Biodegradation is competitive with photochemistry, especially in summer and at night. Formic acid is the most impacted compound.

Iron (Fe) plays a dual role in atmospheric chemistry: it is involved in chemical and photochemical reactivity and serves as a micronutrient for microorganisms that have recently been shown to produce strong organic ligands. These ligands control the reactivity, mobility, solubility and speciation of Fe, which have a potential impact on Fe bioavailability and cloud water oxidant capacity. In this work, the concentrations of Fe-binding ligands and the conditional stability constants were experimentally measured for the first time by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) technique in cloud water samples collected at puy de Dôme (France). The conditional stability constants, which indicate the strength of the Fe-ligand complexes, are higher than those considered until now in cloud chemistry (mainly Fe-oxalate). To understand the effect of Fe complexation on cloud water reactivity, we used the CLEPS cloud chemistry model. According to the model results, we found that Fe complexation impacts the hydroxyl radical formation rate: contrary to our expectations, Fe complexation by natural organic ligands led to an increase in hydroxyl radical production. These findings have important impacts on cloud chemistry and the global iron cycle.

Chemical processes in clouds and fogs can substantially alter atmospheric oxidant budgets and lead to aerosol mass formation. However, many regional and global models do not include detailed aqueous‐phase chemical mechanisms due to the (a) lack of complete understanding of the chemical processes and (b) computational burden of adding constituents. Current gas‐aqueous chemistry 0‐dimensional models were evaluated in a cloud‐chemistry box model intercomparison based on a mid‐September 2016 cloud chemistry event at Whiteface Mountain, New York. Multiphase mechanisms in the five participating models ranged from those appropriate for 3‐d models to highly complex with thousands of reactions. This study focused on oxidant levels in both phases and aqueous‐phase sulfate and organic acid formation. Comparison of gas‐phase‐only chemistry gives very similar oxidant predictions at night but shows significant differences during daytime with the hydroxyl radical (OH) variability of about an order of magnitude. The variability in the model results increases substantially with aqueous chemistry due to different Henry's Law constants, aqueous‐phase reaction rate constants, and chemical mechanisms. Using a prescribed liquid water content and pH value of 4.5, modeled aqueous OH, aldehyde, and organic acid concentrations differ by over an order of magnitude in daytime. Simulations were also conducted at a pH = 5.1, predicted variable pH, and with added transition metal ion chemistry. While we compare predicted and measured inorganic anions and water‐soluble organic carbon, we cannot do so for aqueous‐phase oxidant concentrations due to the lack of measurements. We highlight a need for recommended equilibrium and aqueous‐phase rate constants.

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.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Microorganisms are present in the atmosphere and can survive in cloud droplets. They are able to transform organic compounds and can consequently compete with radical chemistry. Because the cloud system is a complex multiphase medium, the efficiency of biodegradation by cloud microorganisms has to be evaluated by numerical models of different complexity simulating multiphase chemical processes in clouds. However, only abiotic processes are taken into account in these numerical tools. The objective of this work was thus to integrate biological data in an atmospheric cloud chemistry model and to evaluate the effect of microorganisms in the transformation of chemical compounds. For this, experimental biodegradation rates of acetic and formic acids, formaldehyde and hydrogen peroxide by various bacterial representative of cloud biodiversity were experimentally measured. These values have been implemented in a cloud chemistry model describing aqueous phase chemistry with the explicit CLEPS (Cloud Explicit Physico-chemical Scheme) mechanism. Several simulations with and without biodegradation processes have been performed changing temperature (5 °C or 17 ℃) and actinic flux to simulate summer or winter conditions. The chemical scenario (gas concentrations and emission/deposition) is representative of low-NOx emission with significant isoprene emissions.

Secondary organic aerosols (SOA) produced in the atmosphere from sequential oxidation of biogenic or anthropogenic volatile organic compounds (VOC) represent a significant part of the atmospheric organic aerosol. Aerosol particles can act as cloud condensation nuclei or can be scavenged by cloud droplets, where organic carbon undergo chemical and biological transformations. Due to its high complexity, a nontargeted analysis by FT-ICR MS (Fourier-transform ion cyclotron resonance mass spectrometry) has been used to characterize the dissolved organic matter at a molecular level. SOA compounds have been detected in six aqueous samples collected in 2017 at the Puy de Dome station (PUY, France). SOA tracers, produced by VOC oxidation in environmental chambers or in ambient air and characterized in previous studies by mass spectrometry, were searched by FT-ICR MS in cloudwaters. Results clearly indicate the presence of oxidation products of isoprene, α- and β-pinene, and other monoterpenes and sesquiterpenes. I...

