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.

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.

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.

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.

An integrated approach to investigate climate-driven rockfall occurrence in high alpine slopes: the Bessanese glacial basin, Western Italian Alps

Rockfalls are one of the most common instability processes in high mountains.They represent a relevant issue,both for the risks they represent for(infra)structures and frequentation,and for their potential role as terrestrial indicators of climate change.This study aims to contribute to the growing topic of the relationship between climate change and slope instability at the basin scale.The selected study area is the Bessanese glacial basin(Western Italian Alps)which,since 2016,has been specifically equipped,monitored and investigated for this purpose.In order to provide a broader context for the interpretation of the recent rockfall events and associated climate conditions,a cross-temporal and integrated approach has been adopted.For this purpose,geomorphological investigations(last 100 years),local climate(last 30 years)and near-surface rock/air temperatures analyses,have been carried out.First research outcomes show that rockfalls occurred in two different geomorphological positions:on rock slopes in permafrost condition,facing from NW to NE and/or along the glacier margins,on rock slopes uncovered by the ice in the last decades.Seasonal thaw of the active layer and/or glacier debutressing can be deemed responsible for slope failure preparation.With regard to timing,almost all dated rock falls occurred in summer.For the July events,initiation may have been caused by a combination of rapid snow melt and enhanced seasonal thaw of the active layer due to anomalous high temperatures,and rainfall.August events are,instead,associated with a significant positive temperature anomaly on the quarterly scale,and they can be ascribed to the rapid and/or in depth thaw of the permafrost active layer.According to our findings,we can expect that in the Bessanese glacierized basin,as in similar high mountain areas,climate change will cause an increase of slope instability in the future.To fasten knowledge deepening,we highlight the need for a growth of a network of high elevation experimental sites at the basin scale,and the definition of shared methodological and measurement standards,that would allow a more rapid and effective comparison of data.

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.

Application of the Soft X-Ray TSI Advanced Aerosol Neutralizer to Aerosol Measurements Made by a Grimm SMPS

Scanning Mobility Particle Sizer (SMPS) spectrometers are subject to several restrictions when a radioactive source is employed to bring the aerosol to a steady-state charge distribution. An alternative solution, commercially available, is represented by the soft X-ray neutralizer. The present study investigates the outcome of a combination of a Grimm SMPS, which employs a 241Am radioactive source, with the soft X-ray advanced aerosol neutralizer (TSI model 3087). To date, the latter device has been interfaced only with TSI’s Electrostatic Classifiers. Particle size distribution of sodium chloride aerosol was measured with both neutralizers and it was found that the particle number concentration agreed to be within 9% for the 10 - 700 nm range. This difference mainly corresponds to the bias reported by TSI, when the X-ray device is mounted on a TSI spectrometer. It was concluded that the X-ray neutralizer could be usefully employed, as a standalone device, in combination with Grimm Electrostatic Classifiers.

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.

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.

The Use of Ocean-Colour Data to Estimate Chl-a Trends in European Seas

Ocean-colour remote-sensing products have been used to estimate Chl-a trends in European seas with the aim to develop a new indicator based on ocean-colour data for the European Environment Agency (EEA). The new indicator, called CSI023(+), derived from satellite ocean-colour products from the MyOcean Marine Core Service (www.myocean.eu) has been defined and calculated. In our analysis, we have used 3 MyOcean satellite products: 2 global satellite products (SeaWiFS and a merged product) and one regional (adjusted to specific regional Mediterranean conditions) ocean-colour product. We have evaluated the differences among the 3 different products in estimating Chl-a trends. CSI023(+) complements the EEA CSI023 indicator for eutrophication based on chlorophyll-a (Chl-a) in-situ observations. Analysis has revealed the potential of ocean colour as a CSI023(+) indicator to detect large-scale, and in some cases, even local-scale, changes and decreasing trends of Chl-a were observed throughout the Black Sea, the Eastern Mediterranean, the southern part of the Western Mediterranean, the English Channel and the north part of the North Sea. Large areas with increasing trends were observed in the Bay of Biscay, in the North-East Atlantic regions of Ireland and the UK, in the northern part of the North Sea, in the Kattegat and in the Baltic. Specific analysis has been performed in the Mediterranean coastal areas using regional products to investigate local scale results. Validation of ocean-colour products has been carried out through comparison with observations of the Eionet EEA database. The validation results highlight that regional products produced with regional algorithms are recommended for the future.

