Abstract. For the last 25 years, CO-PDD (Cézeaux-Aulnat-Opme-puy de Dôme) has evolved to become a full instrumented platform for atmospheric research. It has received credentials as a national observing platform in France and is internationally recognized as a global station in the GAW (Global Atmosphere Watch) network. It is a reference site of European and national research infrastructures ACTRIS (Aerosol Cloud and Trace gases Research Infrastructure) and ICOS (Integrated Carbon Observing System). The site located on top of the puy de Dôme mountain (1465 m a.s.l.) is completed by additional sites located at lower altitudes and adding the vertical dimension to the atmospheric observations: Opme (660 m a.s.l.), Cézeaux (410 m), and Aulnat (330 m). The integration of different sites offers a unique combination of in situ and remote sensing measurements capturing and documenting the variability of particulate and gaseous atmospheric composition, but also the optical, biochemical, and physical properties of aerosol particles, clouds, and precipitations. Given its location far away from any major emission sources, its altitude, and the mountain orography, the puy de Dôme station is ideally located to sample different air masses in the boundary layer or in the free troposphere depending on time of day and seasons. It is also an ideal place to study cloud properties with frequent presence of clouds at the top in fall and winter. As a result of the natural conditions prevailing at the site and of the very exhaustive instrumental deployment, scientific studies at the puy de Dôme strongly contribute to improving knowledge in atmospheric sciences, including the characterization of trends and variability, the understanding of complex and interconnected processes (microphysical, chemical, biological, chemical and dynamical), and the provision of reference information for climate/chemistry models. In this context, CO-PDD is a pilot site to conduct instrumental development inside its wind tunnel for testing liquid and ice cloud probes in natural conditions, or in situ systems to collect aerosol and cloud. This paper reviews 25 years (1995–2020) of atmospheric observation at the station and related scientific research contributing to atmospheric and climate science.

Abstract. The present study offers the first chemical characterization of the submicron (PM 1 ) fraction in western Africa at a high time resolution, thanks to collocated measurements of nonrefractory (NR) species with an Aerosol Chemical Speciation Monitor (ACSM), black carbon and iron concentrations derived from absorption coefficient measurements with a 7-wavelength Aethalometer, and total PM 1 determined by a TEOM-FDMS (tapered element oscillating microbalance-filtered dynamic measurement system) for mass closure. The field campaign was carried out over 3 months (March to June 2015) as part of the SHADOW (SaHAran Dust Over West Africa) project at a coastal site located in the outskirts of the city of Mbour, Senegal. With an averaged mass concentration of 5.4 µg m −3 , levels of NR PM 1 in Mbour were 3 to 10 times lower than those generally measured in urban and suburban polluted environments. Nonetheless the first half of the observation period was marked by intense but short pollution events (NR PM 1 concentrations higher than 15 µg m −3 ), sea breeze phenomena and Saharan desert dust outbreaks (PM 10 up to 900 µg m −3 ). During the second half of the campaign, the sampling site was mainly under the influence of marine air masses. The air masses on days under continental and sea breeze influences were dominated by organics (36-40 %), whereas sulfate particles were predominant (40 %) for days under oceanic influence. Overall, measurements showed that about threequarters of the total PM 1 were explained by NR PM 1 , BC (black carbon) and Fe (a proxy for dust) concentrations, leaving approximately one-quarter for other refractory species. A mean value of 4.6 % for the Fe / PM 1 ratio was obtained. Source apportionment of the organic fraction, using positive matrix factorization (PMF), highlighted the impact of local combustion sources, such as traffic and residential activities, which contribute on average to 52 % of the total organic fraction. A new organic aerosol (OA) source, representing on average 3 % of the total OA fraction, showed similar variation to nonrefractory particulate chloride. Its rose plot and daily pattern pointed to local combustion processes, i.e., two open waste-burning areas located about 6 and 11 km away from the receptor site and to a lesser extent a traditional fish-smoking location. The remaining fraction was identified as oxygenated organic aerosols (OOA), a factor that prevailed regardless of the day type (45 %) and was representative of regional (approximately three-quarters) but also local (approximately one-quarter) sources due to enhanced photochemical processes.

