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International audience; The objective of this study was to document in situ the particle size distribution of desert dust particles during their long-range transport in the free troposphere, with a particular focus on the coarse size fraction that dominates the mass flux. The basic idea was to deploy a specifically designed optical particle counter/sizer named LOAC (Light Optical Aerosol Counter/sizer) under drifting balloons in order to minimize isokinetic sampling problems. Boundary-layer pressurized balloons (BLBPs) were launched on alert in the western Mediterranean basin in the framework of the Chemistry and Aerosol Mediterranean Experiment (ChArMEx) field campaigns, allowing us to perform original quasi-Lagrangian monitoring of desert dust aerosols over the sea. We illustrate tests and validations of LOAC measurements performed to qualify the instrument that include comparisons with concurrent aircraft and tethered balloon measurements, and remote sensing measurements including an AERONET sun-photometer, and a ground-based and the CALIOP lidar systems. As many as 10 LOAC flights were successfully conducted, mainly from Minorca Isl., Spain, during 4 Saharan dust transport events, with BLPBs drifting at altitudes between 2.0 and 3.3 km above sea level. The longest flight exceeded 700 km and lasted more than 25 h. Aerosol optical depths in the balloon vicinity did not exceed about 0.4 but the presence of turbid dust layers was confirmed thanks to dual scattering angle measurements by LOAC allowing the identification of dust particles. LOAC data from BLBPs could generally be fitted by a 3-mode lognormal distribution at roughly 0.2, 4 and 30 µm in modal diameter. Up to about 0.0001 dust particles per cubic cm larger than 40 µm are reported and no significant evolution of the size distribution was observed during the flights whereas we were expecting to observe gravitational sedimentation of the largest particle size fraction. We also flown LOAC under meteorological sounding balloons during those events to document the dust vertical distribution. Shorter integration time limits the sensitivity to large particles but their presence was confirmed in dust layers. Large particle were also occasionally observed during routine LOAC soundings at Aire-sur-l'Adour in southwestern France. The presence and persistence of a 'giant' mode at about 30 µm in diameter even after several days of transport contradicts calculations of particles sedimentation velocity. From an indirect evidence of the presence of charged particles derived from the LOAC measurements, we speculate that electrical forces might counteract gravitational settling of the coarse particles. More details can be found in the ChArMEx Special Issue (https://www.atmos-chem-phys.net/special_issue334.html; see Renard et al. papers).

International audience; After 43 years of inactivity, the Calbuco volcano which is located in the southern part of Chile erupted on 22 April 2015. The space-time evolutions (distribution and transport) of its aerosol plume are investigated by combining satellite (CALIOP, IASI, OMPS), in situ aerosol counting (LOAC OPC) and lidar observations, and the MIMOSA advection model. The Calbuco aerosol plume reached the Indian Ocean 1 week after the eruption. Over the Reunion Island site (21° S; 55.5° E), the aerosol signal was unambiguously enhanced in comparison with "background" conditions with a volcanic aerosol layer extending from 18 km to 21 km during the May–July period. All the data reveal an increase by a factor of ~ 2 in the SAOD (Stratospheric Aerosol Optical Depth) with respect to values observed before the eruption. The aerosol e-folding time is approximately 90 days. Microphysical measurements obtained before, during and after the eruption reflecting the impact of the Calbuco eruption on the lower stratospheric aerosol content have been analyzed over Reunion site. During the passage of the plume, the volcanic aerosol was characterized by an effective radius of 0.16 ± 0.02 µm with an unimodal lognormal size distribution and the aerosol number concentration appears 20 times higher than before and one year after the eruption. A tendency toward "background" conditions has been observed about one year after the eruption, by April 2016. The volcanic aerosol plume is advected eastward in the Southern Hemisphere and its latitudinal extent is clearly bounded by the subtropical barrier and the polar vortex. The transient behavior of the aerosol layers observed above Reunion Island between May and July 2015 reflects an inhomogeneous geographical distribution of the plume which is controlled by the latitudinal motion of these dynamical barriers.

