by McKay's kinetics and are higher than Larson's.
Journal of Biomaterials and Nanobiotechnology, 2016
Nanoparticles in air are of particular concern for public health and employee exposure in workpla... more Nanoparticles in air are of particular concern for public health and employee exposure in workplaces. Therefore, it is very important to prepare effective filters for their removal. In this work filters were prepared from nanocellulose, i.e. cellulose nanofibrils (CNF). CNF was produced using two methods giving two different qualities of CNF. One quality had negative charges on the fibril surfaces while the other was neutral, and had in addition thinner fibrils compared to the other qualities. Filter samples were produced from water dispersions of CNF, by removal of the water by freeze drying. The performance of the CNF based filters was assessed and compared with filters based on synthetic polymer fibres. The ability to collect NaCl particles with a broad size distribution, ranging from nanometer to micrometer scale, was determined. CNF filters showed quality values comparable with the synthetic polymer based filters. Filters based on both the two CNF qualities had very good filtration efficiency for a given pressure drop across the filter.
Comptes rendus hebdomadaires des séances de l'Académie des sciences. Série D: Sciences naturelles, Jan 19, 1970
Journal of Physics: Conference Series, 2017
The occurrence of ice-nucleating aerosols in the atmosphere has a profound impact on the properti... more The occurrence of ice-nucleating aerosols in the atmosphere has a profound impact on the properties of clouds, and in turn, influences our understanding on weather and climate. Research on this topic has grown constantly over the last decades, driven by advances in online and offline instruments capable of measuring the characteristics of these cloud-modifying aerosol particles. This article presents different aspects to the understanding of how aerosol particles can trigger the nucleation of ice in clouds. In addition, we present some experimental results obtained with the Dynamic Filter Processing Chamber, an off-line instrument that has been applied extensively in the last years and that circumvents some of the problems related to the measurement of Ice Nucleating Particles properties.
Journal of Aerosol Science, 2001
Atmospheric Research, 2019
Please cite this article as: F. Belosi and G. Santachiara, Laboratory investigation of aerosol co... more Please cite this article as: F. Belosi and G. Santachiara, Laboratory investigation of aerosol coating and capillarity effects on particle ice nucleation in deposition and condensation modes, Atmospheric Research,
Atmospheric Chemistry and Physics Discussions, 2018
We present the laboratory results of immersion freezing efficiencies of cellulose particles at su... more We present the laboratory results of immersion freezing efficiencies of cellulose particles at supercooled temperature (T) conditions. Three types of chemically homogeneous cellulose samples are used as surrogates that represent supermicron and submicron ice nucleating plant structural polymers. These samples include micro-crystalline cellulose (MCC), fibrous 5 cellulose (FC) and nano-crystalline cellulose (NCC). Our experimental data show that particles resembling the MCC lab particle occur also in the atmosphere. Our immersion freezing dataset includes data from various ice nucleation measurement techniques available at seventeen different institutions, including nine dry dispersion and eleven aqueous suspension techniques. With a total of twenty methods, we performed systematic accuracy and precision 10 analysis of measurements from all twenty measurement techniques by evaluating T-binned (1 °C) data over a wide T range (-36 °C < T <-4 °C). Specifically, we inter-compared the geometric surface area-based ice nucleation active surface-site (INAS) density data derived from our measurements as a function of T, ns,geo(T). Additionally, we also compared the ns,geo(T) values and the freezing spectral slope parameter (Δlog(ns,geo)/ΔT) from our measurements to previous 15 literature results. Results show that freezing efficiencies of NCC samples agree reasonably well, whereas the diversity for the other two samples spans for ~10 °C. Despite given uncertainties within each instrument technique, the overall trend of the ns,geo(T) spectrum traced by the T-binned average of measurements suggest that predominantly supermicronsized (giant hereafter) cellulose particles (MCC and FC) generally act as more efficient ice-20 nucleating particles than NCC with about one order of magnitude higher ns,geo(T). Further, our results indicate significant diversity between dry and aqueous suspension measurement techniques. The ratios of the individual measurements (ns,ind) to the log average of ns,geo(T) range 0.6-1.4 across the examined T range. In general, the ratios of the log average of dry dispersion measurements are higher than those of aqueous suspension measurements. The 25 observed discrepancy may be due to non-uniform active site density for different sizes and/or the alteration in physico-chemical properties of cellulose by liquid-suspending it. Unless otherwise defined, the cellulose system may not be an ideal calibrant. Given such a distinct difference between two subgroups of immersion freezing techniques, standardization of our methods, especially INP sampling and treatment, may be one approach to reduce the 30 measurement diversity and valiability when we deal with a complex material like cellulose. A community-wide effort to identify specimen-specific limitations and characteristics of each technique, as well as consolidating the ns,geo(T) parameterization, is an alternative approach to achieve overall precise and accurate ice-nucleating particle measurements.
