Urban volatile organic compounds (VOCs) are key precursors of tropospheric ozone and secondary organic aerosols (SOA), yet their long-term dynamics and health implications remain unclear across Europe. Here, we synthesize two decades of VOC observations (2002-2023) from 21 urban monitoring sites in six countries to assess emission trends, oxidation potentials, and human exposure risks. Consistent declines in total hydrocarbons were observed at most sites, reflecting the effectiveness of emission control policies. Aromatic hydrocarbons such as toluene, xylene, and benzene were the dominant contributors to ozone and SOA formation. Physiologically based toxicokinetic (PBTK) modeling suggests that key VOCs preferentially accumulate in the kidney and liver. The integration of atmospheric monitoring with toxicokinetic modeling provides a multi-scale understanding of how urban VOCs influence both air quality and internal human exposure, offering new insight into effective pollution control strategies.

This study presents the first nationwide phenomenological analysis of submicron organic aerosol (OA) sources in France, based on highly time‑resolved Aerosol Chemical Speciation Monitor (ACSM) measurements collected at 12 (sub)urban sites over one to seven years. A harmonized rolling Positive Matrix Factorization (PMF) approach was applied, ensuring consistent constraints and criteria across all datasets. The methodology enabled the identification of major OA sources, including primary OA (POA) factors such as hydrocarbon-like OA (HOA), biomass burning OA (BBOA), and cooking-like OA (COA), which together accounted for approximately 33 % of the OA mass. Secondary or aged OA was represented by oxygenated OA (OOA), which dominated OA at all sites and was further separated into Less Oxidized (LO-OOA) and More Oxidized OOA (MO-OOA), except for Poitiers and Strasbourg. Additional site-specific OA factors were also resolved, including a mixed shipping/industrial OA factor in Marseille Longchamp and an amine-related OA factor in Strasbourg and Creil. Strong seasonality was observed for POA, particularly BBOA, which increased substantially during winter due to residential heating. LO-OOA correlated with BBOA in winter, highlighting the important contribution of biomass combustion to wintertime air quality degradation across France, while summer LO-OOA was mainly associated with biogenic precursors. Comparison with the CHIMERE chemical transport model revealed systematic biases in simulated OA components, underscoring the value of this unique long-term dataset for improving OA representation. These findings are highly relevant for various applications, including epidemiological studies, -in which source-specific exposure indicators can reveal stronger links to health effects-, and near-real-time source apportionment, -by using these identified profiles to constrain the factors in real-time analysis- facilitating the rapid identification of pollution episodes and enable the implementation of air quality management measures. They also offer observational insights for improving air quality, providing valuable information to policymakers for proposing effective mitigation strategies.

Air pollution, especially in urban areas, is the result of a complex mixture of natural and anthropogenic emissions and their atmospheric processing. It causes millions of premature deaths worldwide and affects plant metabolism, which in turn alters the emissions of Biogenic Volatile Organic Compound (BVOCs) by plants. By taking the subtropical Metropolitan Area of São Paulo (MASP) as a natural laboratory, the BIOMASP+ project (BIOsphere-atmosphere interactions in the Metropolitan Area of São Paulo - plus ) aims to evaluate the interplay between the biosphere and secondary pollution (ozone and SOA formation and aging). The Brazilian Atlantic Forest (Mata Atlântica) is the target ecosystem as the fifth biodiversity hotspot in the world. Here we present the scientific motivations of the project, its methodology and the preliminary observations from the Special Observation Periods of year 2023 (SOP1, 2, 3 and 4). BIOMASP+ is (i) integrative, by combining in-situ/remote/laboratory observations and modeling, (ii) multidisciplinary, addressing micrometeorology, urban climate, atmospheric chemistry and biology. The project involves multiple nested scales: from leaf to above-canopy levels, from very short time (microseconds) to multi-year scale, from few millimeters (turbulence scale) to synoptic scale. In particular, the experimental effort relies on the implementation of two contrasting supersites (primary forest and urban forest) with a 30-m and 20-m flux towers, respectively, and a variety of state-of-the-art instruments. Ambient observations and the quantification of BVOC emissions have highlighted the complex interactions between meteorology, atmospheric composition of pollution, biogenic emissions of representative remnants of the Atlantic Forest and anthropogenic emissions.