International audience

The study of the stratospheric aerosols is important to our understanding of the terrestrial radiative budget. Our current comprehension of the different types of stratospheric particles and their spatial and temporal distribution is incomplete. In the present study, we try to show that measuring particle concentrations by the means of a new balloon-borne miniature particle counter, the LOAC, may allow us to determine the local variability in stratospheric aerosols in the size range 0.2 – 100 μm in diameter. In that respect, the PhD thesis consists of a first phase of a more accurate characterisation of the LOAC’s performances under balloon-borne measurement. A second phase consists of comparative analysis of stratospheric aerosol content based on a LOAC dataset obtained during a continuous campaign of balloon launches in France, along with some occasional flights abroad under particular circumstances (volcanic eruption, monsoon). Thus we show that the LOAC has a detection limit that restricts the measurement of submicronic particles in volcanic quiescent periods for concentrations lower than typically 1 particle per cm3. In its current version, the LOAC allows us to characterise aerosols in volcanic plumes in the troposphere and lower stratosphere. And, further, we propose directions concerning possible calibration and analysis strategies for the future data from the next generation of the LOAC currently in development.; L’étude des aérosols stratosphériques est importante pour comprendre le bilan radiatif terrestre. A l’heure actuelle, notre représentation des différents types de particules stratosphériques et leurs répartitions spatiale et temporelle n’est pas complète. Au cours de cette thèse, nous tentons de montrer que la mesure de la concentration en particules sous ballon météorologique au moyen d’un nouveau mini compteur de particules, le LOAC, pourrait permettre de rendre compte de la possible variabilité locale du contenu en aérosols stratosphériques dans la gamme de taille 0,2 à 100 μm en diamètre. La première partie de ce travail consiste à caractériser plus précisément les performances du LOAC sous ballon météorologique appliqué à la mesure en stratosphère. La seconde partie propose une analyse comparée du contenu en aérosols stratosphériques obtenu par LOAC, à partir de lâchers de ballons en France régulièrement depuis 3 ans et plus ponctuellement à l’étranger dans des situations particulières (volcan, mousson), et par d’autres types de données (Observations spatiales, lidar sol et simulation globale). Nous montrons alors que l’instrument possède une limite de détection rendant difficile la mesure des particules submicroniques lors de période de fond en moyenne stratosphère pour des concentrations de l’ordre d’une particule par cm3. Dans sa version actuelle, le LOAC permet de documenter les panaches volcaniques en troposphère ainsi qu’en basse stratosphère. En perspective, nous proposons des directions pour la calibration et l’analyse des futures données d’une nouvelle génération de l’instrument en développement.

International audience; Volcanic eruptions have huge societal and economic consequences. In Iceland, one of the best known examples isthe Laki eruption (1783-84 CE) (Thordarson and Self, 2003) which caused the death of > 20% of the Icelandicpopulations and likely increased European levels of mortality through air pollution (Witham and Oppenheimer,2004). The recent fissure eruption at Holuhraun (31 August 2014 – 27 February 2015) was a major source ofsulfur gases and aerosols and caused also both local and European-wide deteriorations to air quality (Gislason etal. 2015; Schmidt et al. 2015).The capability of atmospheric models to predict volcanic plume impacts is limited by uncertainties in thenear-source plume state. Most in-situ measurements of the elevated plume involve interception of aged plumesthat have already chemically or physically evolved. Small portable sensors airborne drone or balloon platformsoffer a new possibility to characterize volcano plumes near to source.We present the results of a balloon flight through the plume emitted by Baugur the main vent during the nightof the January 22th 2015. The balloon carrying a LOAC (Renard et al. 2015) has intercepted the plume at 8kmdistance downwind from the crater which represents a plume age of approximately 15 minutes. The plume waslocated in altitude between 2 and 3.1km above the sea level. Two layers were observed, a non-condensed lowerlayer and a condensed upper layer. The lower layer of 400m thick was characterized by a mode of fine particlescentered on 0.2m in diameter and a second mode centered on 2.3m in diameter and a total particle concentrationaround 100 particles per cubic centimeter. The upper layer of 800m thick was a cloud-like signature with dropletscentered on 20 m in diameter and a fine mode, the total particles concentrations was 10 times higher than thefirst layer. The plume top height was determined between 2.7 and 3.1 km, the plume height is in good agreementwith an estimate made by analysis of IASI satellite remote sensing data, thus demonstrating in-situ validation ofthis recent satellite algorithm (Carboni et al. 2015).This experimentation shows that under such difficult field campaign conditions (strong wind, low temperatures,only car batteries for power supply, night time and active volcano close to the launch site) it is possible to launchmeteorological balloons with novel payloads to directly sample in-situ the near-source plume, determine theplume altitude, identify dynamical phases of the plume and document the size distribution of particles inside aplume which is only a quarter of an hour old.