Nuovo Cimento della Societa Italiana di Fisica C, 2008
ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined... more ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined by considering both the chemical composition and ice crystal shape. Measured concentrations in snow samples show that nucleation is the dominant aerosol scavenging process. An additional contribution from phoretic forces to aerosol scavenging during growth of ice crystals can be deduced from the correlation between non sea-salt sulphate (nss-SO(4)(2-)) and methanesulfonic acid (MSA) measured in snow samples. The sea-salt contribution is dominant, as usually observed in the coastal Antarctic stations. By determining sea salt from Na(+) concentration, the values of 4400 mu g l(-1); 2400 mu g l(-1); 650 mu g l(-1) were obtained for the examined samples. The NO(3)(-)/Na(+) ratio in fresh snow (range 0.1-0.6), much higher than the value in sea-water (about 10(-4)), excludes a marine origin for NO(3)(-) ion, suggesting a continental and/or stratospheric source. Organic compounds (propionate, acetate, formate, MSA and glycolate) were in addition measured in snow samples. Ice crystal replicas were made by collecting crystals on microscope slides, previously covered with a thin layer of 2% formvar in chloroform. Samples were analyzed by a scanning electron microscope (SEM). A large variety of ice crystal habits (needles, hexagonal plates, crystals with branches, dendritic crystals, etc.) were observed. In the examined replicas of different events, cases are noted in which sample plates are prevalent, others in which prevalently complex crystal shapes are observed, and others again in which simple and complex crystal shapes are present simultaneously.
Atmospheric and Climate Sciences, 2014
The formation of ice in clouds can occur through primary processes, either homogeneously or heter... more The formation of ice in clouds can occur through primary processes, either homogeneously or heterogeneously triggered by aerosol particles called ice nuclei, as well as through secondary processes. The homogeneous ice nucleation process involves only pure water or solution droplets. Homogeneous freezing is crucial for the microphysics in the formation of high-altitude cirrus and polar stratospheric clouds, and also in the glaciation of thunderclouds, at temperatures below about 235 K. Nucleation rates in supercooled water have been measured using different experimental techniques: expansion cloud chambers, water-in-oil emulsions, levitation methods, free falling droplets, supersonic nozzles, field measurements, and molecular dynamics simulations. An important question concerns the possibility that the nucleation process in supercooled water can occur not only in the interior volume of the droplet, but even at or close to its surface. Even if there is no conclusive evidence, the majority of experimental and theoretical results suggest that the contribution of surface nucleation increases with decreasing radius of the supercooled droplets, and the surface (or sub-surface) nucleation rate is prevalent for droplets with radius lower than about 5 μm. If homogeneous freezing initiates at the droplet surface, the freezing rate should depend on the droplet size, and even a slight contamination by molecules within the surface layer could hamper the rate of the nucleation process.