Abstract. Volcanic sulfate aerosols play a key role in air quality and climate. However, the rate of oxidation of sulfur dioxide (SO2) precursor gas to sulfate aerosols (SO42-) in volcanic plumes is poorly known, especially in the troposphere. Here we determine the chemical speciation as well as the intensity and temporal persistence of the impact on air quality of sulfate aerosols from the 2014–2015 Holuhraun flood lava eruption of Icelandic volcano Bárðarbunga. To do so, we jointly analyse a set of SO2 observations from satellite (OMPS and IASI) and ground-level measurements from air quality monitoring stations together with high temporal resolution mass spectrometry measurements of an Aerosol Chemical Speciation Monitor (ACSM) performed far from the volcanic source. We explore month/year long ACSM data in France from stations in contrasting environments, close and far from industrial sulfur-rich activities. We demonstrate that volcanic sulfate aerosols exhibit a distinct chemical signature in urban/rural conditions, with NO3:SO4 mass concentration ratios lower than for non-volcanic background aerosols. These results are supported by thermodynamic simulations of aerosol composition, using the ISORROPIA II model, which show that ammonium sulfate aerosols are preferentially formed at a high concentration of sulfate, leading to a decrease in the production of particulate ammonium nitrate. Such a chemical signature is however more difficult to identify at heavily polluted industrial sites due to a high level of background noise in sulfur. Nevertheless, aged volcanic sulfates can be distinguished from freshly emitted industrial sulfates according to their contrasting degree of anion neutralization. Combining AERONET (AErosol RObotic NETwork) sunphotometric data with ACSM observations, we also show a long persistence over weeks of pollution in volcanic sulfate aerosols, while SO2 pollution disappears in a few days at most. Finally, gathering 6-month long datasets from 27 sulfur monitoring stations of the EMEP (European Monitoring and Evaluation Programme) network allows us to demonstrate a much broader large-scale European pollution, in both SO2 and SO4, associated with the Holuhraun eruption, from Scandinavia to France. While widespread SO2 anomalies, with ground-level mass concentrations far exceeding background values, almost entirely result from the volcanic source, the origin of sulfate aerosols is more complex. Using a multi-site concentration-weighted trajectory analysis, emissions from the Holuhraun eruption are shown to be one of the main sources of SO4 at all EMEP sites across Europe and can be distinguished from anthropogenic emissions from eastern Europe but also from Great Britain. A wide variability in SO2:SO4 mass concentration ratios, ranging from 0.8 to 8.0, is shown at several stations geographically dispersed at thousands of kilometres from the eruption site. Despite this apparent spatial complexity, we demonstrate that these mass oxidation ratios can be explained by a simple linear dependency on the age of the plume, with a SO2-to-SO4 oxidation rate of 0.23 h−1. Most current studies generally focus on SO2, an unambiguous and more readily measured marker of the volcanic plume. However, the long persistence of the chemical fingerprint of volcanic sulfate aerosols at continental scale, as shown for the Holuhraun eruption here, casts light on the impact of tropospheric eruptions and passive degassing activities on air quality, health, atmospheric chemistry and climate.

A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during wintertime (5 February to 15 March 2016) at a suburban site in Douai, northern France, in order to investigate the characteristics and sources of the organic matter (OM). The campaign average concentration of non-refractory submicron particulate matter (NR-PM 1 ) was 11.1 ± 9.3 µg m -3 , and composed of 38% OM, 36% nitrate, 16% ammonium and 9% sulfate. The average values for the OM:OC, O:C and H:C ratios were 1.60 ± 0.15, 0.32 ± 0.11 and 1.55 ± 0.14, respectively, indicating a moderate level of aerosol oxidation. The positive matrix factorization (PMF) source apportionment method was applied to the high-resolution organic aerosol (OA) mass spectra, resulting in four factors: a hydrocarbon-like (HOA) factor; one associated with oxidized biomass burning (oBBOA); and two oxygenated factors (OOA) denoted as less oxidized (LO-OOA) and more oxidized (MO-OOA), with average contributions to OA of 20%, 28%, 17% and 35%, respectively. The oBBOA factor was found to be mainly local as shown by non-parametric wind regression (NWR) analysis, and to correlate well with relative humidity (RH), suggesting fast aqueous processing of locally emitted primary biomass burning emissions. During most part of the campaign, the sampling site was affected by different air masses. However, during the last period of the campaign (5-16 March 2016) the site was heavily impacted by air masses from Eastern Europe which were rich in secondary inorganic and organic aerosols. The H:C versus O:C (Van Krevelen, VK) diagram highlighted that the aerosol followed an oxidation process throughout the whole campaign, with an average slope of -1.05. The impact of continental air masses towards the end of the campaign confined the aerosol towards a narrower space in the VK diagram, suggesting a homogenization of the different aerosol sources due to OA ageing during transport. Several nocturnal NPF (new particle formation)-like events were observed and associated to the fast processing of BBOA emissions and the formation of ammonium nitrate.