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For the last twenty-five years, CO-PDD (Cézeaux-Aulnat-Opme-puy de Dôme) has evolved to become a full instrumented platform for atmospheric research. It is nationally accredited by CNRS, the French national center for scientific research, and recognized as a global station in the GAW network (Global Atmospheric Watch). It is a reference site of the 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). On the sites has been developed 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 situated 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 autumn and winter. As a result of the natural conditions prevailing at the site and of the very exhaustive instrumental deployment, scientific studies at puy de Dôme strongly contribute to improving the 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 providing a reference point for climate models. In this context, CO-PDD is a pilot site to conduct instrumental development inside its wind tunnel for testing liquid/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.

New particle formation (NPF) is a key atmospheric process which may be responsible for a major fraction of the total aerosol number burden at the global scale, including in particular cloud condensation nuclei (CCN). NPF has been observed in various environments around the world, but some specific conditions, such as those encountered in volcanic plumes, remain poorly documented in the literature. Yet, understanding such natural processes is essential to better define preindustrial conditions in climate model simulations, as those form the baseline to calculate the radiative forcing caused by anthropogenic emissions. Here we report observations of NPF performed at the high-altitude observatory of Maïdo (2165 m a.s.l., La Réunion Island) between 1st January and 31st December 2015. During this time period, 3 effusive eruptions of the Piton de la Fournaise, located ~ 39 km away from the station, were observed and documented, resulting in 36 days of measurement in volcanic plume conditions to be compared with 250 non-plume days. This dataset is, to our knowledge, the largest ever reported for the investigation of NPF in tropospheric volcanic plume conditions, and allowed for the first time a statistical approach to characterize the process and also assess its relevance with respect to non-plume conditions. NPF was observed on 86 % of the plume days vs 71 % of the non-plume days during the 4 months when the eruptions occurred. The events were on average detected earlier on plume days, most likely benefiting from larger amounts of precursors available at the site prior to nucleation hours compared to non-plume days, during which condensable species were in contrast transported from lower altitude by the mean of convective processes. Surprisingly, the overall effect of the plume conditions on the particle growth rate was limited. However, with the exception of September, particle formation rates were significantly higher on plume days. The signature of the volcanic plume on the aerosol spectra up to dp = 600 nm was further investigated based on the analysis and fitting of the particle size distributions recorded in the different conditions. The spectra recorded prior to nucleation hours, in absence of freshly formed particles, featured a significant contribution of particles likely formed via heterogeneous processes at the vent of the volcano (and assimilated to volcanic primary particles) to the concentrations of the 2 accumulation modes on plume days. Later on in the morning, the concentrations of the nucleation and Aitken modes showed important variations on plume days compared to event days outside of plume conditions. The spectra recorded on event days, in and off-plume conditions, were further used to provide an average size distribution of the particles of volcanic origin, which clearly highlighted the dominant contribution of secondary over primary particles (96 %) to the total concentration measured on NPF event days within volcanic plume. In a next step, particular attention was paid to the concentration of particles with dp > 50 nm (N50), used as a proxy for potential CCN population. The contribution of secondary particles to the increase of N50 was the most frequent in plume conditions, and the magnitude of the increase was also more important on plume days compared to non-plume days. Last, in order to further evaluate the effect of volcanic plume conditions on the occurrence of NPF, we analysed the variations of the condensation sink (CS) and [H2SO4], previously reported to play a key role in the process. Over the investigated months, higher CS (calculated prior to nucleation hours) were observed in plume conditions, and coincided with high SO2 mixing ratios. Those most likely compensated for the strengthened loss rate of the vapour and favoured the occurrence of NPF, suggesting at the same time a key role of H2SO4 in the process. This last hypothesis was further supported by the correlation between the formation rate of 2 nm particles (J2) and [H2SO4], and by the fair approximation of J2 that was obtained by the mean of a recent parameterisation of the binary nucleation of H2SO4 – H2O. This last result was of high interest as it also demonstrated that in absence of direct measurement of [H2SO4] and sub-2nm particles concentration, estimates of J2 could be obtained from the knowledge of SO2 mixing ratios only.

While primary marine aerosol (PMA) is an important part of global aerosol total emissions, its chemical composition and physical flux as a function of the biogeochemical properties of the seawater still remain highly uncharacterized due to the multiplicity of physical, chemical and biological parameters that are involved in the emission process. Here, nutrient enriched-mesocosms filled with Mediterranean seawater were studied over a three-week period. PMA generated from the mesocosm waters were characterized in term of chemical composition, size distribution and size segregated cloud condensation nuclei (CCN), as a function of the seawater chlorophyll-a (Chl-a) concentration, pigment composition, virus and bacteria abundances. In all mesocosms (enriched and control mesocosms), we detected an enrichment of calcium and a deficit in chloride in the submicron PMA mass compared to the literature inorganic composition of the seawater. A positive linear correlation was found between the aerosol number concentration flux and the seawater temperature, that could be specific to the mediteranean seawater temperature and organic content. We found that the artificial phytoplankton bloom did not affect the condensation nuclei (CN) or CCN number concentration, the normalized size distribution, the CCN activation diameter or the organic fraction of the PMA compared to the control mesocosm. However, we observed correlations between the organic fraction of the Aitken mode particles and heterotrophic flagellates, viruses, and dissolved organic carbon (DOC). No correlation between the particle organic fraction and Chl-a was observed, contrary to previous observations in natural bloom mesocosm experiments. We believe that this could be due to (1) different complexities compared to natural bloom systems, or (2) longer time periods needed to observe correlation trends.

The scientific need to understand nanoparticle dynamics at sizes below 3 nm has pushed companies to develop commercial solutions to measure particles down to 1 nm. In this study we characterize the performance of three new particle counters able to detect particles smaller than 3 nm: Aerosol Dynamics Inc versatile water condensation particle counter (v-WCPC, ADI, Berkeley, USA), TSI 3777 nano enhancer (TSI Inc., Shoreview, USA) and modified and boosted 3010 type CPC from Clermont Ferrand University called as B3010. The 3777 and v-WCPC were characterized using tungsten oxide test particles with all charging states: negative, positive and neutral, and with positively charged tetradodecylammonium bromide. The detection efficiencies of the particle counters were measured with two different temperature settings: low temperature difference settings so that the CPCs did not detect any ions from a radioactive source; and high temperature difference settings so that the supersaturation was at the onset of homogeneous nucleation for the 3777, or confined within the range of liquid water for the ADI v-WCPC. The measured 50 % detection diameters (d50) were in the range of 1.3–2.4 nm for the tungsten oxide particles depending on the particle charging state and CPC temperature settings, and between 2.5 and 3.3 nm for the organic test aerosol for the 3777 and v-WCPC. The d50s were measured for the B3010 with negatively charged tungsten oxide particles with four different inlet flow rates. The v-WCPC and 3777 were also compared side by side by measuring atmospheric aerosol, exhibiting an excellent agreement.