Atmospheric Research, 2017
Nucleation and Atmospheric Aerosols 1996, 1996
Journal of Geophysical Research, 2010
This work reports an analysis of the concentration, size distribution, and deposition velocity of... more This work reports an analysis of the concentration, size distribution, and deposition velocity of atmospheric particles over snow and iced surfaces on the Nansen Ice Sheet (Antarctica). Measurements were performed using the eddy-correlation method at a remote site during the XXII Italian expedition of the National Research Program in Antarctica (PNRA) in 2006. The measurement system was based on a condensation particle counter (CPC) able to measure particles down to 9 nm in diameter with a 50% efficiency and a Differential Mobility Particle Sizer for evaluating particle size distributions from 11 to 521 nm diameter in 39 channels. A method based on postprocessing with digital filters was developed to take into account the effect of the slow time response of the CPC. The average number concentration was 1338 cm −3 (median, 978 cm −3 ; interquartile range, 435-1854 cm −3). Higher concentrations were observed at low wind velocities. Results gave an average deposition velocity of 0.47 mm/s (median, 0.19 mm/s; interquartile range, −0.21 −0.88 mm/s). Deposition increased with the friction velocity and was on average 0.86 mm/s during katabatic wind characterized by velocities higher than 4 m/s. Observed size distributions generally presented two distinct modes, the first at approximately 15-20 nm and the second (representing on average 70% of the total particles) at 60-70 nm. Under strong-wind conditions, the second mode dominated the average size distribution.
Journal of Aerosol Science, 1993
ABSTRACT
Measurements of thermo-and diffusio-phoretic velocities of aerosol particles (carnauba wax, paraf... more Measurements of thermo-and diffusio-phoretic velocities of aerosol particles (carnauba wax, paraffin and sodium chloride) were performed in microgravity conditions (Drop Tower facility, in Bremen, and Parabolic Flights, in Bordeaux). In the case of thermophoresis, a temperature gradient was obtained by heating the upper plate of the cell, while the lower one was maintained at environmental temperature. For diffusiophoresis, the water vapour gradient was obtained with sintered plates imbued with a water solution of MgCl 2 and distilled water, at the top and at the bottom of the cell, respectively. Aerosol particles were observed through a digital holographic velocimeter, a device allowing the determination of 3-D coordinates of particles from the observed volume. Particle trajectories and consequently particle velocities were reconstructed through the analysis of the sequence of particle positions. The experimental values of reduced thermophoretic velocities are between the theoretical values of Yamamoto and Ishihara [Yamamoto, K., Ishihara, Y., 1988. Thermophoresis of a spherical particle in a rarefied gas of a transition regime.
Journal of Aerosol Science, 2007
Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrie... more Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrier gases (helium, nitrogen, argon, xenon) were performed in microgravity conditions (the drop tower facility, in Bremen). The experiments permitted the study of thermophoresis in conditions which minimize the impact of gravity.
Nuovo Cimento della Societa Italiana di Fisica C, 2008
ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined... more ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined by considering both the chemical composition and ice crystal shape. Measured concentrations in snow samples show that nucleation is the dominant aerosol scavenging process. An additional contribution from phoretic forces to aerosol scavenging during growth of ice crystals can be deduced from the correlation between non sea-salt sulphate (nss-SO(4)(2-)) and methanesulfonic acid (MSA) measured in snow samples. The sea-salt contribution is dominant, as usually observed in the coastal Antarctic stations. By determining sea salt from Na(+) concentration, the values of 4400 mu g l(-1); 2400 mu g l(-1); 650 mu g l(-1) were obtained for the examined samples. The NO(3)(-)/Na(+) ratio in fresh snow (range 0.1-0.6), much higher than the value in sea-water (about 10(-4)), excludes a marine origin for NO(3)(-) ion, suggesting a continental and/or stratospheric source. Organic compounds (propionate, acetate, formate, MSA and glycolate) were in addition measured in snow samples. Ice crystal replicas were made by collecting crystals on microscope slides, previously covered with a thin layer of 2% formvar in chloroform. Samples were analyzed by a scanning electron microscope (SEM). A large variety of ice crystal habits (needles, hexagonal plates, crystals with branches, dendritic crystals, etc.) were observed. In the examined replicas of different events, cases are noted in which sample plates are prevalent, others in which prevalently complex crystal shapes are observed, and others again in which simple and complex crystal shapes are present simultaneously.
