Organic aerosols (OA) play a significant role in influencing both climate and human health. However, in source-receptor modelling, a large fraction of OA is typically attributed to highly aged, atmospherically processed species collectively referred to as oxygenated organic aerosol (OOA). Nevertheless, the formation pathways and evolution of OOA as well as their impacts on aerosol optical properties, remain poorly understood. To address this knowledge gap, an experiment was conducted in a suburban site in the Paris region to study the evolution of OOA and their optical properties. Our results show that in regionally transported air masses with mixed biogenic and anthropogenic emissions, the formation of OOA through photochemical processes explains most of the increase in submicron particle mass. Meteorological conditions played a critical role: under dry and strong solar radiation conditions, enhanced formation of more-oxidized OOA (MO-OOA) was observed. BrC absorption increased concurrently, with short-wavelength absorption rising by similar to 35 % over relatively similar to 24 h of photochemical aging. Conversely, under humid, low-radiation conditions, the OA composition shifted toward less-oxidized OOA (LO-OOA). Suppressed photochemistry limited MO-OOA production, resulting in a lower overall OA oxidation state. These findings highlight the role of photochemistry in shaping both the chemical evolution and resultant optical properties of OA, underscoring the need to consider meteorological dynamics when evaluating aerosol-climate interactions in suburban forest environments.

In this study, we present two optimized analytical methods for the quantification of molecular markers to attribute the contribution of various Volatile Organic Compound (VOC) oxidation products to Secondary Organic Aerosol (SOA). Those involve Ultrahigh Performance Liquid Chromatography Electrospray Ionization coupled to Ion Mobility Time-of-Flight Mass Spectrometry (UPLC/ESI-IMS-QTOFMS) and Gas Chromatography Mass Spectrometry (GC-MS). Liquid extraction was performed for both techniques, with an extra derivatization step with N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) containing 1 % trimethylchlorosilane (TMCS) for GC-MS analysis, enhancing the compound detection capacity. Between the two techniques, 23 biogenic and anthropogenic markers were identified, with five common species detected. Recoveries between 40 % and 170 % were observed for nitro-containing compounds and between 70 % and 140 % for aromatic and non-aromatic acids except for 3-methyl-1,2,3-butanetricarboxylic acid. Limits of detection <5 ng were observed by UPLC/ESI-IMS-QTOFMS analysis for 4-nitrophenol and 2-methyl-4-nitrophenol, while GC-MS (with BSTFA derivatization) analysis allowed better detection of lower mass compounds (for example limit of detection for 2-methylerythritol was 0.10 ng). While UPLC/ESI-IMS-QTOFMS allows for the analysis of high molecular weight compounds at high resolution and sensitivity, GC-MS analysis focuses on compounds of lower mass and higher polarity, together, these complementary methods provide a comprehensive tool for the quantification of organic markers arising from the airborne transformation of compounds of both biogenic and anthropogenic origins.

Molecular markers are useful to identify the sources of secondary organic aerosols (SOA) by linking them to their precursor volatile organic compounds (VOCs) through their oxidation processes. To advance the apportionment of the anthropogenic contribution to SOA, this work determines the ratio of markers to total SOA mass (fSOA) by smog chamber experiments on the photo-oxidation of toluene, m-xylene and naphthalene at different experimental conditions. The aerosol chemical composition was determined by a combination of high-resolution mass spectrometry (HRMS), gas chromatography coupled to mass spectrometry (GC-MS) and ultrahigh performance liquid chromatography electrospray ionization coupled to ion mobility time of flight mass spectrometry (UPLC/ESI-IMS-QTOFMS). The mass fraction of dihydroxy-4-oxo-pentanoic acid (DHOPA) in the aerosol (fSOA, DHOPA), associated with toluene and m-xylene oxidation, showed higher values than those previously reported and a dependency on humidity levels during oxidation was found. On the other hand, the fSOA of phthalic acid, associated with naphthalene oxidation, showed the same value as the one previously reported in the literature at different atmospheres. This study highlights that proper selection of the fSOA at relevant atmospheric conditions is required to estimate reliably the contribution of monoaromatics oxidation to SOA.

