Nucléation et formation de nouvelles particules à haute altitude

New particle formation (NPF) results from a complex sequence of multiple processes and contributes to an important fraction of the total atmospheric aerosol number concentration. After they grow, newly formed particles can act as cloud condensation nuclei (CCN), and thus have indirect effect on the Earth radiative balance through cloud related radiative processes. While NPF has often been observed and studied at low altitudes, the occurrence of the process is poorly documented in the literature for high altitude sites. We report a high annual frequency of the NPF process (64%) at the highest measurement site in the world, Chacaltaya (5240 m, Bolivia), in 2012, with frequent multiple events. At the puy de Dôme station (1465 m, ACTRIS, GAW), the occurrence of NPF in the free troposphere was detected using a unique instrumental setup. A complete analysis of the vertical extension of the NPF process was performed based on airborne measurements conducted above the Mediterranean basin in the frame of the HYMEX project (MISTRALS, September – November 2012). Our observations suggest that NPF could be favored at high altitudes with a probability of occurrence increased by 10 above 1000 m. At these altitudes, NPF could significantly contribute to the production of CCN, since 68% of the analyzed events show particle growth up to CCN sizes at Chacaltaya. The high number of observations recorded in various environments also contributed to improve our knowledge regarding the charge of the nucleated clusters, the identity of the gaseous precursors and the atmospheric parameters influencing the NPF process. This will allow a better parameterization of the NPF process in modelling tools.La formation de nouvelles particules est un processus complexe à l’origine d’une fraction importante des concentrations en nombre de particules observées dans l’atmosphère. En jouant le rôle de noyau de condensation (CCN) pour la formation des gouttelettes de nuage, les particules issues de ce processus impactent le bilan radiatif terrestre. Fréquemment observée et documentée à basse altitude, la formation de nouvelles particules a plus rarement fait l’objet d’études à haute altitude. L’analyse des données obtenues en 2012 à la plus haute station du monde, Chacaltaya (5240 m, Bolivie) révèle une fréquence d’observation annuelle du processus remarquablement élevée (64%), avec de nombreux évènements multiples. Les mesures conduites à la station du puy de Dôme (1465 m, ACTRIS, GAW) qui bénéficie d’un dispositif instrumental rare ont plus particulièrement permis de mettre en évidence le déroulement du processus en troposphère libre. Une analyse complète de l’extension verticale du processus rendue possible grâce aux données aéroportées obtenues au-dessus du bassin Méditerranéen dans le cadre du projet HYMEX (MISTRALS, automne 2012) a montré qu’en plus d’être observé à haute altitude, le processus de formation de nouvelles particules semblait y être clairement favorisé, avec une probabilité d’observation multipliée par 10 au-dessus de 1000 m. De plus, à ces altitudes le processus de formation de nouvelles particules pourrait être une source importante de CCN, comme le suggèrent les résultats obtenus à Chacaltaya, où dans 68% des évènements analysés les particules formées atteignent des diamètres suffisants pour jouer le rôle de CCN. La diversité des environnements associés aux bases de données utilisées a également permis d’apporter des éléments relatifs à la compréhension du processus du point de vue de la charge des embryons formés, de l’identité des précurseurs gazeux impliqués et des paramètres atmosphériques influençant le processus. Ces éléments sont déterminants pour une prise en compte optimale du processus de formation de nouvelles particules dans les modèles

In vitro toxicity and photodynamic properties of porphyrinoids bearing imidazolium salts and N-heterocyclic carbene gold(I) complexes

Porphyrins bearing imidazolium salts were synthesized and used as N-heterocylic carbene (NHC) precursors for the preparation of gold(I) complexes. The dark toxicity and phototoxicity of the obtained compounds were investigated in vitro on MCF-7 breast cancer cells. The obtained data showed that porphyrins equipped with imidazolium salts are non-toxic in the dark and present interesting photodynamic properties. On the contrary, corresponding NHC-gold(I) complexes are not suitable photosensitizers for photodynamic therapy (PDT) applications. Their dark toxicity strongly depends on the nature of the linker between the porphyrin core and the NHC. This work was extended to the synthesis of a pyropheophorbide a derivative with a pendant imidazolium group for PDT applications using excitation wavelengths of 450 nm, 545 nm, and importantly of 650 nm

Quantified effect of seawater biogeochemistry on the temperature dependence of sea spray aerosol fluxes

Future change in sea surface temperature may influence climate via various air-sea feedbacks and pathways. In this study, we investigate the influence of surface seawater biogeochemical composition on the temperature dependence of sea spray number emission fluxes. Dependence of sea spray fluxes was investigated in different water masses (i.e. subantarctic, subtropical and frontal bloom) with contrasting biogeochemical properties across a temperature range from ambient (13&ndash;18 &deg;C) to 2 &deg;C, using seawater circulating in a plunging jet sea spray generator. We observed sea spray total concentration to increase significantly at temperatures below 8 &deg;C, with an average 4-fold increase at 2 &deg;C relative to initial concentration at ambient temperatures. This temperature dependence was more pronounced for smaller size sea spray particles (i.e. nucleation and Aitken modes). Moreover, temperature dependence varied with water mass type and so biogeochemical properties. While the sea spray flux at moderate temperatures (8&ndash;11 &deg;C) was highest in frontal bloom waters, the effect of low temperature on the sea spray flux was highest with subtropical seawaters. The temperature dependence of sea spray flux was also inversely proportional to the seawater cell abundance of the cyanobacterium Synechococcus, which facilitated parameterization of temperature dependence of sea spray emission fluxes as a function of Synechococcus for future implementation in modelling exercises.</p