Atmospheric Research, 2020
Laboratory generated aerosols and aerosol sampled in a field campaign were studied to assess the ... more Laboratory generated aerosols and aerosol sampled in a field campaign were studied to assess the repeatability of heterogeneous ice activation processes and verify if the ice nuclei particles maintained their nucleating capability. Experiments were performed using a filter technique, i.e. aerosol particles were sampled on blackgridded cellulose nitrate membrane filters and assessed for ice nucleating particles in a dynamic filter-processing chamber at our ISAC-CNR laboratory. The laboratory investigations used Arizona Test Dust (ATD) generated by nebulizing a suspension of ATD. After nebulization, the wet aerosol in the carrier gas was dried and then sampled on a filter. Aerosol field sampling (size range PM1 and PM10) was carried out in the Arctic (Ny-Ålesund, Svalbard) from 12 to 23 April 2019. The activation processes were repeated twice for each filter at temperatures of −22°C and −18°C. For aerosol, ATD runs were performed only in conditions of supersaturation (saturation ratio S w = 1.02), while for the Arctic aerosol, conditions below saturation were also considered (S w = 0.96). The experiments showed that approximately 40% of the ATD particles maintain their initial nucleating efficiency, both at T = −22°C and − 18°C, and at S w = 1.02, i.e. in the condensation freezing mode. The percentage of repeatability was higher for ATD with respect to the Artic-sampled aerosol processed in the condensation mode. This may depend on greater particle morphology variation of the mixed organic-inorganic aerosol due to the hydration and dehydration cycle. Recent experiments on nucleation of ice from water vapor on atmospheric aerosol by Kiselev et al. (2017), Campbell and Christenson (2018), David et al. (2019) have evidenced that active sites are related to local surface defects, such as steps, cracks, cavities, atomic lattice distortions caused by impurities, and crystallographic dislocations, leading to a better structure matching between the ice and the particle surface. In addition, the influence of surface charge was evidenced by Abdelmonem et al. (2017) and Marcolli et al. (2016). Holden et al. (2019) have shown that the activity of active sites derives from a combination of nanoscale topography and surface chemistry, e.g. the
Il Nuovo Cimento C
SO 2 oxidation by oxygen in monodisperse water droplets was studied in a cylindrical chamber, wit... more SO 2 oxidation by oxygen in monodisperse water droplets was studied in a cylindrical chamber, without and in the presence of ammonia. The range of SO 2 concentration was from about 10 22 to 5 ppmv, while the NH 3 input concentration was kept constant at about 4 .7 3 10 22 ppmv. The contact time between gases and droplets was 210 s. The experimental results were compared with the theoretical values predicted by the kinetics of Larson et al. (Atmos. Environ., 12 (1978) 1597) and McKay (Atmos. Environ., 5 (1971) 7). Much higher sulphate concentrations were obtained in experiments run in the presence of NH 3 , as opposed to those without NH 3. The experimental results agree with the values predicted by McKay's kinetics and are higher than Larson's.
Journal of Biomaterials and Nanobiotechnology, 2016
Nanoparticles in air are of particular concern for public health and employee exposure in workpla... more Nanoparticles in air are of particular concern for public health and employee exposure in workplaces. Therefore, it is very important to prepare effective filters for their removal. In this work filters were prepared from nanocellulose, i.e. cellulose nanofibrils (CNF). CNF was produced using two methods giving two different qualities of CNF. One quality had negative charges on the fibril surfaces while the other was neutral, and had in addition thinner fibrils compared to the other qualities. Filter samples were produced from water dispersions of CNF, by removal of the water by freeze drying. The performance of the CNF based filters was assessed and compared with filters based on synthetic polymer fibres. The ability to collect NaCl particles with a broad size distribution, ranging from nanometer to micrometer scale, was determined. CNF filters showed quality values comparable with the synthetic polymer based filters. Filters based on both the two CNF qualities had very good filtration efficiency for a given pressure drop across the filter.
Comptes rendus hebdomadaires des séances de l'Académie des sciences. Série D: Sciences naturelles, Jan 19, 1970
Journal of Physics: Conference Series, 2017
The occurrence of ice-nucleating aerosols in the atmosphere has a profound impact on the properti... more The occurrence of ice-nucleating aerosols in the atmosphere has a profound impact on the properties of clouds, and in turn, influences our understanding on weather and climate. Research on this topic has grown constantly over the last decades, driven by advances in online and offline instruments capable of measuring the characteristics of these cloud-modifying aerosol particles. This article presents different aspects to the understanding of how aerosol particles can trigger the nucleation of ice in clouds. In addition, we present some experimental results obtained with the Dynamic Filter Processing Chamber, an off-line instrument that has been applied extensively in the last years and that circumvents some of the problems related to the measurement of Ice Nucleating Particles properties.