Cloud droplets contain dynamic and complex pools of highly heterogeneous organic matter, resulting from the dissolution of both water-soluble organic carbon in atmospheric aerosol particles and gas-phase soluble species, and are constantly impacted by chemical, photochemical, and biological transformations. Cloud samples from two summer events, characterized by different air masses and physicochemical properties, were collected at the Puy de Dôme station in France, concentrated on a strata-X solid-phase extraction cartridge and directly infused using electrospray ionization in the negative mode coupled with ultrahigh-resolution mass spectrometry. A significantly higher number (n = 5258) of monoisotopic molecular formulas, assigned to CHO, CHNO, CHSO, and CHNSO, were identified in the cloud sample whose air mass had passed over the highly urbanized Paris region (J1) compared to the cloud sample whose air mass had passed over remote areas (n = 2896; J2). Van Krevelen diagrams revealed that lignins/CRAM-like, aliphatics/proteins-like, and lipids-like compounds were the most abundant classes in both samples. Comparison of our results with previously published data sets on atmospheric aqueous media indicated that the average O/C ratios reported in this work (0.37) are similar to those reported for fog water and for biogenic aerosols but are lower than the values measured for aerosols sampled in the atmosphere and for aerosols produced artificially in environmental chambers.

The new detailed aqueous-phase mechanism Cloud Explicit Physico-chemical Scheme (CLEPS 1.0), which describes the oxidation of isoprene-derived water-soluble organic compounds, is coupled with a warm microphysical module simulating the activation of aerosol particles into cloud droplets. CLEPS 1.0 was then extended to CLEPS 1.1 to include the chemistry of the newly added dicarboxylic acids dissolved from the particulate phase. The resulting coupled model allows the prediction of the aqueous-phase concentrations of chemical compounds originating from particle scavenging, mass transfer from the gas-phase and in-cloud aqueous chemical reactivity. The aim of the present study was more particularly to investigate the effect of particle scavenging on cloud chemistry. Several simulations were performed to assess the influence of various parameters on model predictions and to interpret long-term measurements conducted at the top of Puy de Dôme (PUY, France) in marine air masses. Specific attention was paid to carboxylic acids, whose predicted concentrations are on average in the lower range of the observations, with the exception of formic acid, which is rather overestimated in the model. The different sensitivity runs highlight the fact that formic and acetic acids mainly originate from the gas phase and have highly variable aqueous-phase reactivity depending on the cloud acidity, whereas C3–C4 carboxylic acids mainly originate from the particulate phase and are supersaturated in the cloud.

A new detailed aqueous phase mechanism named the Cloud Explicit Physico-chemical Scheme (CLEPS 1.0) is proposed to describe the oxidation of water soluble organic compounds resulting from isoprene oxidation. It is based on structure activity relationships (SARs) which provide global rate constants together with branching ratios for HO⋅ abstraction and addition on atmospheric organic compounds. The GROMHE SAR allows the evaluation of Henry's law constants for undocumented organic compounds. This new aqueous phase mechanism is coupled with the MCM v3.3.1 gas phase mechanism through a mass transfer scheme between gas phase and aqueous phase. The resulting multiphase mechanism has then been implemented in a model based on the Dynamically Simple Model for Atmospheric Chemical Complexity (DSMACC) using the Kinetic PreProcessor (KPP) that can serve to analyze data from cloud chamber experiments and field campaigns. The simulation of permanent cloud under low-NOx conditions describes the formation of oxidized monoacids and diacids in the aqueous phase as well as a significant influence on the gas phase chemistry and composition and shows that the aqueous phase reactivity leads to an efficient fragmentation and functionalization of organic compounds.