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.

QA/QC Procedures for <i>in-Situ</i>Calibration of a High Altitude Automatic Weather Station: The Case Study of the AWS Pyramid, 5050 m asl, Khumbu Valley, Nepal

In-situ calibrations of weather stations are usually performed by positioning standard instruments close to the station under calibration and comparing the obtained results. This procedure could be useful to evaluate the proper functioning of the monitoring equipments, but do not allowed the determination of a calibration curve that allow the corrections of the acquired parameters. Thus, the development of a dedicated facility for in-situ calibration of weather stations, enabling simultaneous generation of a wide range of temperatures and pressures could offer important improvements in this framework, particularly if this facility is applied to high mountains monitoring stations where the weather stations calibrations could be very difficult. This paper will present the calibration chamber developed in the framework of the EMRP-METEOMET (Metrology for Meteorology) Project, which aims is to bring metrological traceability to high altitude meteorological instruments and through this experience will provide a general overview on the importance of the application of this methodology at different levels.

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.

Comparison of PM_(10) concentrations and metal content in three different sites of the Venice Lagoon: An analysis of possible aerosol sources

The Venice Lagoon is exposed to atmospheric pollutants from industrial activities, thermoelectric power plants, petrochemical plants, incinerator, domestic heating, ship traffic, glass factories and vehicular emissions on the mainland. In 2005, construction began on the mobile dams （MOSE）, one dam for each channel connecting the lagoon to the Adriatic Sea as a barrier against high tide. These construction works could represent an additional source of pollutants. PM10 samples were taken on random days between 2007 and 2010 at three different sites： Punta Sabbioni, Chioggia and Malamocco, located near the respective dam construction worksites. Chemical analyses of V, Cr, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, Sb, T1 and Pb in PM10 samples were performed by Inductively coupled plasma- quadrupole mass spectrometry （ICP-QMS） and results were used to identify the main aerosol sources. The correlation of measured data with meteorology, and source apportionment, failed to highlight a contribution specifically associated to the emissions of the MOSE construction works. The comparison of the measurements at the three sites showed a substantial homogeneity of metal concentrations in the area. Source apportionment with principal component analysis （PCA） and positive matrix factorization （PMF） showed that a four principal factors model could describe the sources of metals in PM10. Three of them were assigned to specific sources in the area and one was characterised as a source of mixed origin （anthropogenic and crustal）. A specific anthropogenic source of PM10 rich in Ni and Cr, active at the Chioggia site, was also identified.

Air Pollution Steady-State Advection-Diffusion Equation: The General Three-Dimensional Solution

Atmospheric air pollution turbulent fluxes can be assumed to be proportional to the mean concentration gradient. This assumption, along with the equation of continuity, leads to the advection-diffusion equation. Many models simulating air pollution dispersion are based upon the solution (numerical or analytical) of the advection-diffusion equation as- suming turbulence parameterization for realistic physical scenarios. We present the general steady three-dimensional solution of the advection-diffusion equation considering a vertically inhomogeneous atmospheric boundary layer for arbitrary vertical profiles of wind and eddy-diffusion coefficients. Numerical results and comparison with experimental data are shown.

An Analytical Air Pollution Model with Time Dependent Eddy Diffusivity

Air pollution transport and dispersion in the atmospheric boundary layer are modeled by the advection-diffusion equation, that is, essentially, a statement of conservation of the suspended material in an incompressible flow. Many models simulating air pollution dispersion are based upon the solution (numerical or analytical) of the advection-diffusion equation assuming turbulence parameterization for realistic physical scenarios. We present the general time dependent three-dimensional solution of the advection-diffusion equation considering a vertically inhomogeneous atmospheric boundary layer for arbitrary vertical profiles of wind and eddy-diffusion coefficients. Numerical results and comparison with experimental data are shown.