Journal of Aerosol Science, 2001
Atmospheric Research, 2019
Please cite this article as: F. Belosi and G. Santachiara, Laboratory investigation of aerosol co... more Please cite this article as: F. Belosi and G. Santachiara, Laboratory investigation of aerosol coating and capillarity effects on particle ice nucleation in deposition and condensation modes, Atmospheric Research,
Atmospheric Chemistry and Physics Discussions, 2018
We present the laboratory results of immersion freezing efficiencies of cellulose particles at su... more We present the laboratory results of immersion freezing efficiencies of cellulose particles at supercooled temperature (T) conditions. Three types of chemically homogeneous cellulose samples are used as surrogates that represent supermicron and submicron ice nucleating plant structural polymers. These samples include micro-crystalline cellulose (MCC), fibrous 5 cellulose (FC) and nano-crystalline cellulose (NCC). Our experimental data show that particles resembling the MCC lab particle occur also in the atmosphere. Our immersion freezing dataset includes data from various ice nucleation measurement techniques available at seventeen different institutions, including nine dry dispersion and eleven aqueous suspension techniques. With a total of twenty methods, we performed systematic accuracy and precision 10 analysis of measurements from all twenty measurement techniques by evaluating T-binned (1 °C) data over a wide T range (-36 °C < T <-4 °C). Specifically, we inter-compared the geometric surface area-based ice nucleation active surface-site (INAS) density data derived from our measurements as a function of T, ns,geo(T). Additionally, we also compared the ns,geo(T) values and the freezing spectral slope parameter (Δlog(ns,geo)/ΔT) from our measurements to previous 15 literature results. Results show that freezing efficiencies of NCC samples agree reasonably well, whereas the diversity for the other two samples spans for ~10 °C. Despite given uncertainties within each instrument technique, the overall trend of the ns,geo(T) spectrum traced by the T-binned average of measurements suggest that predominantly supermicronsized (giant hereafter) cellulose particles (MCC and FC) generally act as more efficient ice-20 nucleating particles than NCC with about one order of magnitude higher ns,geo(T). Further, our results indicate significant diversity between dry and aqueous suspension measurement techniques. The ratios of the individual measurements (ns,ind) to the log average of ns,geo(T) range 0.6-1.4 across the examined T range. In general, the ratios of the log average of dry dispersion measurements are higher than those of aqueous suspension measurements. The 25 observed discrepancy may be due to non-uniform active site density for different sizes and/or the alteration in physico-chemical properties of cellulose by liquid-suspending it. Unless otherwise defined, the cellulose system may not be an ideal calibrant. Given such a distinct difference between two subgroups of immersion freezing techniques, standardization of our methods, especially INP sampling and treatment, may be one approach to reduce the 30 measurement diversity and valiability when we deal with a complex material like cellulose. A community-wide effort to identify specimen-specific limitations and characteristics of each technique, as well as consolidating the ns,geo(T) parameterization, is an alternative approach to achieve overall precise and accurate ice-nucleating particle measurements.
Nuovo Cimento della Societa Italiana di Fisica C, 2008
ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined... more ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined by considering both the chemical composition and ice crystal shape. Measured concentrations in snow samples show that nucleation is the dominant aerosol scavenging process. An additional contribution from phoretic forces to aerosol scavenging during growth of ice crystals can be deduced from the correlation between non sea-salt sulphate (nss-SO(4)(2-)) and methanesulfonic acid (MSA) measured in snow samples. The sea-salt contribution is dominant, as usually observed in the coastal Antarctic stations. By determining sea salt from Na(+) concentration, the values of 4400 mu g l(-1); 2400 mu g l(-1); 650 mu g l(-1) were obtained for the examined samples. The NO(3)(-)/Na(+) ratio in fresh snow (range 0.1-0.6), much higher than the value in sea-water (about 10(-4)), excludes a marine origin for NO(3)(-) ion, suggesting a continental and/or stratospheric source. Organic compounds (propionate, acetate, formate, MSA and glycolate) were in addition measured in snow samples. Ice crystal replicas were made by collecting crystals on microscope slides, previously covered with a thin layer of 2% formvar in chloroform. Samples were analyzed by a scanning electron microscope (SEM). A large variety of ice crystal habits (needles, hexagonal plates, crystals with branches, dendritic crystals, etc.) were observed. In the examined replicas of different events, cases are noted in which sample plates are prevalent, others in which prevalently complex crystal shapes are observed, and others again in which simple and complex crystal shapes are present simultaneously.