The atmospheric dispersion Gaussian puff model developed by CEA has been coupled with a new cloud chemistry model using the Kinetic PreProcessor KPP. The purpose is to take into account a more complex chemical reactivity during dispersion in order to better assess the impact in case of an accidental release. The preprocessor and chemical mechanism have also been improved by supplementary chemical reactivity (NH3, HCN…) and by adding new photolysis calculation. The mass transfer of chemical compounds between gas phase, cloud droplet and aqueous chemical reactivity of organic and inorganic compounds has been implemented. Several sensitivity tests show the importance of the atmospheric chemical reactivity for the spatial dispersion of the release. Some of these tests are presented in this study.

Concentrations of 33 metal elements were determined by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis for 24 cloud water samples (corresponding to 10 cloud events) collected at the puy de Dôme station. Clouds present contrasted chemical composition with mainly marine and continental characteristics; for some cloud events, a further anthropogenic source can be superimposed on the background level. In this context, measurements of trace metals may help to evaluate the impact of anthropogenic and natural sources on the cloud and to better discriminate the origin of the air masses. The metal concentrations in the samples are low (between 16.4 µg L−1 and 1.46 mg L−1). This could be explained by the remoteness of the puy de Dôme site from local sources. Trace metals are then used to confirm and refine a previous sample classification. A principal component analysis (PCA) using the pH value and the concentrations of Cl−, NO3−, SO42−, Na+ and NH4+ is performed considering 24 cloud samples. This first analysis shows that 18 samples are of marine origin and 6 samples are classified as continental. The same statistical approach is used adding trace metal concentration. Zn and Mg elements are the most abundant trace metals for all clouds. A higher concentration of Cd is mainly associated to clouds from marine origins. Cu, As, Tl and Sb elements are rather found in the continental samples than in the marine ones. Mg, V, Mn and Rb elements mainly found in soil particles are also more concentrated in the samples from continental air mass. This new PCA including trace metal confirms the classification between marine and continental air masses but also indicates that one sample presenting low pH and high concentrations of SO42−, Fe, Pb and Cu could be rather attributed to a polluted event.

The high altitude Puy de Dôme (PUY) research station, located at 1465 m a.s.l. in central France, hosts many instruments allowing continuous measurements and intensive campaigns to measure meteorological parameters, gas-phase species, aerosol and cloud properties. Aerosol chemical composition measurements provided by a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS) are used to evaluate the WRF-Chem model and more precisely its ability to simulate organic aerosol (OA) for a particular event in summer 2010. Using the Volatility Basis Set approach (VBS), dedicated to the formation of secondary organic aerosol (SOA), the WRF-Chem model strongly underestimates the high concentration levels of OA observed at the PUY station: 12.5 µg m−3 were observed and only 2.6 µg m−3 were simulated. By means of several measurements of gas-phase volatile organic compounds (VOCs), the robustness of both emissions and SOA formation processes in the WRF-Chem model was tested. The underestimation of the OA mass concentration appears to be mainly due to a misrepresentation of the oxidation rate of the organic condensable vapours (OCVs) and the SOA yields of both anthropogenic and biogenic VOCs in the VBS parameterization.

