With dry deposition and below-cloud scavenging, in-cloud scavenging is one of the three components of aerosol transfer from the atmosphere to the ground. There is no experimental validation of in-cloud particle scavenging models for all cloud types that is not impacted by uncertainties concerning below-cloud scavenging. In this article, the choice was made to start with a recognised and validated microphysical cloud formation model (the DEtailed SCAvenging Model, DESCAM) to extract a scheme of aerosol scavenging by clouds, valid for different cloud types. The resulting model works for the two most extreme precipitation clouds: from cumulonimbus to stratus. It is based on data accessible a priori from numerical weather prediction (NWP) outputs, i.e. the intensity of the rain and the relative humidity in the cloud. The diagnostic of the altitude of the cloud base proves to be a key parameter, and accuracy in this regard is vital. This new in-cloud scavenging scheme is intended for use in long-distance (> 100 km) atmospheric transport models (ATMs) or global climate models (GCMs).

A new In-Cloud Aerosol Scavenging Experiment (In-CASE) has been conceived to measure the collection efficiency (CE) of submicron aerosol particles by cloud droplets. In this setup, droplets fall at their terminal velocity through a 1 m high chamber in a laminar flow containing aerosol particles. At the bottom of the In-CASE chamber, the droplet train is separated from the aerosol particle flow – droplets are collected in an impaction cup, whereas aerosol particles are deposited on a high-efficiency particulate air (HEPA) filter. The collected droplets and the filter are then analysed by fluorescence spectrometry since the aerosol particles are atomised from a sodium fluorescein salt solution (C20H10Na2O5). In-CASE fully controls all the parameters which affect the CE – the droplets and aerosol particles size distributions are monodispersed, the electric charges of droplets and aerosol particles are controlled, and the relative humidity is indirectly set via the chamber's temperature. This novel In-CASE setup is presented here as well as the first measurements obtained to study the impact of relative humidity on CE. For this purpose, droplets and particles are electrically neutralised. A droplet radius of 49.6±1.3 µm has been considered for six particle dry radii between 50 and 250 nm and three relative humidity levels of 71.1±1.3 %, 82.4±1.4 % and 93.5±0.9 %. These new CE measurements have been compared to theoretical models from literature which adequately describe the relative humidity influence on the measured CE.

A new In-Cloud Aerosol Scavenging Experiment (In-CASE) has been developed to measure the collection efficiency (CE) of submicron aerosol particles by cloud droplets. Droplets fall at their terminal velocity through a 1 m high chamber in a laminar flow containing aerosol particles. At the bottom of the In-CASE chamber, the droplet train is separated from the aerosol particles flow, and the droplets are collected in an impaction cup, whereas aerosol particles are deposited on a high-efficiency particulate air (HEPA) filter. The collected droplets and the filter are then analysed by fluorescence spectrometry since the aerosol particles are atomised from a sodium fluorescein salt solution (C20H10Na2O5). In-CASE fully controls all the parameters which affect the CE – the droplets and aerosol particles size distributions are monodispersed, the electric charges of droplets and aerosol particles are known and set, and the relative humidity is indirectly controlled via the chamber's temperature. This paper details the In-CASE setup and the dataset of 70 measurements obtained to study the impact of the electric charges on CE. For this purpose, droplets and particles charges are controlled through two charging systems developed in this work – both chargers are detailed below. The droplet charge varies from -3.0×104±1.4×103 to +9.6×104±4.3×103 elementary charges, while the particle charge ranges from zero to -90±9 elementary charges depending on the particle radius. A droplet radius of 48.5±1.1 µm has been considered for four particle dry radii between 100 and 250 nm while the relative humidity level during experiments is 95.1±0.2 %. The measurements are then compared to theoretical models from literature – showing good agreement.

This article presents an experimental study, performed in the BERGAME setup, dedicated to studying the collection of submicron aerosol particles by raindrops. The initial aim was to focus on the influence of the electrical charges of raindrops on the efficiency with which they collect aerosol particles. However, in the relative humidity range considered in this article (26–36%), measurements highlight a first-order role of phoretic effect for submicron aerosol particles. Indeed, measurements highlight a 100% increase in the collection efficiency for each percentage decrease in the atmospheric relative humidity. Phoretic effects are known to play a role in collection by drops; however, none of the models found in the literature predicts the same magnitude as the one presently measured. Characterization of the aerosol trajectories around the drop, accelerated to terminal velocity, seems to show a coupling between phoretic effects and rear capture. This interaction, already suggested by Grover et al., is a line of explanation for such a sharp unpredicted increase of the collection efficiency with moisture decrease.