Observations of biogenic ion-induced cluster formation in the atmosphere

A substantial fraction of aerosols, which affect air quality and climate, is formed from gaseous precursors. Highly oxygenated organic molecules (HOMs) are essential to grow the newly formed particles and have been evidenced to initiate ion-induced nucleation in chamber experiments in the absence of sulfuric acid. We investigate this phenomenon in the real atmosphere using an extensive set of state-of-the-art ion and mass spectrometers deployed in a boreal forest environment. We show that within a few hours around sunset, HOMs resulting from the oxidation of monoterpenes are capable of forming and growing ion clusters even under low sulfuric acid levels. In these conditions, we hypothesize that the lack of photochemistry and essential vapors prevents the organic clusters from growing past 6 nm. However, this phenomenon might have been a major source of particles in the preindustrial atmosphere and might also contribute to particle formation in the future and consequently affect the climate.Peer reviewe

Evidence of nitrate-based nighttime atmospheric nucleation driven by marine microorganisms in the South Pacific

9 pags. 3 figs.Our understanding of ocean-cloud interactions and their effect on climate lacks insight into a key pathway: do biogenic marine emissions form new particles in the open ocean atmosphere? Using measurements collected in ship-borne air-sea interface tanks deployed in the Southwestern Pacific Ocean, we identified new particle formation (NPF) during nighttime that was related to plankton community composition. We show that nitrate ions are the only species for which abundance could support NPF rates in our semicontrolled experiments. Nitrate ions also prevailed in the natural pristine marine atmosphere and were elevated under higher sub-10 nm particle concentrations. We hypothesize that these nucleation events were fueled by complex, short-term biogeochemical cycling involving the microbial loop. These findings suggest a new perspective with a previously unidentified role of nitrate of marine biogeochemical origin in aerosol nucleation.We acknowledge the support and expertise of the Officers and Crew of the R/V Tangaroa and National Institute of Water and Atmospheric Research (NIWA) Vessel Services. This research received funding from the European Research Council (ERC) under the Horizon 2020 research and innovation program (Sea2Cloud grant agreement number - 771369 and grant agreement number 101002728) and was supported by NIWA SSIF funding to the Ocean-Climate Interactions, and Flows and Productivity Programs. The support from the Academy of Finland (331207) and the German Academic Exchange Service (DAAD) is greatly appreciated.Peer reviewe

Differing mechanisms of new particle formation at two Arctic sites.

New particle formation in the Arctic atmosphere is an important source of aerosol particles. Understanding the processes of Arctic secondary aerosol formation is crucial due to their significant impact on cloud properties and therefore Arctic amplification. We observed the molecular formation of new particles from low-volatility vapors at two Arctic sites with differing surroundings. In Svalbard, sulfuric acid (SA) and methane sulfonic acid (MSA) contribute to the formation of secondary aerosol and to some extent to cloud condensation nuclei (CCN). This occurs via ion-induced nucleation of SA and NH3 and subsequent growth by mainly SA and MSA condensation during springtime and highly oxygenated organic molecules during summertime. By contrast, in an ice-covered region around Villum, we observed new particle formation driven by iodic acid but its concentration was insufficient to grow nucleated particles to CCN sizes. Our results provide new insight about sources and precursors of Arctic secondary aerosol particles.Peer reviewe

Seasonality of the particle number concentration and size distribution : a global analysis retrieved from the network of Global Atmosphere Watch (GAW) near-surface observatories