Organic aerosol (OA) is a major component of atmospheric particulate matter (PM), affecting both human health and climate. However, high-resolution estimates of OA exposure needed for exposure analysis remain scarce. Here, we integrate a chemical transport model (CAMx) with a random forest (RF) machine learning approach to bias-correct and downscale daily OA concentrations across Europe. CAMx OA simulations at ∼15 km resolution show moderate agreement with observations (r = 0.55). By combining these outputs with high-resolution land-use data and training the RF model on ∼48,000 daily OA measurements from 137 sites, prediction accuracy improved (r = 0.65), with ∼l5% reduction in root mean square error. The resulting maps provide European daily OA concentrations at ∼250 m resolution for alternate years from 2011 to 2019. The model captures key spatial features, including elevated OA in the Po Valley, Southeastern, and Central Europe, as well as intracity variations due to local hotspots. Seasonal analysis reveals higher concentrations in winter, while long-term trends indicate a general decline in OA levels. Exposure estimates show that half of the European population experiences OA levels above 3 µg/m3, and ∼50 million people are exposed to more than 5 µg/m3, which is the current guideline level recommended by the world health organization for total PM2.5. These high-resolution OA maps offer vital critical support for epidemiological research and air quality policy.

Organic aerosol particles (OA) can absorb solar radiation with varying efficiencies depending on their chemical composition and physical properties. This light-absorbing fraction of OA, commonly referred to as brown carbon (BrC), is difficult to accurately represent in climate models due to the inherent diversity of its optical properties. This variability arises from differences in emission sources and atmospheric processing, as well as from variations in experimental design and the analytical methods used to quantify BrC absorption. As a result, the climate effect of BrC remains uncertain. Here, we studied the light absorption properties of surface ambient OA using measurements from 17 sites across Europe. Combining multi-wavelength absorption measurements from filter-based photometers with OA mass concentrations and source apportionment derived from ACSM/AMS data, we derive empirical estimates of the OA mass absorption cross section (MACOA), its wavelength dependence (AAEOA), the OA density (⍴OA), and the MAC associated with different primary and secondary OA sources. We further develop parameterizations that relate MACOA, AAEOA and ⍴OA to the ambient black carbon-to-organic aerosol ratio (eBC/OA) and propose a corresponding parameterization for the imaginary refractive index (kOA). Given the widespread availability of eBC and OA measurements in global monitoring networks, the framework presented here provides a practical approach for estimating the absorptive properties of surface OA particles under real-world conditions.

In the framework of the International Network to study Deposition and Atmospheric chemistry in AFrica (INDAAF) program, we report a 15-year dataset (2006-2020) of total carbon (TC), organic carbon (OC) and elemental carbon (EC) aerosols in PM2.5 and PM10 at the Lamto site (Cote d'Ivoire), a representative West African natural wet savanna. Harmonized protocols ensure global comparability, making this dataset the longest continuous African record and allowing us to document carbonaceous aerosol variability from weekly up to annual scales. Annual EC concentrations in PM2.5 and PM10 ranged from 1 to 1.35 mu g m-3 and OC from 3 to 5.6 mu g m-3 respectively, with dry-season (November-March) values about twice those of the wet season (April-October). Strong seasonal peaks were linked to biomass burning and Harmattan transport, while higher wet-season OC/EC ratios reflected incomplete combustion and secondary organic carbon formation. Back trajectories and OC/EC emission inventories indicate a decline in large-scale savanna fires but rising agricultural waste burning and growing local anthropogenic contributions. Upward OC and EC trends (+3-5 % yr-1) highlight shifting emission sources and the growing anthropogenic impact on aerosol burdens over West African savannas. This integrated approach combining in-situ monitoring, emission inventories, and trajectory analysis-offers valuable insight into regional aerosol dynamics and supports the evaluation of satellite products, air quality strategies, and chemistry-transport models.