A comparison between retrieved properties of the rain drop size distributions (DSDs) from multifrequency cloud radar observations and WRF Model results using either the Morrison or the Thompson bulk microphysics scheme is performed in order to evaluate the model’s ability to predict the rain microphysics. This comparison reveals discrepancies in the vertical profile of the rain DSDs for the stratiform region of the squall-line system observed on 12 June 2011 over Oklahoma. Based on numerical sensitivity analyses, this study addresses the bias at the top of the rain layer and the vertical evolution of the DSD properties (i.e., of Dm and N0*). In this way, the Thompson scheme is used to explore the sensitivity to the melting process. Moreover, using the Thompson and Morrison schemes, the sensitivity of the DSD vertical evolution to different breakup and self-collection parameterizations is studied. Results show that the DSDs are strongly dependent on the representation of the melting process in the Thompson scheme. In the Morrison scheme, the simulations with more efficient breakup reproduce the DSD properties with better fidelity. This study highlights how the inaccuracies in simulated Dm and N0* for both microphysics schemes can impact the evaporation rate, which is systematically underestimated in the model.

This paper describes a theoretical model which computes the collection efficiency of aerosol particles by droplets, due to the combined action of dynamic (inertia, weight and drag) and electrophoresis forces acting on an aerosol particle of radius 0.004 <= a <= 1.3 mu m around a droplet of radius 15 <= A <= 100 mu m. The electrostatic forces are defined following the concept of image charges. In the given particle range, the Brownian motion must be considered and was consequently added to the model. A novel approach is developed, based on the Langevin's theory and solved using an Ito process. Results of electroscavenging related to natural atmospheric ionisation, as well as for strongly charged aerosol particles released after a nuclear accident, are presented with pressure and temperature representative of the mid-troposphere (-17 degrees C, 540 hPa). The droplet charges considered in the paper are representative of weakly and strongly electrified clouds. The values of collection efficiency computed are convenient for incorporation into cloud models and to study scavenging of aerosol particles by clouds whether for climate, pollution or nuclear safety issues.

This study aims to characterize the microphysical and optical properties of ice crystals and 15 supercooled liquid droplets within low-level Arctic mixed-phase clouds (MPCs). We compiled and analyzed 16 cloud in situ measurements from 4 airborne spring campaigns (representing 18 flights and 71 vertical profiles in 17 MPCs) over the Greenland and Norwegian Seas mainly in the vicinity of the Svalbard Archipelago. Cloud phase 18 discrimination and representative vertical profiles of number, size, mass and shape of ice crystals and liquid 19 droplets are established. The results show that the liquid phase dominates the upper part of the MPCs. High 20 concentrations (120 cm in average) of small droplets (mean values of 15 μm), with an averaged LWC of 0.2 21 g.m are measured at cloud top. The ice phase dominates the microphysical properties in the lower part of the 22 cloud and beneath it, in the precipitation region (mean values of 100 μm, 3 L and 0.025 g.m for diameter, 23 particle concentration and IWC respectively). The analysis of the ice crystal morphology shows that the majority 24 of ice particles are irregularly shaped or rimed particles, the prevailing regular habits found are stellars and 25 plates. We hypothesize that riming and diffusional growth processes (including the Wegener-Bergeron-Findeisen 26 mechanism) are the main growth mechanisms involved in the observed MPCs. The impact of larger scale 27 meteorological conditions on the vertical profiles of MPC properties was also investigated. Large values of LWC 28 and high concentration of smaller droplets are possibly linked to polluted situations and air masses origins from 29 the South, which lead to very low values of ice crystal size and IWC. On the contrary, clean situations with low 30 temperatures exhibit larger values of ice crystal size and IWC. Several parameterizations relevant for remote 31 sensing or modeling studies are also determined, such as IWC (and LWC) – extinction relationship, ice and 32 liquid integrated water paths, ice concentration and liquid water fraction according to temperature. 33

This article presents new measurements of the efficiency with which aerosol particles of accumulation mode size are collected by a 1.25 mm sized raindrop. These laboratory measurements provide the link to reconcile the scavenging coefficients obtained from theoretical approaches with those from experimental studies. We provide here experimental proof of the rear capture mechanism in the flow around drops, which has a fundamental effect on sub-microscopic particles. These experiments thus confirm the efficiencies theoretically simulated by Beard (1974). Finally, we propose a semi-analytical expression to take into account this essential mechanism to calculate the collection efficiency for drops within the rain size range.