Abstract. Organic compounds of both anthropogenic and natural origin are ubiquitous in the multiphasic atmospheric medium. Their transformation in the atmosphere affects air quality and the global climate. Modelling provides a useful tool to investigate the chemistry of organic compounds in the tropospheric multiphase system. While several comprehensive explicit mechanisms exist in the gas phase, explicit mechanisms are much more limited in the aqueous phase. Recently, new empirical methods have been developed to estimate HO• reaction rates in the aqueous phase: structure-activity relationships (SARs) provide global rate constants and branching ratios for HO• abstraction from and addition to atmospheric organic compounds. Based on these SARs, a new detailed aqueous-phase mechanism, named the cloud explicit physico-chemical scheme (CLEPS), to describe the oxidation of hydrosoluble organic compounds resulting from isoprene oxidation is proposed. In this paper, a protocol based on reviewed experimental data and evaluated prediction methods is described in detail. The current version of the mechanism includes approximately 850 aqueous reactions and 465 equilibria. Inorganic reactivity is described for 67 chemical species (e.g., transition metal ions, HxOy, sulphur species, nitrogen species, and chlorine). For organic compounds, 87 chemical species are considered in the mechanism, corresponding to 657 chemical forms that are individually followed (e.g., hydrated forms, anionic forms). This new aqueous-phase mechanism is coupled with the detailed gas phase mechanism MCM v3.3.1 through mass transfer parameterization for the exchange between the gas phase and aqueous phase. The GROMHE SAR enables the evaluation of the Henry's law constants for undocumented organic compounds. The resulting multiphase mechanism is implemented in a model based on the Dynamically Simple Model for Atmospheric Chemical Complexity (DSMACC) using the Kinetic PreProcessor (KPP). This model allows simulation of the time evolution of the concentrations of each individual chemical species in addition to detailed time-resolved flux analyses. The variable photolysis in both phases is calculated using the TUV 4.5 radiative transfer model. To evaluate our chemical mechanism, an idealized cloud event with fixed microphysical cloud parameters is simulated. The simulation is performed for a low-NOx situation. The results indicate the formation of oxidized mono- and diacids in the aqueous phase, as well as a significant influence on the gas phase chemistry and composition. For this particular simulation, the aqueous phase mechanism is responsible for the efficient fragmentation and functionalization of organic compounds. This new cloud chemistry model allows for the analysis of individual aqueous sub systems and can be used to analyze the results from cloud chamber experiments and field campaigns.

Organic aerosols (OA) represent a large fraction (from 20 to 90 %) of the submicron particulate mass and it is mainly composed of secondary organic aerosols (SOA). Despite the ubiquity of OA in the atmosphere, there are still large uncertainties in understanding the formation pathways of SOA. Consequently, OA sources and physico-chemical transformations during their transport are poorly represented in chemistry-transport models and large gaps still remain between simulated and measured OA concentrations. The ability of the WRF-CHEM model to reproduce the organic aerosol mass concentration originated from anthropogenic or/and biogenic emissions is evaluated. From this perspective, simulations for two contrasted air masses are performed with WFR-Chem using the Volatility Basis Set (VBS) approach dedicated to the formation of SOA. Simulations results are compared to aerosol measurements performed at the puy de Dôme station with a compact Time-of-Flight Aerosol Mass Spectrometer for two episodes in autumn 2008 and in summer 2010. Moreover, measurements of both anthropogenic and biogenic VOCs are used to access the capacity of the WRF-Chem model to correctly simulate the concentrations levels of the gas precursors of the SOA.

Gas chemistry and aerosol chemistry of 10km-resolution mesoscale models Meso-NH and WRF/CHEM were evaluated on three cases over Europe. These one-day duration cases were selected from Freney et al. (2011) and occurred on contrasted meteorological conditions and at different seasons: a cyclonic circulation with a well-marked frontal zone on winter, an anti-cyclonic situation with local storm precipitations on summer and a cold front in the northwest of Europe associated to a convergence of air masses over eastern Europe and conflicting air masses over Spain and France on autumn. To assess the performance of the two models, surface hourly databases from observation stations over Europe were used, together with airborne measurements. For both models, the meteorological fields were in good agreement with the measurements for the three days. Winds presented the largest normalised mean bias integrated over all European stations for both models. Daily gas chemistry was reproduced with normalised mean biases between −14 and 11%, a level of accuracy that is acceptable for policy support. The two models' performances were degraded during night-time quite likely due to the constant primary species emissions. The PM2.5 bulk mass concentration was overestimated by Meso-NH over Europe and slightly underestimated by WRF/CHEM. The absence of wet deposition in the models partly explains the local discrepancies with the observations. More locally, the systematic low mixing ratio of volatile organic compounds in the gas phase simulated by WRF/CHEM at three stations was correlated with the underestimation of OM (organic matter) mass in the aerosol phase. Moreover, this mass of OM was mainly composed of anthropogenic POAs (primary organic aerosols) in WRF/CHEM, suggesting a missing source for SOAs (secondary organic aerosols) mass in WRF/CHEM aerosol parameterisation. The contribution of OM was well simulated by Meso-NH, with a higher contribution for the summer case. For Meso-NH, SOA made the major contribution to the OM mass. The simulation of the mass of SO42− in particles by both models was often overestimated and correlated with an underestimation of the SO2 mixing ratio. The simulated masses of NO3− and NH4+ in particles were always higher for Meso-NH than for WRF/CHEM, which was linked to a difference in NOX mixing ratio between the models. Finally, computations of model performance criterion and model performance goals show that both models can be considered acceptable for standard modelling applications. In particular, Meso-NH model, using a gaseous chemical mechanism designed to compute the organic precursors of aerosols, shows comparable simulated amounts of SOA to observations at local sites.