An interlaboratory comparison (ILC) was conducted for levoglucosan, mannosan, and galactosan, as widely used organic tracers for assessing biomass burning aerosol in ambient air. Organized as part of the European research infrastructure ACTRIS (Aerosol, Clouds and Trace Gases Research Infrastructure) activities the OrGanic Tracers and Aerosol Constituents-Calibration Centre (OGTAC-CC) distributed aliquots from three ambient PM2.5 filter samples and two prepared aqueous standard solutions to ten research laboratories across Europe, each using its own analytical protocol. Overall agreement was good for the ambient filter samples, with relative standard deviations relative to the general mean of 14% for levoglucosan, 22% for mannosan, and 33% for galactosan. Individual measurement accuracy, expressed as mean percentage error, ranged from-33% to 13% for levoglucosan,-51% to 15% for mannosan, and-54% to 42% for galactosan. Laboratory performance was also assessed using z-scores, showing that despite methodological diversity, nearly all results were classified as acceptable. This ILC provides a timely snapshot of current European laboratory capability for key biomass burning tracers. The joint intercomparison study demonstrates the readiness of European laboratories to provide harmonized levoglucosan measurements at a continental scale, meeting the comparability needs arising from the inclusion of levoglucosan in the revised EU Ambient Air Quality Directive (AAQD), and supporting requirements across European (Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP), ACTRIS) and national monitoring networks.

Organic aerosols are an important and highly dynamic component of fine particulate matter, yet their long-term response to emission controls is poorly constrained. We analyze a decade (2013-2023) of wintertime aerosol mass spectrometry data from urban Nanjing, eastern China, using a developed machine learning framework that disentangles anthropogenic emission-driven changes from meteorology-driven changes. The mean organic aerosol concentrations decreased from 24.6 to 16.5 mu g m-3 during the period of 2013-2017. After accounting for meteorological influences, emission controls account for similar to 94% of the observed decrease. However, the effectiveness of anthropogenic emission controls on the total organic aerosol was weakened by a factor of approximately 2-8 times in the subsequent emission control phases, particularly with more oxidized secondary organic aerosol showing a minimal further decrease. Machine learning-based attribution analysis reveals that reductions in fossil fuel combustion and traffic-related aromatic precursors explain, on average, similar to 50-60% of the long-term variability in secondary organic aerosol, while the meteorological influence plays a minor role. These results provide observationally source-resolved evidence that current measures are reaching diminishing returns and that effective future controls must target overlooked precursors and secondary formation pathways.

Since organic aerosols (OA) account for a significant fraction of PM worldwide, source apportionment is essential for effective air quality mitigation and policymaking. In the present study, we developed a novel method based on a chemical mass balance and an elastic net regressor (EN-CMB), using positive matrix factorization (PMF) as prior knowledge for near real-time source apportionment of OA. EN-CMB has been integrated into a so-called continuous aerosol source apportionment (CASA) software package, which has been evaluated against state-of-the-art rolling-PMF data at three contrasted urban sites in Europe. CASA exhibits very satisfactory performance for primary OA components, with, at all sites, R 2 values of 0.87-0.97 and mean bias error (MBE) of between -0.15 and 0.14 μg/m3. Secondary OA (SOA) fractions showed similar R 2 values (0.81-0.97), but slightly higher MBE values (ranging from -0.71 to 0.04 μg/m3), which can be related to the complex nature of SOA and is still acceptable regarding bulk trends. CASA allows near real-time operation and, as it is open source, represents a promising example of timely and efficient air pollution management with applications in real-time air quality monitoring. The next steps will enable community-driven initiatives to improve and expand the application of such open-source methodologies across diverse regions and emission sources.

Hygroscopicity strongly influences aerosol properties and multiphase chemistry, which is essential in several atmospheric processes. Although CCN (cloud condensation nuclei) properties are commonly measured, sub-saturated hygroscopicity measurements remain rare. During the ACROSS campaign (Atmospheric ChemistRy Of the Suburban foreSt, conducted in Paris in summer 2022), particles' hygroscopic growth rates at 90 % relative humidity (RH) and chemical composition were measured at the sub-urban site using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA, scanning at 100, 150, 200, and 250 nm) and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). Growth factor probability density functions (GF-PDFs) revealed two distinct modes, namely hydrophobic and hygroscopic, suggesting a combination of internal and external particle mixing, with the split at GF 1.2. The prevalence of the hygroscopic mode increased with particle size, with mean hygroscopicity (κ) values of 0.23 and 0.38 for 100 and 200 nm particles, respectively. Using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule, the agreement between measured and chemically derived hygroscopicity was approximately 51% for 100 nm particles, which declined for 200 and 250 nm. These emphasise the large effect of external particle mixing and its influence on predicting hygroscopicity. The ZSR approach proves to be unreliable in predicting the wide growth distribution of externally mixed particles. In this measurement, 80 %–90 % of the particles were externally mixed and influenced by fresh emission, which affected the hygroscopicity prediction by a factor of 2. A cluster analysis based on backward trajectories and meteorological conditions gives valuable insights into the chemical composition and variations in the hygroscopicity of different air masses.