Atmospheric and Climate Sciences, 2014
The formation of ice in clouds can occur through primary processes, either homogeneously or heter... more The formation of ice in clouds can occur through primary processes, either homogeneously or heterogeneously triggered by aerosol particles called ice nuclei, as well as through secondary processes. The homogeneous ice nucleation process involves only pure water or solution droplets. Homogeneous freezing is crucial for the microphysics in the formation of high-altitude cirrus and polar stratospheric clouds, and also in the glaciation of thunderclouds, at temperatures below about 235 K. Nucleation rates in supercooled water have been measured using different experimental techniques: expansion cloud chambers, water-in-oil emulsions, levitation methods, free falling droplets, supersonic nozzles, field measurements, and molecular dynamics simulations. An important question concerns the possibility that the nucleation process in supercooled water can occur not only in the interior volume of the droplet, but even at or close to its surface. Even if there is no conclusive evidence, the majority of experimental and theoretical results suggest that the contribution of surface nucleation increases with decreasing radius of the supercooled droplets, and the surface (or sub-surface) nucleation rate is prevalent for droplets with radius lower than about 5 μm. If homogeneous freezing initiates at the droplet surface, the freezing rate should depend on the droplet size, and even a slight contamination by molecules within the surface layer could hamper the rate of the nucleation process.
Atmospheric Research, 2017
Nucleation and Atmospheric Aerosols 1996, 1996
Journal of Geophysical Research, 2010
This work reports an analysis of the concentration, size distribution, and deposition velocity of... more This work reports an analysis of the concentration, size distribution, and deposition velocity of atmospheric particles over snow and iced surfaces on the Nansen Ice Sheet (Antarctica). Measurements were performed using the eddy-correlation method at a remote site during the XXII Italian expedition of the National Research Program in Antarctica (PNRA) in 2006. The measurement system was based on a condensation particle counter (CPC) able to measure particles down to 9 nm in diameter with a 50% efficiency and a Differential Mobility Particle Sizer for evaluating particle size distributions from 11 to 521 nm diameter in 39 channels. A method based on postprocessing with digital filters was developed to take into account the effect of the slow time response of the CPC. The average number concentration was 1338 cm −3 (median, 978 cm −3 ; interquartile range, 435-1854 cm −3). Higher concentrations were observed at low wind velocities. Results gave an average deposition velocity of 0.47 mm/s (median, 0.19 mm/s; interquartile range, −0.21 −0.88 mm/s). Deposition increased with the friction velocity and was on average 0.86 mm/s during katabatic wind characterized by velocities higher than 4 m/s. Observed size distributions generally presented two distinct modes, the first at approximately 15-20 nm and the second (representing on average 70% of the total particles) at 60-70 nm. Under strong-wind conditions, the second mode dominated the average size distribution.
Journal of Aerosol Science, 1993
ABSTRACT
Measurements of thermo-and diffusio-phoretic velocities of aerosol particles (carnauba wax, paraf... more Measurements of thermo-and diffusio-phoretic velocities of aerosol particles (carnauba wax, paraffin and sodium chloride) were performed in microgravity conditions (Drop Tower facility, in Bremen, and Parabolic Flights, in Bordeaux). In the case of thermophoresis, a temperature gradient was obtained by heating the upper plate of the cell, while the lower one was maintained at environmental temperature. For diffusiophoresis, the water vapour gradient was obtained with sintered plates imbued with a water solution of MgCl 2 and distilled water, at the top and at the bottom of the cell, respectively. Aerosol particles were observed through a digital holographic velocimeter, a device allowing the determination of 3-D coordinates of particles from the observed volume. Particle trajectories and consequently particle velocities were reconstructed through the analysis of the sequence of particle positions. The experimental values of reduced thermophoretic velocities are between the theoretical values of Yamamoto and Ishihara [Yamamoto, K., Ishihara, Y., 1988. Thermophoresis of a spherical particle in a rarefied gas of a transition regime.