The oxidative capacity of the cloud aqueous phase is investigated during three field campaigns from 2013 to 2014 at the top of the puy de Dôme station (PUY) in France. A total of 41 cloud samples are collected and the corresponding air masses are classified as highly marine, marine and continental. Hydroxyl radical (HO•) formation rates (RHO•f) are determined using a photochemical setup (xenon lamp that can reproduce the solar spectrum) and a chemical probe coupled with spectroscopic analysis that can trap all of the generated radicals for each sample. Using this method, the obtained values correspond to the total formation of HO• without its chemical sinks. These formation rates are correlated with the concentrations of the naturally occurring sources of HO•, including hydrogen peroxide, nitrite, nitrate and iron. The total hydroxyl radical formation rates are measured as ranging from approximately 2 × 10−11 to 4 × 10−10 M s−1, and the hydroxyl radical quantum yield formation (ΦHO•) is estimated between 10−4 and 10−2. Experimental values are compared with modelled formation rates calculated by the model of multiphase cloud chemistry (M2C2), considering only the chemical sources of the hydroxyl radicals. The comparison between the experimental and the modelled results suggests that the photoreactivity of the iron species as a source of HO• is overestimated by the model, and H2O2 photolysis represents the most important source of this radical (between 70 and 99 %) for the cloud water sampled at the PUY station (primarily marine and continental).

Abstract. Simulations with the chemistry-transport model WRF-Chem are compared to aerosol measurements performed at the puy de Dôme station with a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS) for two episodes in autumn 2008 and in summer 2010. The WRF-Chem model is used with the Volatility Basis Set (VBS) approach dedicated to the formation of secondary organic aerosol (SOA). The model systematically underestimates the observed concentrations of organic aerosol (OA), with significant differences observed for the summer case. For this event, where high OA concentrations were observed (12.5 μg m-3 in average), simulated OA mass concentration is 2.0 μg m-3. For the autumn event, observed OA concentrations reached 2.6 μg m-3. The simulated concentrations reached only 0.6 μg m-3. During the summer event, several gas-phase volatile organic compounds (VOCs) were measured and were used to test the robustness of both emission/dry deposition and SOA formation processes. The concentrations of VOCs, and more specifically the anthropogenic ones, calculated by the model are estimated to be much lower than the observed ones. Hence, the emissions of all SOA precursors are multiplied by 2 in the model: this induces an increase of the mean OA mass concentration of 25% (+0.5 μg m-3) and 18% (+0.4 μg m-3), respectively for anthropogenic and biogenic VOCs. The dry deposition of gas-phase organic condensable vapours (OCVs) is also critical to predict the SOA mass concentrations: dividing the deposition factor by 2 leads to an increase of OA mass by an additional 12% (+0.2 μg m-3). However, these increases were not sufficient to explain the observed OA concentration, and the underestimation of the OA concentration levels seems to be principally related to a lack in the parameterization of SOA formation. Changing the oxidation rate of OCVs from 1.0 × 10-11 to 4.0 × 10-11 cm3 molecule−1 s-1, doubling the SOA yields for anthropogenic VOCs and finally doubling the SOA yields for biogenic VOCs results in an increase of the mean OA mass concentration by 56% (+1.1 μg m-3), 61% (+1.2 μg m-3) and 40% (+0.8 μg m-3), respectively. The consideration of both emission/dry deposition and SOA formation processes tests lead to a mean OA mass concentration of 10.7 μg m-3 for 2010, a value that is close to the observations. For 2008, modifying solely the oxidation rate of OCVs and the SOA yields is sufficient to reproduce the observed level of mean OA mass (a mass of 2.4 μg m-3 is obtained by the model whereas a mass of 2.6 μg m-3 was observed).