Gaseous and particulate organic compounds are key components of atmospheric chemistry and better understanding their composition, sources and processes essential to limit their impacts. Source apportionment using positive matrix factorization (PMF) is customarily performed for such studies. Combining organic aerosol data with their gaseous precursors was shown to be a promising approach, however very rarely attempted so far, to refine their origins using PMF. In this study, co located continuous proton-transfer-reaction mass spectrometer (PTR-MS) and aerosol chemical speciation monitor (ACSM) measurements were performed at the suburban SIRTA station located in the Paris region. A combined dataset using both instruments during summer (June-August 2020) was then used in an exploratory PMF analysis to investigate the sources and processes of organic compounds, particularly the influences of biogenic emissions and important photochemical reactions on the formation of secondary organic aerosol (SOA) in this period of the year. Specific parameters and evaluation procedures were needed to ensure an equivalent representation of both instruments in the PMF model. A weighing factor was applied to the PTR-MS uncertainties which was controlled and optimized based on the analysis of the modelled scaled residuals. Seven main factors were obtained, describing anthropogenic sources (hydrocarbon-like organics, cooking-like organics), primary biogenic volatile organics, nighttime VOC, oxidized organics, aged organics and a specific isoprene oxidation factor which contributed 11 % to VOC and 4 % to OA. Compared to single-instrument PMFs, more factors were obtained, notably including the cooking-like factor which is not usually resolved using only ACSM data. This method also showed a better mathematical performance for the PTR-MS variables (mean absolute scaled residuals lower for the combined PMF (11.2) than for the PTR-MS-only PMF (38.2)) and a better separation of the factors for the ACSM variables in the combined PMF.

Carbonaceous aerosols (CA), composed of black carbon (BC) and organic matter (OM), significantly impact the climate. Light absorption properties of CA, particularly of BC and brown carbon (BrC), are crucial due to their contribution to global and regional warming. We present the absorption properties of BC (b(Abs,BC)) and BrC (b(Abs,BrC)) inferred using Aethalometer data from 44 European sites covering different environments (traffic (TR), urban (UB), suburban (SUB), regional background (RB) and mountain (M)). Absorption coefficients showed a clear relationship with station setting decreasing as follows: TR > UB > SUB > RB > M, with exceptions. The contribution of b(Abs,BrC) to total absorption (b(Abs)), i.e. %Abs(BrC), was lower at traffic sites (11-20 %), exceeding 30 % at some SUB and RB sites. Low AAE values were observed at TR sites, due to the dominance of internal combustion emissions, and at some remote RB/M sites, likely due to the lack of proximity to BrC sources, insufficient secondary processes generating BrC or the effect of photobleaching during transport. Higher b(Abs) and AAE were observed in Central/Eastern Europe compared to Western/Northern Europe, due to higher coal and biomass burning emissions in the east. Seasonal analysis showed increased b(Abs), b(Abs,BC), b(Abs,BrC) in winter, with stronger %Abs(BrC), leading to higher AAE. Diel cycles of b(Abs,BC) peaked during morning and evening rush hours, whereas b(Abs,BrC), %Abs(BrC), AAE, and AAE(BrC) peaked at night when emissions from household activities accumulated. Decade-long trends analyses demonstrated a decrease in b(Abs), due to reduction of BC emissions, while b(Abs,BrC) and AAE increased, suggesting a shift in CA composition, with a relative increase in BrC over BC. This study provides a unique dataset to assess the BrC effects on climate and confirms that BrC can contribute significantly to UV-VIS radiation presenting highly variable absorption properties in Europe.

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.