Abstract. Between 2012 and 2019, measurements of aerosol mass concentration, composition of the inorganic fraction, optical properties, meteorology and ozone were conducted at the Henties Bay Aerosol Observatory (HBAO) located at the Sam Nujoma Marine and Coastal Resources Research Centre of the University of Namibia at Henties Bay (22.09° S, 14.26° E; 30 m above mean sea level) in Namibia. The site aimed at filling a gap in observation of the boundary layer aerosol at the coastal interface between the hyper-arid Namib desert and the south East Atlantic Ocean, characterised by haziness due to frequent mineral dust plumes and persistent sea spray. Some of the data were already validated and analysed by Formenti et al. (2018), Klopper et al. (2020), Desboeufs et al. (2024). In this paper we present the entire quality-controlled dataset which is now made available to the public with the following DOI: Aerosol elemental and water-soluble ionic composition during 2016 (https://doi.org/10.57932/3998ece8-9629-44bf-a826-b7ae1b25e8e6; Formenti et al., 2026) Aerosol elemental, water-soluble ionic and dissolved elemental composition during 2017 (https://doi.org/10.57932/2ac79cd1-282a-4004-87d5-38f0ebcaf40c; Formenti, 2023) Aerosol mass concentration between 2012 and 2019 (https://doi.org/10.57932/d5bcf2ef-1c1f-4f87-9945-e5ec9a948112; Valorso et al., 2026a) Ozone mixing ratio between 2012 and 2019 (https://doi.org/10.57932/59ed8db1-2a18-4429-a153-fffa61cdbb41; Valorso et al., 2026b) Aerosol particle scattering between 2015 and 2019 (https://doi.org/10.57932/4d3613b6-4c49-417f-ac1d-fd825602fbfb; Valorso et al., 2026c) Aerosol equivalent black carbon concentration between 2012 and 2019 (https://doi.org/10.57932/c8c755ef-bf7a-441e-a4c6-1ee875e5f570; Valorso et al., 2026d) Wind speed and direction between 2012 and 2019 (https://doi.org/10.57932/99b707a5-1cba-4b61-915b-829aa742a726; Valorso et al., 2026e) Basic meteorology between 2018 and 2019 (https://doi.org/10.57932/9d344c70-2f09-4fff-a2d9-ce72f5cf1757; Valorso et al., 2026f) These research-quality data, unique for the area, have many uses, including the determination of seasonal changes in the atmospheric composition and the validation of climate and weather modeling output and satellite-retrieved aerosol products.

Abstract. Urban air pollution involves complex mixtures of gases and particulate matter whose molecular-level composition and gas-particle partitioning remain poorly characterized, limiting our understanding of secondary organic aerosol (SOA) formation. We address this gap using controlled atmospheric simulations combined with detailed molecular characterization. Two distinct urban atmospheric scenarios were simulated in the CESAM smog chamber: a standard urban (traffic emissions and biogenic precursors) and a biomass burning enhanced. Both scenarios were aged under controlled irradiation with NOx to simulate tropospheric photochemistry. PM1 concentrations reached 15 ± 7 µg.m-3 for the standard scenario and 63 ± 24 µg.m-3 for the biomass burning scenario, with organic aerosol fractions of approximately 17 % and 40 %, respectively. Gas-phase analysis via proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) identified 23 volatile organic compounds (VOCs), dominated by oxygenated species (74–77 %). Particle-phase molecular analysis using ultrahigh-performance liquid chromatography electrospray ionization ion mobility quadrupole time-of-flight mass spectrometry (UPLC/ESI-IMS-QTOFMS) revealed 32 distinct compounds. The biomass burning scenario showed elevated source-specific tracers, including a levoglucosan isomer, nitrophenolic compounds (e.g., 3-methyl-4-nitrocatechol, 4-nitroguaiacol), and oxidized aromatics. Volatility distributions estimated via group contribution methods placed most compounds in the semi-volatile, low-volatility, and extremely low-volatility organic compound ranges (C* < 300 µg m-3), indicating substantial functionalization and partitioning. These results demonstrate the capacity of simulation chambers to generate reproducible urban aerosol analogues with distinct source-specific molecular signatures and well-characterized volatility distributions. This detailed molecular speciation provides a robust basis for process-oriented model evaluation and opens perspectives for systematic investigations of SOA formation pathways under controlled urban photochemical conditions.

