A Review of Thermo- and Diffusio-Phoresis in the Atmospheric Aerosol Scavenging Process. Part 2: Ice Crystal and Snow Scavenging

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.

Airborne Nanoparticles Filtration by Means of Cellulose Nanofibril Based Materials

Nanoparticles in air are of particular concern for public health and employee exposure in work-places. 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.

Surface Snow, Firn and Ice Core Composition in Polar Areas in Relation to Atmospheric Aerosol and Gas Concentrations: Critical Aspects

The paper addresses some of the problems surrounding the relation between ice core chemical signals and atmospheric chemical composition in polar areas. The topic is important as the reconstruction of past climate and past atmospheric chemical composition is based on the assumption that chemical concentrations in the air, snow, firn and ice core are correlated. Ice core interpretation of aerosol is more straightforward than that of reactive gases. The transfer functions of gaseous species strongly interacting with ice are complex and additional field and laboratory experiments are required. Ice core chemical signals depend on the chemical composition of precipitations, which are related to the physics of precipitation formation, the chemical composition of the atmosphere, and post-depositional processes. Published papers reporting data on the chemical composition of snow seldom consider the fact that crystal formation and growth in cloud (rimed or unrimed) or near the ground (clear-sky precipitations), hoar-frost formation and surface riming determine different chemical concentrations, even assuming constant background concentration in the atmosphere. This paper discusses the physical and chemical processes affecting the formation of precipitations in polar areas, and the process of scavenging gases from non-growing and growing crystals. Attention is mainly focused on the processes involving nitrate anion in snow, hoar frost and firn. Knowledge of the chemical relationship between surface snow and atmospheric chemical concentration could be enhanced by considering specific events, such as snow falling from cloud, clear sky precipitation, and surface hoar or riming surface, with simultaneous air sampling. In conclusion, field and laboratory experiments are still required to study the scavenging processes during crystal formation.

A Review of Termo- and Diffusio-Phoresis in the Atmospheric Aerosol Scavenging Process. Part 1: Drop Scavenging

The role of phoretic forces in providing in-cloud and below-cloud scavenging due to falling drop is reviewed by considering published papers dealing with theoretical models, laboratory and field measurements. Theoretical analyses agree that Brownian diffusion appears to dominate drop scavenging of aerosol with radius less than 0.1 μm, and inertial impaction dominates scavenging of aerosol with radius higher than 1 μm. Thus, there is a minimum collection efficiency for particles in the approximate range 0.1 μm - 1 μm, where phoretic forces are felt. Generally speaking, published papers report not uniform evaluations of the contribution of thermo- and diffusiophoretic forces. This disagreement is partially due to the different laboratory and field conditions, and different theoretical approaches.

Performance Evaluation of Four Commercial Optical Particle Counters

The performances of four optical particles counters, Aerosol Spectrometer (Grimm 1.108), Enviro Check (Grimm 1.107), DustMonit and ParticleScan, were evaluated in laboratory tests employing monodisperse aerosol particles. The study focused on how commercial instruments perform during routine measurements respect to OPC scientific understanding, because it is important for users of such instruments to be aware of their limitations. Measurements were performed using aerosol generated by a Monodisperse Aerosol Generator (MAGE), which produced carnauba wax particles of diameter (1.00 ± 0.08) μm and (1.40 ± 0.15) μm, and monodisperse Polystyrene Latex (PSL) aerosol with nominal diameter of 1.0mm. The results show comparable total particle number concentrations for all the counters, when the count of the first size channel (0.3 - 0.4 μm) for the 1.108 Grimm counter was left out. In the said channel the Grimm counter 1.108 always showed much higher particle counts than those inferred from the tested aerosols. The overcount was proved by the fact that the aerosol sampled in each test on a Nuclepore filter showed no particles in the 0.3 - 0.4 μm range when examined under Scanning Electronic Microscope (SEM). The presence of an artefact produced by the counter was assumed as a likely explanation. For all the counters, the Count Median Diameters (CMDs) of aerosol size distributions, were far below the expected value for the aerosol used. The nearest CMD values to the expected ones were shown by the Grimm 1.107 counter.