Brown carbon (BrC) is a fraction of organic aerosol (OA) that absorbs radiation in the ultraviolet and short visible wavelengths. Its contribution to radiative forcing is uncertain due to limited knowledge of its imaginary refractive index (k). This study investigates the variability of k for OA from wildfires, residential, shipping, and traffic emission sources over Europe. The Multiscale Online Nonhydrostatic Atmosphere Chemistry (MONARCH) model simulated OA concentrations and source contributions, feeding an offline optical tool to constrain k values at 370 nm. The model was evaluated against OA mass concentrations from aerosol chemical speciation monitors (ACSMs) and filter sample measurements, as well as aerosol light absorption measurements at 370 nm derived from an Aethalometer™ from 12 sites across Europe. Results show that MONARCH captures the OA temporal variability across environments (regional, suburban, and urban background). Residential emissions are a major OA source in colder months, while secondary organic aerosol (SOA) dominates in warmer periods. Traffic is a minor primary OA contributor. Biomass and coal combustion significantly influence OA absorption, with shipping emissions also notable near harbors. Optimizing k values at 370 nm revealed significant variability in OA light absorption, influenced by emission sources and environmental conditions. Derived k values for biomass burning (0.03 to 0.13), residential (0.008 to 0.13), shipping (0.005 to 0.08), and traffic (0.005 to 0.07) sources improved model representation of OA absorption compared to a constant k. Introducing such emission source-specific constraints is an innovative approach to enhance OA absorption in atmospheric models.

Abstract. CO2 is the main contributor to global warming, and cities now account for more than two-thirds of emissions of this gas. Atmospheric observatories located on the outskirts of cities are therefore important facilities for measuring the impact on atmospheric composition of the emission reductions planned by cities. The Saclay observatory, part of the ICOS and ACTRIS research infrastructures and located 20 km southwest of Paris, has been monitoring greenhouse gases (CO2, CH4), reactive gases (NOx, O3, CO), and carbonaceous aerosols (eBC) since 2012. This study presents 10 years of monitoring of these compounds, characterizing diurnal, seasonal cycles and decadal trends. In order to best characterize the impact of Parisian emissions, we defined two sets of data depending on whether the station is downwind of Paris or, conversely, in background conditions with westerly winds. This strategy allows us to characterize the urban offset in the Saclay measurement series. The results show a significant decrease in the urban offset of compounds mainly linked to traffic emissions: -35.6 %, -52.3 %, and -56.7 % for CO, NOx, and eBClf. There was also a 15 % decrease in urban offset of CO2 between the 2012–2017 and 2019–2022 periods, a figure consistent with the Airparif inventories' estimate of the decrease in emissions in Paris over the same period.

The apportionment of equivalent black carbon (eBC) to combustion sources from liquid fuels (mainly fossil; eBC(LF)) and solid fuels (mainly non-fossil; eBC(SF)) is commonly performed using data from Aethalometer instruments (AE approach). This study evaluates the feasibility of using AE data to determine the absorption Angstrom exponents (AAEs) for liquid fuels (AAE(LF)) and solid fuels (AAE(SF)), which are fundamental parameters in the AE approach. AAEs were derived from Aethalometer data as the fit in a logarithmic space of the six absorption coefficients (470-950 nm) versus the corresponding wavelengths. The findings indicate that AAE(LF) can be robustly determined as the 1st percentile (PC1) of AAE values from fits with R-2 > 0.99. This R-2-filtering was necessary to remove extremely low and noisy-driven AAE values commonly observed under clean atmospheric conditions (i.e., low absorption coefficients). Conversely, AAE(SF) can be obtained from the 99th percentile (PC99) of unfiltered AAE values. To optimize the signal from solid fuel sources, winter data should be used to calculate PC99, whereas summer data should be employed for calculating PC1 to maximize the signal from liquid fuel sources. The derived PC1 (AAE(LF)) and PC99 (AAE(SF)) values ranged from 0.79 to 1.08, and 1.45 to 1.84, respectively. The AAE(SF) values were further compared with those constrained using the signal at mass-to-charge 60 (m/z 60), a tracer for fresh biomass combustion, measured using aerosol chemical speciation monitor (ACSM) and aerosol mass spectrometry (AMS) instruments deployed at 16 sites. Overall, the AAE(SF) values obtained from the two methods showed strong agreement, with a coefficient of determination (R-2) of 0.78. However, uncertainties in both approaches may vary due to site-specific sources, and in certain environments, such as traffic-dominated sites, neither approach may be fully applicable.