Aerosol particles are a complex component of the atmospheric system which influence climate directly by interacting with solar radiation, and indirectly by contributing to cloud formation. The variety of their sources, as well as the multiple transformations they may undergo during their transport (including wet and dry deposition), result in significant spatial and temporal variability of their properties. Documenting this variability is essential to provide a proper representation of aerosols and cloud condensation nuclei (CCN) in climate models. Using measurements conducted in 2016 or 2017 at 62 ground-based stations around the world, this study provides the most up-to-date picture of the spatial distribution of particle number concentration (N-tot) and number size distribution (PNSD, from 39 sites). A sensitivity study was first performed to assess the impact of data availability on N-tot's annual and seasonal statistics, as well as on the analysis of its diel cycle. Thresholds of 50% and 60% were set at the seasonal and annual scale, respectively, for the study of the corresponding statistics, and a slightly higher coverage (75 %) was required to document the diel cycle. Although some observations are common to a majority of sites, the variety of environments characterizing these stations made it possible to highlight contrasting findings, which, among other factors, seem to be significantly related to the level of anthropogenic influence. The concentrations measured at polar sites are the lowest (similar to 10(2) cm(-3)) and show a clear seasonality, which is also visible in the shape of the PNSD, while diel cycles are in general less evident, due notably to the absence of a regular day-night cycle in some seasons. In contrast, the concentrations characteristic of urban environments are the highest (similar to 10(3)-10(4) cm(-3)) and do not show pronounced seasonal variations, whereas diel cycles tend to be very regular over the year at these stations. The remaining sites, including mountain and non-urban continental and coastal stations, do not exhibit as obvious common behaviour as polar and urban sites and display, on average, intermediate N-tot (similar to 10(2)-10(3) cm(-3)). Particle concentrations measured at mountain sites, however, are generally lower compared to nearby lowland sites, and tend to exhibit somewhat more pronounced seasonal variations as a likely result of the strong impact of the atmospheric boundary layer (ABL) influence in connection with the topography of the sites. ABL dynamics also likely contribute to the diel cycle of N-tot observed at these stations. Based on available PNSD measurements, CCN-sized particles (considered here as either >50 nm or >100 nm) can represent from a few percent to almost all of N-tot, corresponding to seasonal medians on the order of similar to 10 to 1000 cm(-3), with seasonal patterns and a hierarchy of the site types broadly similar to those observed for N-tot. Overall, this work illustrates the importance of in situ measurements, in particular for the study of aerosol physical properties, and thus strongly supports the development of a broad global network of near surface observatories to increase and homogenize the spatial coverage of the measurements, and guarantee as well data availability and quality. The results of this study also provide a valuable, freely available and easy to use support for model comparison and validation, with the ultimate goal of contributing to improvement of the representation of aerosol-cloud interactions in models, and, therefore, of the evaluation of the impact of aerosol particles on climate.Peer reviewe

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Nucleation and new particle formation at high altitudes

La formation de nouvelles particules est un processus complexe à l’origine d’une fraction importante des concentrations en nombre de particules observées dans l’atmosphère. En jouant le rôle de noyau de condensation (CCN) pour la formation des gouttelettes de nuage, les particules issues de ce processus impactent le bilan radiatif terrestre. Fréquemment observée et documentée à basse altitude, la formation de nouvelles particules a plus rarement fait l’objet d’études à haute altitude. L’analyse des données obtenues en 2012 à la plus haute station du monde, Chacaltaya (5240 m, Bolivie) révèle une fréquence d’observation annuelle du processus remarquablement élevée (64%), avec de nombreux évènements multiples. Les mesures conduites à la station du puy de Dôme (1465 m, ACTRIS, GAW) qui bénéficie d’un dispositif instrumental rare ont plus particulièrement permis de mettre en évidence le déroulement du processus en troposphère libre. Une analyse complète de l’extension verticale du processus rendue possible grâce aux données aéroportées obtenues au-dessus du bassin Méditerranéen dans le cadre du projet HYMEX (MISTRALS, automne 2012) a montré qu’en plus d’être observé à haute altitude, le processus de formation de nouvelles particules semblait y être clairement favorisé, avec une probabilité d’observation multipliée par 10 au-dessus de 1000 m. De plus, à ces altitudes le processus de formation de nouvelles particules pourrait être une source importante de CCN, comme le suggèrent les résultats obtenus à Chacaltaya, où dans 68% des évènements analysés les particules formées atteignent des diamètres suffisants pour jouer le rôle de CCN. La diversité des environnements associés aux bases de données utilisées a également permis d’apporter des éléments relatifs à la compréhension du processus du point de vue de la charge des embryons formés, de l’identité des précurseurs gazeux impliqués et des paramètres atmosphériques influençant le processus. Ces éléments sont déterminants pour une prise en compte optimale du processus de formation de nouvelles particules dans les modèles.New particle formation (NPF) results from a complex sequence of multiple processes and contributes to an important fraction of the total atmospheric aerosol number concentration. After they grow, newly formed particles can act as cloud condensation nuclei (CCN), and thus have indirect effect on the Earth radiative balance through cloud related radiative processes. While NPF has often been observed and studied at low altitudes, the occurrence of the process is poorly documented in the literature for high altitude sites. We report a high annual frequency of the NPF process (64%) at the highest measurement site in the world, Chacaltaya (5240 m, Bolivia), in 2012, with frequent multiple events. At the puy de Dôme station (1465 m, ACTRIS, GAW), the occurrence of NPF in the free troposphere was detected using a unique instrumental setup. A complete analysis of the vertical extension of the NPF process was performed based on airborne measurements conducted above the Mediterranean basin in the frame of the HYMEX project (MISTRALS, September – November 2012). Our observations suggest that NPF could be favored at high altitudes with a probability of occurrence increased by 10 above 1000 m. At these altitudes, NPF could significantly contribute to the production of CCN, since 68% of the analyzed events show particle growth up to CCN sizes at Chacaltaya. The high number of observations recorded in various environments also contributed to improve our knowledge regarding the charge of the nucleated clusters, the identity of the gaseous precursors and the atmospheric parameters influencing the NPF process. This will allow a better parameterization of the NPF process in modelling tools