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The Pyrenean Platform for Observation of the Atmosphere (P2OA) is a coupled plain–mountain instrumented platform in southwestern France. It is composed of two physical sites: the "Pic du Midi" mountaintop observatory (2877 m a.s.l.) and the "Centre de Recherches Atmosphériques" (600 m a.s.l). Both sites are complementarily instrumented for the monitoring of climate-relevant variables and the study of meteorological processes in a mountainous region. The scientific topics covered by P2OA include surface–atmosphere interactions in heterogeneous landscapes and complex terrain, the physics and chemistry of atmospheric trace species at a large scale, the influence of local- and regional-scale emissions and transport on the atmospheric composition, and transient luminous events above thunderstorms. With a large number of instruments and a high hosting capacity, P2OA contributes to atmospheric sciences through (i) building long-term series of atmospheric observations, (ii) hosting experimental field campaigns and instrumental tests, and (iii) educational training in atmospheric observation techniques. In this context, P2OA is part of the French component of the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS-Fr) and also contributes to the Integrated Carbon Observation System (ICOS) research infrastructure and to several European or international networks. Here, we present the complete instrumentation of P2OA and the associated datasets, give a meteorological characterization of the platform, and illustrate the potential of P2OA and its dataset with past or ongoing studies and projects. [ABSTRACT FROM AUTHOR]

Black-carbon-containing particles strongly absorb light, causing substantial radiative heating of the atmosphere. The climate-relevant properties of black carbon (BC) are poorly constrained in high-altitude mountain regions, where many complex interactions between BC, radiation, clouds and snow have important climate implications. This study presents 2-year measurements of BC microphysical and optical properties at the Pic du Midi (PDM) research station, a high-altitude observatory located at 2877 m above sea level in the French Pyrenees. Among the long-term monitoring sites in the world, PDM is subject to limited influence from the planetary boundary layer (PBL), making it a suitable site for characterizing the BC in the free troposphere (FT). The classification of the dominant aerosol type using aerosol spectral optical properties indicates that BC is the predominant aerosol absorption component at PDM and controls the variation in single-scattering albedo (SSA) throughout the 2 years. Single-particle soot photometer (SP2) measurements of refractory BC (rBC) show a mean mass concentration (MrBC) of 35 ngm-3 and a relatively constant rBC core mass-equivalent diameter of about 180 nm, which are typical values for remote mountain sites. Combining the MrBC with in situ absorption measurements, a rBC mass absorption cross-section (MAC rBC) of 9.2 ± 3.7 m2g-1 at λ=880 nm has been obtained, which corresponds to an absorption enhancement (Eabs) of ∼2.2 compared to that of bare rBC particles with equal rBC core size distribution. A significant reduction in the ΔMrBC/ΔCO ratio when precipitation occurred along the air mass transport suggests wet removal of rBC. However we found that the wet removal process did not affect the rBC size, resulting in unchanged Eabs. We observed a large seasonal contrast in rBC properties with higher MrBC and Eabs in summer than in winter. In winter a high diurnal variability in MrBC (Eabs) with higher (lower) values in the middle of the day was linked to the injection of rBC originating from the PBL. On the contrary, in summer, MrBC showed no diurnal variation despite more frequent PBL conditions, implying that MrBC fluctuations are rather dominated by regional and long-range transport in the FT. Combining the ΔMrBC/ΔCO ratio with air mass transport analysis, we observed additional sources from biomass burning in summer leading to an increase in MrBC and Eabs. The diurnal pattern of Eabs in summer was opposite to that observed in winter with maximum values of ∼2.9 observed at midday. We suggest that this daily variation may result from a photochemical process driving the rBC mixing state rather than a change in BC emission sources. Such direct 2-year observations of BC properties provide quantitative constraints for both regional and global climate models and have the potential to close the gap between model-predicted and observed effects of BC on the regional radiation budget and climate. The results demonstrate the complex influence of BC emission sources, transport pathways, atmospheric dynamics and chemical reactivity in driving the light absorption of BC. [ABSTRACT FROM AUTHOR]