The source of nitrous acid (HONO) and its importance in island or marine environments are poorly understood. Herein, based on comprehensive field measurements at a hilltop on Corsica Island, we find an inverse diel variation of HONO with higher concentrations during daytime. Night-time HONO budget analysis indicates significant HONO formation during air mass transport along the hillside. In the daytime, although photosensitized NO2 uptake on the ground and NO + OH make considerable contributions (26% and 5%, respectively), a large part of HONO formation (67%, 320 pptv h(-1)) still cannot be explained with state-of-the-art parametrization. Nevertheless, photosensitized heterogeneous NO2 reactions are likely to account for the missing source, due to underestimation of the source by typical parametrizations at low NO2 levels. Furthermore, we demonstrate a significant role of HONO formation as a OH primary source at this island site, with a OH production rate exceeding one-fourth of that of O-3 photolysis. Our findings underscore a potential role of heterogeneous surface reactions in the oxidizing capacity of the island environments.

It is important to study aerosols and their origins, as they pose various negative health and environmental impacts. In this study, we combined year-long datasets from 15 different countries with Trajectory Statistical Methods (TSMs) for the first time at this comprehensive scale. We found possible source regions and seasonal variations of various particulate matter (PM) components in Europe, including total organic aerosol (OA), biomass burning OA (BBOA), oxygenated OA (OOA), ammonium (NH4), nitrate (NO3), and sulphate (SO4). We found that for all of the studied components, Eastern Europe was among the highest contributors. For NO3, other important source regions were Northern France and the Benelux, while for SO4 there were significant contributions from the Mediterranean region. We also compared our measurement-based model with simulated concentrations of an atmospheric chemistry transport model (CAMx). We observed a satisfactory agreement in regions where we had sufficient coverage with air pollution monitoring stations. The main deviations for OA were found around the Po Valley, where CAMx consistently estimated higher concentrations, while the TSM analysis did not highlight it as a hotspot because long-term monitoring datasets in this region are lacking. CAMx also underestimated the concentrations around Poland, mainly from residential burning. Our results provide opportunities to refine European emission inventories and deliver valuable information on long-range transported air pollutants. This work suggests that policies mitigating air pollution in Eastern Europe and the Benelux could help improve overall air quality in entire Europe more efficiently.

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).

Today, around 7 million deaths a year worldwide are linked to air pollution. Moreover, urban planning projections show that by 2050, there will be 2 billion more people in cities, further increasing the contribution of cities to rising CO2 emissions. In addition, this could lead to health problems linked to deteriorating air quality. Our study focuses on the city of Paris and the Ile de France region, where the main pollutant and CO2 emission sectors are road traffic and the residential sector. Depending on the emission sector, there is co-emission between pollutants and greenhouse gases which can be used to link atmospheric observations of those components to a particular sector. This so-called multi-component atmospheric approach can therefore complement the information provided by sectorial inventories. This study uses data measured on the Saclay peri-urban site to analyze long-term time series of CO2 and pollutants with meteorological conditions and surface emissions. At Saclay, the ACTRIS SIRTA station measures reactive gases, and aerosol while the ICOS tall tower measures greenhouse gases. The two stations are not co-located but are only 2 km apart. The compounds chosen for this study are CO2, CO, CH4, from the ICOS tower, and NOx, O3, and black carbon (BC) from the SIRTA site. The BC has been separated into wood burning (BCwb) and fossil fuel (BCff) contributions. The data from the two stations cover more than ten years of measurements for all the compounds mentioned. Since Saclay is located about 20km from southwest Paris, it was necessary to distinguish between two geographical sectors. An urban sector with the main contribution from Paris and a rural sector for comparison with concentrations close to background levels. The two sectors account for more than 44% of total data. The diurnal cycles of the studied compounds show similar patterns in the urban and rural sectors but with very contrasted amplitudes. The NO2, BCff, and CO cycles in the urban sector are strongly driven by the traffic with morning and evening peaks corresponding to the rush hours. On the other hand, BCwb diurnal cycle peaks mainly in the evening as expected with the timing of the residential heating, and contrary to other species the amplitudes of the maximum are higher in the rural sector. This can be explained by the larger use of wood burning in the rural areas, whereas it is strictly regulated in Paris. The diurnal cycle of CO2 peaks in the morning mainly due to photosynthesis, but a share comes from emissions from transport or the tertiary sector. Species ratios, such as CO/CO2 and NO2/CO2, are calculated for further study and comparison with emission inventories. The study will also include analysis of seasonal cycles and long-term trends for the two geographical sectors.