The study of the stratospheric aerosols is important to our understanding of the terrestrial radiative budget. Our current comprehension of the different types of stratospheric particles and their spatial and temporal distribution is incomplete. In the present study, we try to show that measuring particle concentrations by the means of a new balloon-borne miniature particle counter, the LOAC, may allow us to determine the local variability in stratospheric aerosols in the size range 0.2 – 100 μm in diameter. In that respect, the PhD thesis consists of a first phase of a more accurate characterisation of the LOAC’s performances under balloon-borne measurement. A second phase consists of comparative analysis of stratospheric aerosol content based on a LOAC dataset obtained during a continuous campaign of balloon launches in France, along with some occasional flights abroad under particular circumstances (volcanic eruption, monsoon). Thus we show that the LOAC has a detection limit that restricts the measurement of submicronic particles in volcanic quiescent periods for concentrations lower than typically 1 particle per cm3. In its current version, the LOAC allows us to characterise aerosols in volcanic plumes in the troposphere and lower stratosphere. And, further, we propose directions concerning possible calibration and analysis strategies for the future data from the next generation of the LOAC currently in development., L’étude des aérosols stratosphériques est importante pour comprendre le bilan radiatif terrestre. A l’heure actuelle, notre représentation des différents types de particules stratosphériques et leurs répartitions spatiale et temporelle n’est pas complète. Au cours de cette thèse, nous tentons de montrer que la mesure de la concentration en particules sous ballon météorologique au moyen d’un nouveau mini compteur de particules, le LOAC, pourrait permettre de rendre compte de la possible variabilité locale du contenu en aérosols stratosphériques dans la gamme de taille 0,2 à 100 μm en diamètre. La première partie de ce travail consiste à caractériser plus précisément les performances du LOAC sous ballon météorologique appliqué à la mesure en stratosphère. La seconde partie propose une analyse comparée du contenu en aérosols stratosphériques obtenu par LOAC, à partir de lâchers de ballons en France régulièrement depuis 3 ans et plus ponctuellement à l’étranger dans des situations particulières (volcan, mousson), et par d’autres types de données (Observations spatiales, lidar sol et simulation globale). Nous montrons alors que l’instrument possède une limite de détection rendant difficile la mesure des particules submicroniques lors de période de fond en moyenne stratosphère pour des concentrations de l’ordre d’une particule par cm3. Dans sa version actuelle, le LOAC permet de documenter les panaches volcaniques en troposphère ainsi qu’en basse stratosphère. En perspective, nous proposons des directions pour la calibration et l’analyse des futures données d’une nouvelle génération de l’instrument en développement.