Discrepancy between Ice Particles and Ice Nuclei in Mixed Clouds: Critical Aspects

Measurements of ice crystal concentrations in mixed clouds tend to exceed ice nucleus concentrations measured in nearby clear air. This discrepancy is a source of uncertainty in climate change projections as the radiative properties of mixed phase clouds are largely determined by their liquid and ice water content. The ice enhancement process can sometimes depend on secondary ice production, which can occur through ice crystal fracture during sublimation, cloud drop shattering during freezing or following collision with ice particles. However, the discrepancy is observed even in mixed clouds where only primary ice nucleation processes occur. Several hypotheses have been suggested for the observed discrepancies. One factor could be the existence in clouds of pockets of high vapor supersaturation formed by droplet freezing or removal of small droplets by collision with larger droplets, associated with the fact that ice crystal concentration increases with water supersaturation. However, ice crystal concentrations are usually measured at near water saturation. Additional factors could be drop freezing during evaporation and activation of droplet evaporation residues. Here we suggest that a major factor could be underestimation of the contact freezing mode as it is not measured in experimental campaigns and seldom considered in nucleation models. Laboratory experiments give only incomplete answers to the important questions concerning the contact freezing mode, e.g. what fraction of the aerosol particles that come into contact with the droplet surface results in a freezing event and what is the influence of particle type and size, air temperature and relative humidity. As supercooled droplets grow or evaporate in mixed clouds, phoretic forces should play an important role in the collision efficiency between aerosol and droplets, and consequently in contact freezing. A further question is the possibility that aerosol, usually not active in deposition or condensation/immersion freezing, can trigger ice nucleation by colliding with supercooled droplets.

Aerosol Scavenging during the Early Growth Stage of Ice Crystal Formation

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.

Further Laboratory Experiments on Aerosol Scavenging in Mixed Clouds to Assess the Role of Phoretic Forces and Particle Solubility

Scavenging experiments have been performed in a cloud chamber inside a cold room with different aerosol particles: Paraffin particles, NaCl particles, Magnesium oxide particles, Carbon particles, Sahara dust particles. Essentially the experimental tests were carried on following the sequence of operations: the generation of the aerosol particles, their injection in the lower part of the cloud chamber, injection of water droplets in the whole chamber volume, nucleation of ice crystals, collection of ice crystals and their examination as for resulting scavenging efficiency. Evidence is given of the peculiar behaviour of soluble particles, individual and eventually inside mixed particles, leading to very much important scavenging efficiency, probably to be ascribed to aerodynamic capture. The evident peculiar behaviour of deliquescent particles can be oriented towards applications to an efficient abatement of specific effluents, on one side, and to weather modification experiments, both rain enhancement and hail prevention experiments.

Eyjafjallajökull Volcanic Eruption: Ice Nuclei and Particle Characterization

The Eyjafjallaj?kull 2010 eruption was an extraordinary event in that it led to widespread and unprecedented disruption to air travel over Europe – a region generally considered to be free from the hazards associated with volcanic eruptions, excluding the extreme south influenced by Mt. Etna. In situ measurements were performed at the research centre of the National Research Council (CNR) area of Bologna (44?31′ N;11?20′ E), an urban background site, in order to contribute to knowledge concerning the impact of the volcanic emission.Aerosol size distributions measured with a Differential Mobility Particle Sizer (DMPS) and an Optical Particle Counter (OPC) show an increase in concentration of the accumulation and coarse fraction during the transit of the ash cloud, with respect to the subsequent period of the event, while particles smaller than 0.3 μm seem not to be affected by volcanic ash. Ice nuclei measured in the sampled air during and after the ash cloud transit, show an higher concentration during the ash cloud transit, with a ratio of about 1:110 with respect to the aerosol number concentration measured with the OPC.The elemental composition of aerosol particles, performed with SEM-EDX, gives about 30% of the inorganic coarse particles (geometric diameter larger than 1 μm) of volcanic origin on the 20 April. Si and Al concentrations result prevalently much higher than Ca and Fe ones. A large number of particles contained sulphur, indicating secondary processes of sulphate/sulphuric acid formation due to sulphur dioxide oxidation during transport in the volcanic plume.

Does the Homogeneous Ice Nucleation Initiate in the Bulk Volume or at the Surface of Super-Cooled Water Droplets? A Review

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.

Experimental Study on the Dependency of Ice Nucleation Active Surface Site Density on ATD Aerosol Size

In light of the percentage of Earth’s cloud coverage, heterogeneous ice nucleation in clouds is the most important global-scale pathway. More recent parameterizations of ice nucleation processes in the atmosphere are based on the concept of ice nucleation active surface site density (ns). It is usually assumed that ns is independent of time and aerosol size distribution, i.e. that the surface properties of aerosols of the same species do not vary with size. However, the independence of ns on aerosol size for every species has been questioned. This study presents the results of ice nucleation processes of ATD laboratory-generated aerosol (particle diameters of 0 - 3 μm). Ice nucleation in the condensation mode was performed in a Dynamic Filter Processing Cham- ber at temperatures of -18°C and -22°C, with a saturation ratio with respect to water of 1.02. Results show that ns increased by lowering the nucleation temperature, and was also dependent on the particle size. The ns of particles collected on the filters, after a 0.5 μm D50 cut-off cyclone, resulted statistically higher with respect to the values obtained from the particles collected on total filters. The results obtained suggest the need for further investigation of ns dependence of same composition aerosol particles with a view to support weather and climate predictions.