At high-altitude stations worldwide, atmospheric composition measurements aim to represent the free troposphere and intercontinental scale. The high-altitude environment favours local and regional air mass transport, impacting the sampled air composition. Processes like mixing, source–receptor pathways, and chemistry rely on local and regional weather patterns, necessitating station-specific characterization. The Pic du Midi (PDM) is a mountaintop observatory at 2850 m above sea level in the Pyrenees. The PDM and the Centre de Recherches Atmosphériques (CRA) in the foothills form the Pyrenean Platform for the Observation of the Atmosphere (P2OA). This study aimed to identify recurring weather patterns at P2OA and relate them to the PDM's atmospheric composition. We combined 5 years of data from PDM and CRA, including 23 meteorological variables (temperature, humidity, cloud cover, and wind at different altitudes). We used hierarchical clustering to classify the data set into six clusters. Three of the clusters represented common weather conditions (fair, mixed, disturbed weather), one highlighted winter north-westerly windstorms, and the last two denoted south foehn conditions. Additional diagnostic tools allowed us to study specific phenomena such as foehns and thermally driven circulations and to affirm our understanding of the clusters. We then analysed the PDM's atmospheric composition statistics for each cluster. Notably, radon measurements indicated a regional background dominance in the lower troposphere, overshadowing diurnal thermal effects. Cluster differences emerged for the anomalies in CO, CO 2 , CH 4 , O 3 , and aerosol concentrations, and we propose interpretations in relation to chemical sources and sinks. [ABSTRACT FROM AUTHOR]

Black Carbon containing particles (BC) are strong light absorbers, causing substantial radiative heating of the atmosphere. The climate-relevant properties of BC are poorly constrained in high-elevation mountain regions, where numerous complex interactions between BC, radiation, clouds and snow have important climate implications. This study presents twoyear measurements of BC microphysical and optical properties at the research station of Pic du Midi (PDM), a high-altitude observatory located at 2877 m above sea level in the French Pyrenees. Among the worldwide existing long-term monitoring sites, PDM has experiences limited influence of the planetary boundary layer (PBL), making it an appropriate site for characterizing free tropospheric (FT) BC. The classification of the dominant aerosol type using the spectral optical properties of the aerosols indicates that BC was the predominant absorption component of aerosols at PDM and controlled the variation of Single Scattering Albedo (SSA) throughout the two years. Single-particle soot photometer (SP2) measurements showed a mean mass concentrations of BC (MBC) of 35 ng m-3 and a relatively constant BC core mass-equivalent diameter of around 180 nm, which are typical values for remote mountain sites. Combining the MBC with in situ absorption measurements yielded BC mass absorption coefficient (MACBC) of 9.8 ± 2.7 m² g-1 at 880 nm, which corresponds to an absorption enhancement Eabs) of 2.4 compared to that of bare BC particles with equal BC core size distribution. A significant reduction of the ratio ΔBC/ΔCO when precipitation occurred along the air mass transport suggests wet removal of BC. However we found that the wet removal process did not affect the size of BC, resulting in unchanged Eabs. We observed a large seasonal contrast in BC properties with higher MBC and Eabs in summer than winter. In winter a strong diurnal variability of MBC (Eabs) with higher lower) values in the middle of the day was linked to the injection of BC originating from the PBL. During summer in contrast, MBC showed no diurnal variation was rather constant despite more frequent PBL-conditions, implying that MBC fluctuations were rather dominated by regional and long-range transport in the FT. A body of evidence suggests that biomass burning emissions effectively altered the concentration and optical properties of BC at PDM, leading to higher Eabs in summer compared to winter. The diurnal pattern of Eabs in summer was opposite to that observed in winter with maximum values of 2.9 observed at noon. We suggest that this daily variation results from photochemical processing driving BC mixing state rather than a change in BC emission source. Such direct two-year observations of BC properties provide quantitative constraints for both regional and global climate models and have the potential to close the gap between model predicted and observed effects of BC on regional radiation budget and climate. The results demonstrates the complex influence of BC emission sources, transport pathways, atmospheric dynamics and chemical reactivity in driving the light absorption of BC. [ABSTRACT FROM AUTHOR]