The complex refractive index (CRI; n−ik) and the single scattering albedo (SSA) are key parameters driving the aerosol direct radiative effect. Their spatial, temporal, and spectral variabilities in anthropogenic–biogenic mixed environments are poorly understood. In this study, we retrieve the spectral CRI and SSA (370–950 nm wavelength range) from in situ surface optical measurements and the number size distribution of submicron aerosols at three sites in the greater Paris area, representative of the urban city, as well as its peri-urban and forested rural environments. Measurements were taken as part of the ACROSS (Atmospheric Chemistry of the Suburban Forest) campaign in June–July 2022 under diversified conditions: (1) two heatwaves leading to high aerosol levels, (2) an intermediate period with low aerosol concentrations, and (3) an episode of long-range-transported fire emissions. The retrieved CRI and SSA exhibit an urban-to-rural gradient, whose intensity is modulated by the weather conditions. A full campaign average CRI of 1.41−0.037i (urban), 1.52−0.038i (peri-urban), and 1.50−0.025i (rural) is retrieved. The imaginary part of the CRI (k) increases and the SSA decreases at the peri-urban and forest sites when exposed to the influence of the Paris urban plume. Values of k > 0.1 and SSA < 0.6 at 520 nm are related to a black carbon mass fraction larger than 10 %. Organic aerosols are found to contribute to more than 50 % of the aerosol mass and up to 10 % (urban), 17 % (peri-urban), and 22 % (forest) of the aerosol absorption coefficient at 370 nm. A k value of 0.022 (370 nm) was measured at the urban site for the long-range-transported fire episode.

Environmental monitoring and air quality survey during long-term field campaign tests, especially in lowaccessibility or extreme environments, requires robust, standalone and autonomous analyzer with low gas consumption and minimal human intervention. The current states of art emphasize the need to develop miniaturized GC based on novel detectors that offers the best compromise between carrier gas consumption, detection limits and panel of measured VOCs. This work presents the development and optimization of MAVERIC, a miniaturized and autonomous Gas Chromatograph system coupled to an innovative Nano Gravimetric Detector (NGD) based on NEMS (nano-electromechanical-system) resonator. A homemade software is developed to control the instrument as well as the electronics modules. The system operates at low flow rate (2 mL.min-1) of helium used as carrier gas, and allows the measurements of VOCs from C6 to C10 in less than 30 min. A mixture of isoprene, benzene, toluene and alpha-pinene is used to optimize experimental conditions. Under optimal conditions, the detection limit, the stability, the repeatability and the linearity of the analytical system are assessed. A detection limit of sub-ppb to few ppt level was determined for C6 and C9 compounds, respectively. The lowest detection limit corresponds to the highest molecular weight compounds due to NGD sensitivity at ambient temperature. The system is standalone, portable, robust and equipped with 4G connection that allows remote control of the instrument and easy data export. It is very adapted for a long period field campaign test, environmental monitoring and air quality survey outdoors and in low-accessibility or extreme environments.