International audience; We present findings from a measurement campaign that deployed a range of in-situ real-time atmospheric measurement techniques to characterise aerosols and gases in Mt Etna plume in October 2013. The LOAC (Light Optical Aerosol Counter) instrument for size-resolved particle measurements was deployed alongside two Multi-Gas instruments (measuring SO 2 , H2S, HCl, CO 2) and an ozone sensor. Measurements were performed at the summit craters (in cloudy-and non-cloudy conditions) and in grounding downwind plume on the volcano flank. These high frequency measurements (acid gases: 1 to 0.1 Hz, aerosol: 0.1 Hz) provide a detailed in-situ dataset for time-resolved plume characterisation and volcano monitoring. The LOAC measurement of sized-resolved aerosol (over a 0.2 to 50 µm particle diameter range) alongside SO 2 (10's ppbv to 10's ppmv) provides a valuable dataset for determining the volcanic aerosol volume and surface area to SO 2 ratios. These parameters are presently poorly defined but are important for atmospheric models of the reactive halogen chemistry that occurs on volcanic aerosol surfaces to convert volcanic HBr into reactive bromine, including BrO. The LOAC's patented optical design can also provide insights into particle properties. The two Multi-Gas SO 2 time-series show good agreement, detecting co-varying plume fluctuations in the downwind plume, which also correlate with the LOAC total aerosol volume time-series. An estimate of HCl/SO 2 in Etna emissions was made by Multi-Gas electrochemical sensor, using a novel design to limit absorption/desorption effects and low-noise electronics for improved resolution. The detection of volcanic HCl by electrochemical sensor brings new possibilities for Multi-Gas monitoring of volcanic halogen emissions. Electrochemical sensor response times are not instantaneous, particularly for sticky gases such as HCl (T90 ∼min), but also even for " fast " response (T90 ∼ 10 to 30 s) sensors such as SO 2 and H2S. However, in a volcanic plume environment, Multi-Gas instruments are exposed to very rapidly fluctuating gas concentrations due to turbulent plume eddies. The combination of these effects can introduce measurement errors, emphasizing a need for sensor response modelling approaches for accurate determination of gas ratios from Multi-Gas instruments.

International audience; LOAC (Light Optical Aerosol Counter) is a small optical particle counter/sizer of ∼250 grams that can fly under all kinds of balloons. The measurements are conducted at two scattering angles: the first one, at 12 • , is used to determine the aerosol particle concentrations in 19 size classes within a diameter range of ∼0.2-100 micrometers. The second angle is at 60 • , is used to discriminate between the different types of particles dominating the different size classes. The sensor particularly discriminates wet or liquid particles, mineral dust and carbon particles. 30 flights of LOAC have been conducted during the ChAMEx campaign (Chemistry Aerosol Mediterranean Experiment) on summer 2013, from Minorca Island (Spain) and Ile du Levant (south of France): 19 flights under meteorological balloons and 12 flights under low altitude drifting balloons. Most of the flights were also coupled with ozone concentration measurements. LOAC balloons were especially, but not only, dedicated to study the various Saharan dust events that occurred during the campaign. In particular, flights were conducted every 12 hours during the 15-19 June dust event. Turbid air masses from North America were also sampled in late June over Minorca. The flights allow us to determine the vertical extent of the dust plume and various aerosol layers, and to follow the particle size distribution and the concentration evolution along the vertical. The low altitude drifting balloons, which stayed at constant altitude (between 0.4 and 3 km) for several hours, allow us to study the time-evolution of the aerosol concentrations in the same air mass. Under both balloon types, LOAC has detected larges particles up to ∼30 micrometers in diameter. The flights drifting within dust layers indicate that there is a relatively stable particle size distribution during transport over the sea, with no clear sedimentation loss of large particles. LOAC is used to tentatively identify the various kinds of particles (marine salt close to the sea, pure sand or more heterogeneous layers). Continuous surface measurements have also been conducted on Minorca Island with a LOAC. The LOAC number concentration measurements are converted to mass concentrations, in order to evaluate the effect of the mid-June dust event on the ambient air quality. Coincident ground-based remote sensing (lidar, sun-photometer) and in situ measurements (aethalometer, TEOM.. .) performed on Minorca Island and occasionally close airborne data are also used for comparison with balloon data.