Journal of Aerosol Science, 2007
Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrie... more Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrier gases (helium, nitrogen, argon, xenon) were performed in microgravity conditions (the drop tower facility, in Bremen). The experiments permitted the study of thermophoresis in conditions which minimize the impact of gravity.
Nuovo Cimento della Societa Italiana di Fisica C, 2008
ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined... more ABSTRACT Fresh snow samples collected in a, coastal Antarctic site (Terra Nova Bay) were examined by considering both the chemical composition and ice crystal shape. Measured concentrations in snow samples show that nucleation is the dominant aerosol scavenging process. An additional contribution from phoretic forces to aerosol scavenging during growth of ice crystals can be deduced from the correlation between non sea-salt sulphate (nss-SO(4)(2-)) and methanesulfonic acid (MSA) measured in snow samples. The sea-salt contribution is dominant, as usually observed in the coastal Antarctic stations. By determining sea salt from Na(+) concentration, the values of 4400 mu g l(-1); 2400 mu g l(-1); 650 mu g l(-1) were obtained for the examined samples. The NO(3)(-)/Na(+) ratio in fresh snow (range 0.1-0.6), much higher than the value in sea-water (about 10(-4)), excludes a marine origin for NO(3)(-) ion, suggesting a continental and/or stratospheric source. Organic compounds (propionate, acetate, formate, MSA and glycolate) were in addition measured in snow samples. Ice crystal replicas were made by collecting crystals on microscope slides, previously covered with a thin layer of 2% formvar in chloroform. Samples were analyzed by a scanning electron microscope (SEM). A large variety of ice crystal habits (needles, hexagonal plates, crystals with branches, dendritic crystals, etc.) were observed. In the examined replicas of different events, cases are noted in which sample plates are prevalent, others in which prevalently complex crystal shapes are observed, and others again in which simple and complex crystal shapes are present simultaneously.
Gli aerosol ultrafini rivestono un ruolo importante per la salute della popolazione e per i cambi... more Gli aerosol ultrafini rivestono un ruolo importante per la salute della popolazione e per i cambiamenti climatici. La misura della loro distribuzione dimensionale rappresenta pertanto una informazione fondamentale, e può essere effettuata con il Differential Mobility Analyzer (DMA), che è il più diffuso sistema di campionamento e caratterizzazione dimensionale. Nel DMA l’aerosol campionato viene portato all’equilibrio di Boltzmann generalmente mediante l’utilizzo di una sorgente radioattiva. L’acquisizione e la detenzione della sorgente radioattiva rappresenta però un notevole ostacolo all’impiego del DMA, per la complessità delle norme che regolano il trasporto e la detenzione di materiale radioattivo. Una soluzione alternativa è costituita dal neutralizzatore a raggi X deboli sviluppato da TSI (mod. 3087) con intensità < di 9.5 KeV. Sebbene il neutralizzatore sia stato concepito per essere utilizzato con il classificatore DMA della TSI, esso può essere montato, in linea di principio, anche su altri DMA. Allo stato attuale sono stati pubblicati risultati relativi a confronti effettuati fra il neutralizzatore a raggi X e quelli radioattivi utilizzando strumentazione TSI. La comunicazione presenta invece i risultati ottenuti in laboratorio di un confronto fra un sistema Grimm (L-DMA, mod. 5400) con sorgente radioattiva Am-241 (configurazione standard) e con il nuovo neutralizzatore a raggi X. Le prove sono state effettuate con aerosol polidisperso generato in laboratorio nebulizzando con un Collison una soluzione di cloruro di sodio. La modalità di generazione è stabile nel tempo. Il confronto fra il neutralizzatore a raggi X e la sorgente radioattiva Am-241 mostra un errore relativo percentuale inferiore al 20%. Verranno presentate anche le perdite per diffusione dell’aerosol all’interno del neutralizzatore e la conseguente funzione di correzione della distribuzione dimensionale. Il lavoro è stato finanziato nell’ambito del Progetto Europeo Smart Nano-structured Devices Hierarchically Assembled by Mineralization Processes ( SMILEY, NMP.2012.1.4-2 FP7 SMALL-6-310637).