This study aims to characterize the microphysical and optical properties of ice crystals and supercooled liquid droplets within low-level Arctic mixed-phase clouds (MPCs). We compiled and analyzed cloud in situ measurements from four airborne spring campaigns (representing 18 flights and 71 vertical profiles in MPCs) over the Greenland and Norwegian seas mainly in the vicinity of the Svalbard archipelago. Cloud phase discrimination and representative vertical profiles of the number, size, mass and shape of ice crystals and liquid droplets are established. The results show that the liquid phase dominates the upper part of the MPCs. High concentrations (120 cm−3 on average) of small droplets (mean values of 15 µm), with an averaged liquid water content (LWC) of 0.2 g m−3 are measured at cloud top. The ice phase dominates the microphysical properties in the lower part of the cloud and beneath it in the precipitation region (mean values of 100 µm, 3 L−1 and 0.025 g m−3 for diameter, particle concentration and ice water content (IWC), respectively). The analysis of the ice crystal morphology shows that the majority of ice particles are irregularly shaped or rimed particles; the prevailing regular habits found are stellars and plates. We hypothesize that riming and diffusional growth processes, including the Wegener–Bergeron–Findeisen (WBF) mechanism, are the main growth mechanisms involved in the observed MPCs. The impact of larger-scale meteorological conditions on the vertical profiles of MPC properties was also investigated. Large values of LWC and high concentration of smaller droplets are possibly linked to polluted situations and air mass origins from the south, which can lead to very low values of ice crystal size and IWC. On the contrary, clean situations with low temperatures exhibit larger values of ice crystal size and IWC. Several parameterizations relevant for remote sensing or modeling studies are also determined, such as IWC (and LWC) – extinction relationship, ice and liquid integrated water paths, ice concentration and liquid water fraction according to temperature.

3709, L 26-27, The authors state that "despite low emission rates disconnecting the concentration of INA bacteria existing at the surface of plants from their concentration in the air above (Garcia et al., 2012): : :: : : ". It should be noted that the data of Garcia et al are not proof of low emission rate. The lack of detection by these authors can be readily explained by the relative insensitivity of their technique, which is much less sensitive than classical microbiological methods that were used many years previously by Lindemann and coworkers to detect INA bacteria in aerosols and for measuring flux. The comparative calculations of these methods are presented in detail in :

Abstract. The analysis of radioactive aerosol scavenged by rain after the Chernobyl accident highlights certain differences between the modelling studies and the environmental measurements. Part of these discrepancies can probably be attributed to uncertainties in the efficiencies used to calculate aerosol particle collection by raindrops, particularly drops with a diameter larger than one millimetre. In order to improve the issue of these uncertainties, an experimental study was performed to close the gaps still existing for this key microphysical parameter. In the present article, attention is first focused on the efficiency with which aerosol particles, in the accumulation mode are collected by raindrops with a diameter of 2 mm. The collections efficiencies measured for aerosol particle in the sub-micron range are quantitatively consistent with previous theoretical model developed by Beard (1974) and thus highlight the major role of rear capture in the submicron range.

The distribution, abundance and nature of ice nucleation active particles in the atmosphere are major sources of uncertainty in the prediction of cloud coverage, precipitation patterns and climate. Some biological ice nuclei (IN) induce freezing at temperatures at which most other atmospheric particles exhibit no detectable activity (> −10 °C). Their actual contribution to the pool of IN in clouds remains poorly known, but numerical studies have suggested a probable significance of biological IN in atmospheric processes. In this study, cloud water was collected aseptically from the summit of Puy de Dôme (1465 m a.s.l., France) within contrasted meteorological and physico-chemical situations. Total and biological (i.e. heat-sensitive) IN were quantified by droplet-freezing assay between −5 °C and −14 °C. We observed that freezing was systematically induced by biological material, between −6 °C and −8 °C in 92% of the samples. Its removal by heat treatment consistently led to a decrease of the onset freezing temperature, by 3 °C or more in most samples. At −10 °C, 0 to ~ 220 biological IN mL−1 of cloud water were measured (i.e. 0 to ~ 22 m−3 of cloud air based on cloud liquid water content estimates), and these represented 65% to 100% of the total IN. Based on back-trajectories and on physico-chemical analyses, the high variability observed resulted probably from a source effect, with IN originating mostly from continental sources. Assuming that biological IN were all bacteria, at maximum 0.6% of the bacterial cells present in cloud water samples could have acted as IN at −8 °C, 1.5% at −10 °C, and 3.1% at −12 °C. The data set generated here will help elucidate the role of biological and bacterial IN on cloud microphysics by numeric modelling, and their impact on precipitation at local scale.