Understanding the interaction of anthropogenic and biogenic emissions around large urban agglomerations remains an important question for atmospheric research and the key question of the ACROSS (Atmospheric Chemistry of the Suburban Forest) project. ACROSS is based on an intensive field campaign in the Paris area, including ground-based measurements in the urban inner centre and suburban and forested sites and measurements made on board aircraft, during the exceptionally hot and dry summer of 2022. In addition, 3D modelling represents an important tool in ACROSS, and here we use the available measurements from the campaign together with observations from air quality and meteorological networks to evaluate the WRF–CHIMERE model simulation for the ACROSS period. We find that the WRF model is able to reproduce the meteorological variability during the campaign, in particular during two heatwaves. The model reproduces the daily ozone maxima well but overestimates PM2.5 by a factor of 1.5–2, partly due to an overestimation of secondary aerosol, both organic and inorganic. For organic aerosol in the Île-de-France area, the biases are reduced to about ±20 %. These differences are in contrast with those of the existing literature and might have been increased by the hot conditions of summer 2022. For case studies during two heatwave days, the model shows the sources for two organic aerosol peaks above 20–30 µg m−3, on one occasion due to biogenic secondary organic aerosol formation in different forests around Paris and on another occasion due to the advection of wildfire aerosols in combination with secondary formation mainly from forest-emitted biogenic volatile organic compounds (BVOCs).

In order to study aerosols in environments influenced by anthropogenic and biogenic emissions to variable extents, PM1 samples were collected during summer 2022 in the greater Paris area (ACROSS campaign, Atmospheric Chemistry Of the Suburban Forest, 14 June to 25 July) at two locations that represent the urban Paris and the suburban forested areas. They were analyzed using high-resolution mass spectrometry (HRMS) together with total carbon (TC) with a thermo-optical method. Both sites are compared here to explore differences in aerosol composition from urban and forested environments. The TC analysis shows similar organic carbon (OC) concentrations at both sites (3.2 ± 1.8 µg m−3 for Paris and 2.9 ± 1.5 µg m−3 for Rambouillet) and higher elemental carbon (EC) values in the urban area. Both OC and EC concentrations did not show significant variations for daytime and nighttime conditions. This work highlights the influence of anthropogenic inputs on the chemical composition of urban and forested areas, derived from the presence of CHO and CHON compounds but also the detection of two sulfur-containing compounds (C5H12SO7 and C10H17NSO7), which could be tentatively assigned as organosulfates. A smaller number of aromatic compounds were observed for clean periods that better represent the local biogenic and anthropogenic contributions in Rambouillet and Paris, respectively.

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The complex refractive index (CRI) is one of the key parameter driving aerosol spectral optical properties and direct radiative effects (DRE). Its value and spectral variation under different conditions, such as anthropogenic− and biogenic−dominated environments and anthropogenic−biogenic mixing situations, remains not fully understood. As a consequence, oversimplified representations of aerosol optical properties are generally used in climate models. Therefore, measurements of aerosol CRI in different environments and their inclusion in models are needed. The field observations from the ACROSS campaign, performed in June-July 2022 in the Ile de France region, are used in this study to deepen the knowledge of aerosol optical properties, aiming to improve the aerosol representation in the CHIMERE model and provide the best constraint for DRE simulations. Measurements obtained both at the Paris city center and the Rambouilllet rural forest sites during ACROSS are considered, in order to explore the CRI variability from anthropogenic−dominated to biogenic−dominated environments, including anthropogenic−biogenic mixing situations. The CRI retrievals at seven different wavelengths, performed by combining the Mie theory with optical and size distribution measurements, are representative of different atmospheric conditions, aerosol loadings as well as type and chemical compositions. In fact, the June-July 2022 period was characterized by highly diversified weather conditions: 1) two strong heatwaves, promoting SOA build-up and favoring the export of the Paris pollution plume towards the forest site; 2) Saharan dust events transported from the upper atmosphere to the ground; 3) biomass burning episode; 4) periods with reduced anthropogenic influence. The CRI retrievals under these different conditions and their link to particulate chemical composition is investigated. Hence, the CRI dataset presented here constitutes a unique dataset from which models can benefit to validate and constrain simulations and DRE estimations, under both urban and biogenic emissions influence. These data, in conjunction with those from the aircraft observations during ACROSS, are used to initialize and perform sensitivity studies on the aerosol DRE, using the CHIMERE−WRF coupled model, the OPTSIM model for the aerosol optical properties and the Rapid Radiative Transfer Model for GCMs (RRTMG).Keywords: Complex refractive index, direct radiative effect, aerosol mixing, urban, forest

