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Le site instrumenté Cézeaux-Aulnat Opme Puy de Dôme (CO-PDD) est une plateforme d'observation de l'atmosphère labellisée à l'échelle nationale par le CNRS et le Cnes, impliquée dans des réseaux internationaux pour le climat tels que GAW (Global Atmospheric Watch) et dans des infrastructures de recherche telles que Actris (Aerosol Cloud and Trace gases Research Infrastructure) ou Icos (Integrated Carbon Observation System). Composé de quatre stations de mesures in situ et par télédétection dans la région clermontoise, il produit sur le long terme des mesures de la composition atmosphérique gazeuse et particulaire, des nuages et des précipitations en incluant la dimension verticale. Sa localisation géographique lui permet d'échantillonner des masses d'air présentant des origines très diverses, dans des conditions d'ennuagement très fréquentes. Cela en fait un site exceptionnel pour caractériser la variabilité environnementale de la composition atmosphérique. The instrumented site Cézeaux-Aulnat Opme Puy de Dôme (COPDD) is an atmosphere observation platform labeled at the national level by CNRS and CNES institutes and involved in international climate networks such as GAW (Global Atmospheric Watch) and in research infrastructures such as ACTRIS (Aerosol Cloud and Trace gas Research Infrastructure) or ICOS (Integrated Carbon Observation System). Composed of four in situ and remote sensing measurement points around the city of Clermont-Ferrand, it produces long-term measurements of gaseous and aerosol atmospheric composition, clouds and precipitation, including the vertical dimension. Its geographical location allows various origin air masses to be sampled, in very frequent cloudy conditions. This makes it an exceptional site for characterizing the environmental variability of the atmospheric composition.

Les nuages représentent un milieu de dissolution de nombreux composés chimiques à la fois sous la forme de gaz et de particules. Ces composés peuvent subir dans cet environnement multiphasique de nombreuses transformations complexes induites par le rayonnement solaire, les processus microphysiques ou encore l'activité des micro-organismes vivants dans les nuages. Cet article présente de façon résumée les activités d'observation et de recherche sur les nuages menées sur le site atelier du puy de Dôme au cours des 20 dernières années. Il se focalise plus particulièrement sur les développements méthodologiques tant en termes de prélèvements que de caractérisations du milieu et des processus de transformation. Ce travail est le résultat de la création d'un consortium unique regroupant des physiciens, des biologistes et des chimistes oeuvrant à une question commune autour du rôle des nuages dans les processus atmosphériques. Many chemical compounds coming from both the gaseous and particulate phases dissolve into cloud droplets. These compounds can undergo in this multiphasic environment many complex transformations induced by light, microphysical processes or even by the activity of microorganisms living in clouds. This article summarizes the observation and research activities on clouds sampled at the puy de Dôme site over the past 20 years. A particular focus is made to highlight methodological developments both in terms of sampling and characterization of the cloud medium and transformation processes occurring within it. This has been achieved by the creation of a unique consortium grouping together physicists, biologists and chemists working on a common question on the role of clouds on atmospheric processes.

The Forests gAses aeRosols Clouds Exploratory (FARCE) campaign was conducted in March–April 2015 on the tropical island of La Réunion. For the first time, several scientific teams from different disciplines collaborated to provide reference measurements and characterization of LaRéunionvegetation,volatileorganiccompounds(VOCs), biogenic VOCs (BVOCs), (bio)aerosols and composition of clouds, with a strong focus on the Maïdo mountain slope area. The main observations obtained during this 2-month intensivefieldcampaignaresummarized.Theyincludecharacterizations of forest structure, concentrations of VOCs and precursors emitted by forests, aerosol loading and optical properties in the planetary boundary layer (PBL), formation of new particles by nucleation of gas-phase precursors, icenucleatingparticlesconcentrations,andbiologicalloadingin bothcloud-freeandcloudyconditions.SimulationsandmeasurementsconfirmthattheMaïdoObservatorylieswithinthe PBL from late morning to late evening and that, when in the PBL, the main primary sources impacting the Maïdo Observatory are of marine origin via the Indian Ocean and of biogenic origin through the dense forest cover. They also show that(i)themarinesourceprevailslessandlesswhilereaching the observatory; (ii) when in the PBL, depending on the localizationofahorizontalwindshear,theMaïdoObservatory canbeaffectedbyairmassescomingdirectlyfromtheocean and passing over the Maïdo mountain slope, or coming from inland; (iii) bio-aerosols can be observed in both cloud-free and cloudy conditions at the Maïdo Observatory; (iv) BVOC emissions by the forest covering the Maïdo mountain slope can be transported upslope within clouds and are a potential cause of secondary organic aerosol formation in the aqueous phase at the Maïdo Observatory; and (v) the simulation of dynamicsparameters,emittedBVOCsandcloudlifecyclein the Meso-NH model are realistic, and more advanced MesoNH simulations should use an increased horizontal resolution (100m) to better take into account the orography and improve the simulation of the wind shear front zone within which lies the Maïdo