A numerical study is presented to evaluate the possible impact of recently measured collection scavenging efficiencies of aerosol particles by raindrops on the calculation of wet removal of pollution plumes by rain. These new collection efficiencies cover the previously undocumented range between 0.3 mu m and 3.5 mu m diameter for the aerosol particles and 2 to 2.6 mm diameter for the drops. They show for particles between 03 mu m and 0.7 mu m a region of an important increase of the efficiencies with decreasing particle size probably due to the capture of particles in the rear vortex developing behind the falling drop. This hypothesis was motivated by measurements using particle image velocimetry (PIV). Also for the particles larger than 1.5 mu m the efficiencies exceed old approximations by up to a factor of two.Typical continental and maritime aerosol particle distributions were used for background and plume distributions and each time the deposited particle mass was calculated with the old and the new efficiencies for the different resulting precipitation rates. In the simulations the new efficiencies increased the calculated wet removal rate of pollution plume particles between 5% and 17%, with respect to the simulations with the old efficiencies, whereby one third of this increase could be attributed to rear capture. This phenomenon has not been observed in these size ranges before.The study also highlights the weakness of certain methods to determine the scavenging coefficient of a pollution plume by rain. Instead of linking the scavenging coefficient to the decrease of the particle spectrum in the air, an operational approach, e.g. in case of accidental releases, needs to be developed that links it to the rainfall intensity, as well as other variables like meteorological parameters, cloud size and plume characteristics.Considering the still persisting gaps of measurements in the collection efficiencies more laboratory measurements of collection scavenging efficiencies between aerosol particles and raindrops are also necessary. (C) 2013 Elsevier B.V. All rights reserved.

The understanding and the anticipation of the environmental fallout in case of severe nuclear accidents with radioactive releases is crucial for the environment. In this study we aim to improve our knowledge on the aerosol particles scavenging, in particular the washout by raindrops with a diameter larger than 1 mm.

In this study, we use a Tandem Differential Mobility Analyser (TDMA) system combining particle volatilization and humidification conditioning (VH-TDMA) to test the effect of the gentle volatilization of a small fraction of the atmospheric particles on the particle hygroscopic growth in several environments (urban to remote). We first give an overview of the Hygroscopic Growth Factors (HGF) in these various environments, showing that in most of them, aerosol particles are externally mixed. We then show that the particle hygroscopicity can either be increased or decreased after thermal conditioning of the particle at moderate temperatures (50-110 degrees C). The hygroscopic growth factor changes induced by volatilization indicate that some volatile compounds, although present at low concentrations, can significantly influence the hygroscopic growth of particles in a way that can most of time be theoretically explained if simplified assumptions are used. However, simplified assumptions occasionally fail over several hours to explain hygroscopic changes, kinetic/surface effects observed at remote environments are suspected to be important. (C) 2013 Elsevier Ltd. All rights reserved.

The hygroscopic growth and droplet activation of uncoated soot particles and such coated with succinic acid and sulfuric acid were investigated during the IN-11 campaign at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility. A GFG-1000 soot generator applying either nitrogen or argon as carrier gas and a miniCAST soot generator were utilized to generate soot particles. Different organic carbon (OC) to black carbon (BC) ratios were adjusted for the CAST-soot by varying the fuel to air ratio. The hygroscopic growth was investigated by means of the mobile Leipzig Aerosol Cloud Interaction Simulator (LACIS-mobile) and two different Hygroscopicity Tandem Differential Mobility Analyzers (HTDMA, VHTDMA). Two Cloud Condensation Nucleus Counter (CCNC) were applied to measure the activation of the particles. For the untreated soot particles neither hygroscopic growth nor activation was observed at a supersaturation of 1%, with exception of a partial activation of GFG-soot generated with argon as carrier gas. Coatings of succinic acid lead to a detectable hygroscopic growth of GFG-soot and enhanced the activated fraction of GFG- (carrier gas: argon) and CAST-soot, whereas no hygroscopic growth of the coated CAST-soot was found. Sulfuric acid coatings led to an OC-content dependent hygroscopic growth of CAST-soot. Such a dependence was not observed for activation measurements. Coating with sulfuric acid decreased the amount of Polycyclic Aromatic Hydrocarbons (PAH), which were detected by AMS-measurements in the CAST-soot, and increased the amount of substances with lower molecular weight than the initial PAHs. We assume that these reaction products increased the hygroscopicity of the coated particles in addition to the coating substance itself.

The analysis of the radioactive aerosol scavenging by rain after the Chernobyl accident highlights some differences between the modelling studies and the environmental measurements. Part of this gap is due to the uncertainties on the scavenging efficiency of aerosol particles by raindrops, in particular for drops with a diameter larger than one millimeter. The IRSN (Institut de Radioprotection et de Sûreté Nucléaire) has decided to launch an experimental study to measure with a better accuracy the scavenging efficiency of large raindrops. The scavenging efficiency of aerosol has been determined by measuring precisely the mass of aerosol particles collected by a single drop after its path through an atmosphere loaded with particles. The collection efficiencies for drop diameters of 2 mm and 2.6 mm (previously unknown for atmospheric aerosols) are measured. The impact of these new data on modeling of the washout of the atmosphere by the rain is noticed.