Mineral dust has radiative and biogeochemical impacts, affects human health and soil fertility. The mineral dust cycle, i.e., dust emission, transport and deposition depends on meteorological parameters, in particular surface wind speed and precipitation. Climate change has lead to measurable change in surface temperature and precipitation regimes in the Sahel (e.g., Panthou et al., 2018) and is also expected to modify the surface winds that controls dust emissions and transport. Since 2006, mineral dust is monitored in the Sahel by the stations of the INDAAF network (https://indaaf.obs-mip.fr/). We used the PM10 surface concentrations and the Aerosol Optical Depth (AOD) from the AERONET network measured in Cinzana (Mali) and Banizoumbou (Niger) to detect possible changes in the Sahelian atmospheric dust content. The Angstrom exponent is used to select situations where mineral dust is the dominant contributor to the AOD. PM10 concentrations and AOD are significantly correlated but have distinct seasonal cycles, with a ratio PM10/AOD peaking in August. No clear trend on the annual and seasonal mean concentrations or AODs has been identified. When subtracting the mean seasonal cycle to the monthly median PM10 concentration we observe a slight decrease of the residuals in Cinzana (Mali) but no trend in the AOD. No correlation was found between the AOD or the PM10 concentrations and the North Atlantic Oscillation Index but the PM10 concentration tends to increase with the Sahelian drought index. For most of the years, the PM10 concentrations and AODs are lower when the maximum of the vegetation cover of the previous year (represented by satellite Normalized Vegetation Index) is higher. This may reflect the protective effect of the dry vegetation residues on dust emission. These results suggest that, for the measurement period (2006-2019), the variability of the dust content is mainly due to the seasonal cycle and that the year to year variability is so large that no trends can be detected. Longer time series, with a better temporal sampling, seem to be necessary to have a chance to detect a significant change.

In the semi-arid Sahel region, wet deposition can represent more than half of the total annual deposition and are associated to different rainfall types, from stratiform precipitation to convective systems. Surface parameters such as temperature, wind speed, wind direction as well as rainfall rate can be used to distinguish these situations. We investigate the behaviour of dust wet deposition at the event-scale based on a multiannual (2007 to 2016) monitoring of wet deposition fluxes, PM10 concentration, precipitation and meteorological parameters in two Sahelian stations Banizoumbou (Niger, 13.54°N, 2.66 E) and Cinzana (Mali, 13.28°N, 5.93°W) of the INDAAF network. Rainfall events have been classified into three types: (i) stratiform, convective associated with (ii) weak precipitation or (iii) intense precipitation. This classification is based on selected criteria regarding evolutions of surface temperature, of wind speed and direction before and after the rainfall onset as well as on the event rainfall rate. Based on an interpretation of hundreds of single events, almost 25% of wet deposition events are associated with non-convective situation, more than 40% with atmospheric convective situation and weak precipitation, and more than 35% events with atmospheric convective situation combined with intense precipitation. This exhaustive work over a long-time period of measurements illustrates the predominance of convective situations regarding wet deposition in the two Sahelian stations. Washout ratios (WR) have been computed from PM10 concentrations, precipitation and deposition fluxes for each kind of events when data were concomitant. The dependency of WR to precipitation amount is shown to differ depending on the rain types. For instance, the decreasing dependency of WR with the precipitation amount of non-convective events has been quantified and could be explained by a dilution effect of the deposition. On the contrary, no clear dependency of WR with the precipitation has been observed for atmospheric convective conditions associated with intense rainfall rate.

The role of relative humidity (RH) in secondary organic aerosol (SOA) formation from high-NOx photooxidation of long-chain alkanes was investigated by performing simulation chamber experiments on n-dodecane (C12H26). This molecule was chosen as a model compound for the class of long-chain alkanes. The experiments span a wide range of RH conditions from <1 to 70%. The humidity was found to reduce the SOA production yield by a factor of 2, from extremely dry (<1%) to humid (RH >= 5%) conditions. Measurements of major oxidation products in the gaseous and aerosol phases revealed the effect of water on a series of multiphasic reaction mechanisms and SOA production. Under very low RH, an acid-catalyzed "dehydration" reaction of polyfunctional molecules leads to lower volatility products. This pathway was found to be inhibited from dry to ambient RH conditions, explaining the SOA yield reduction. In addition to the previously established carbonyls and hydroxycarbonyls, novel tracers, that is, lactone and carboxylic acid, were identified using authentic standards, providing evidence for water-sensitive multiphasic pathways. Among the species formed via the dehydration channel, we find not only volatile species but also very reactive intermediates (e.g., dihydrofurans) which partition back to the gas phase. Owing to its double bond, it can be further oxidized to lower volatility secondary products, which forms a complex multiphasic scheme involving many condensation reaction-evaporation steps.

Summary Using CESAM, an atmospheric simulation chamber (cesam.cnrs.fr), we have developed a totally innovative platform for exposing mice to realistic atmospheric conditions. Here we present the first toxicological analyses of the organs of these mice after 48 hours to several days of exposure, carried out as part of feasibility experiments aimed at testing this experimental concept. This platform has received funding from the European Union’s Horizon 2020 research and innovation programme through the EUROCHAMP-2020 Infrastructure Activity under grant agreement N° 730997, and is now supporting the new REMEDIA H-2020 project (call H2020 “Exposome”) Introduction The World Health Organization (WHO) estimated that there were 3.7 million premature deaths due to air pollution in 2014, and confirmed that air pollution is the greatest environmental risk to health (responsible for a loss of more than 3% of productivity). The studies conducted so far show that the effects of air pollution on health depend not only on the quality of the surrounding air, but also on the subjects exposed and their individual vulnerability (asthma, obesity, period of life, etc.). Despite the evidence on the adverse health effects of exposure to air micro-pollutants, there are still uncertainties about the nature of these effects, and progress need to be made on their quantification. This limitation of knowledge is mainly attributed to the complexity of the polluted atmospheres, and to the great difficulty to model the impact of realistic situations of exposure. Methodology The innovative approach we set up is to realistically simulate, at the laboratory, the atmospheric mixture in all its complexity, thus keeping the ability to control, reproduce and carefully characterize the experimental conditions. We used the CESAM chamber (4.2 m3 stainless steel atmospheric simulation, evacuable down to a few 10-7 atm, temperature controlled between +15°C and +60°C) in order to study the myriad of products arising from the atmospheric oxidation of primary organic compounds. The experimental protocol consists in the continuous injection of relevant mixtures of primary pollutants (mainly nitrogen oxides, organic compounds from a representative mix of anthropogenic emissions, sulphur dioxide, soot, inorganic salts and potentially mineral dust particles if needed - e.g. to simulate Beijing’s atmosphere) at low concentrations (ppb levels) in air in the CESAM simulation chamber operated as a slow flow reactor. The residence time of simulated air parcels in the experimental volume is fixed to 4 hours, in order to represent air masses of regional scale. During this time the synthetic mixture is exposed to an artificial solar irradiation, allowing secondary pollutants such as ozone, nitric acid, formaldehyde, peroxyacetyl nitrate as well as complex polyfunctional organics including SOA to be produced and to reach their chemical steady state. Mice are exposed to constant flows of such a mixture during time scales of week to address their effects on health. Conclusions Here we present the first toxicological analyses related to organs/tissue of these mice after exposure of 48h to several day, carried out with a representative atmosphere of Beijing or a representative atmosphere of Paris. References Coll P. et al., 2018, WIT Transactions on Ecology and the Environment, 230.