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 investigates the geochemical partitioning of arsenic (As) and antimony (Sb) in deposited dust collected from ventilation outlets of ten underground coal mines in Shanxi, with particular emphasis on potential mobility and associated ecological risks. Particle-size distribution, mineralogical characteristics, and bulk geochemical composition indicate that, relative to the corresponding parent coal, the deposited dust is dominated by fine mineral particles and enriched in clay minerals, carbonates, Fe-bearing phases, sulfate minerals, As, and Sb relative to the corresponding parent coal. Sequential extraction, TESCAN Integrated Mineral Analyzer (TIMA), and X-ray photoelectron spectroscopy (XPS) results further show that As is mainly partitioned into the residual (F8), surface-associated (F3), and acid-soluble fractions (F4), whereas Sb is predominantly concentrated in F8 and F4. These operationally defined fractions should be regarded as indicators of potential reactivity rather than direct measures of field mobility. A morphology- and scenario-adjusted risk index (MARI), derived from RI by assigning fraction-specific mobility-potential weights to As and Sb, shows that dust samples containing higher proportions of readily soluble and potentially reactive pools exhibit extremely high baseline relative risk (MARI = 781-813), and that this risk increases further under scenario-based acidic and low-oxygen conditions (MARI = 932-1016). Collectively, these findings suggest that environmental stressors may enhance the potential reactivity and relative ecological concern of As- and Sb-bearing dust, rather than directly demonstrating release at the field scale. Accordingly, effective management should prioritize focus on dust suppression, targeted environmental monitoring, and stabilization of reactive dust particles. Risk assessments should also incorporate operational partitioning and scenario-dependent sensitivity to improve the identification and prioritization of high-risk deposited dust from coal mine ventilation outlets.

BACKGROUND:Maternal air pollution exposure has been associated with impaired fetal growth, yet most studies have overlooked microenvironmental and personal exposures. OBJECTIVES:To examine associations between maternal air pollution exposure in key microenvironments (home, workplace, and commuting route) and fetal growth (birth weight and small for gestational age) using three modelling approaches and personal, home-indoor, and home-outdoor monitoring. METHODS:We used data from 1024 pregnant women in the Barcelona Life Study Cohort (2018-2021). Exposure to nitrogen dioxide, black carbon, and fine particulate matter (PM2.5) and its metallic constituents (copper, iron, and zinc) in each microenvironment were estimated using land use regression models, dispersion models, and hybrid land use regression-dispersion models, and combined with time-activity data to estimate total microenvironment exposures. Personal, home-indoor, and home-outdoor nitrogen dioxide concentrations were measured using passive samplers. Associations with birth weight and small for gestational age were evaluated using linear and logistic mixed-effects models. RESULTS:Higher nitrogen dioxide and black carbon exposure at home and in total microenvironments, estimated by land use regression, dispersion, and hybrid models, were associated with lower birth weight. Increased black carbon exposure in the workplace (hybrid model) and PM2.5 exposure both at home (land use regression model) and in total microenvironments (land use regression and dispersion models) were also associated with reduced birth weight, as were higher home, workplace, and total microenvironmental exposure to metallic components of PM2.5 in land use regression models, although higher workplace PM2.5-zinc was associated with higher birth weight in hybrid models. Higher personal and home-outdoor nitrogen dioxide exposure were further associated with reduced birth weight. Similar patterns were observed for small for gestational age. CONCLUSION:Maternal air pollution exposure was associated with impaired fetal growth. Home-based exposure estimates and short-term nitrogen dioxide measurements may serve as practical exposure proxies during pregnancy.

Neuroimaging studies suggest that air pollution exposure interferes with brain functional development, potentially affecting long-term cognitive abilities. Early life may represent a critical window of vulnerability, yet most research has assessed exposure effects later in childhood, when postnatal influences accumulate. This study examined the early impact of prenatal exposure to airborne particulate matter (PM2.5) on the functional structure of local connections in the cerebral cortex of neonates. This population-based study was conducted in Barcelona between 2018 and 2021. PM₂.₅ exposure was estimated using land-use regression models incorporating time-weighted maternal mobility data across distinct environments. Resting-state functional MRI scans were obtained from 61 neonates at 29 days postnatally. Brain functional connectivity was assessed using local Iso-Distant Average Correlation (IDAC) measures across different spatial distances. Voxel-wise regression analyses were performed to examine associations between PM2.5 exposure and IDAC measures. Higher maternal PM2.5 exposure during gestation was associated with stronger local functional connectivity in the neonatal cortex, particularly in brain regions involved in sensorimotor function. In the premotor and supplementary motor cortices, these effects varied by spatial scale, showing stronger associations at longer local distances. The findings suggest that prenatal exposure to air pollution interferes with early brain functional organization. Notably, the observed connectivity changes could reflect both a delay in the segregation of primary sensorimotor areas and the acceleration of functional maturation in higher levels of the sensorimotor system. Longitudinal studies are needed to track how these exposure-related alterations evolve over time and their potential impact on developmental outcomes.

The highly variable spatial composition of desert soils remains a major source of uncertainty in estimates of dust radiative forcing. Existing soil mineralogy data sets used in climate models are limited by sparse ground observations and coarse spatial resolution. NASA's Earth surface Mineral dust source InvesTigation (EMIT) addresses this challenge by providing near-global maps of surface mineralogy across dust source regions using imaging spectroscopy at similar to 60 m spatial resolution. Here, we evaluate EMIT 60-m mineral mass fraction estimates against ground-based measurements derived from X-ray diffraction and iron wet chemistry analyses of soil samples collected from active dust sources in the Sonoran and Mojave Deserts of Southern California. We also compare particle size distribution measurements with EMIT 60-m soil texture estimates used for estimating mineral mass fractions, including spectrally inactive and thus unretrieved minerals, mainly quartz and feldspar. Both EMIT 60-m mineralogy and soil texture estimates exhibit limited variability compared with ground measurements. Ground measurements reveal a strong Spearman correlation (r s = 0.84) between the mass fraction of minerals unretrieved by EMIT and the sand-size mass fraction. We apply empirical adjustments that constrain the unretrieved mineral fraction using sand content and rescale iron oxide mass fractions, significantly improving agreement with ground mineralogy, including an increase in the iron oxide correlation from 0.15 to 0.68. We further develop an empirical method to partition unretrieved minerals into quartz and feldspar. Together, these empirical adjustments provide an efficient approach to enhance EMIT 60-m mineralogy estimates and better represent fine-scale spatial variability in soil mineralogical composition.

Atmospheric aerosols play a critical role in air quality, human health, and climate. Ultrafine particles (UFPs) are of particular concern due to their strong health impact and their dominant contribution to particle number concentrations. While receptor models such as Positive Matrix Factorization (PMF) are widely used to estimate the contribution of various sources to aerosol mass, their application to aerosol number remains challenging. In this work, we apply PMF to the number size distributions predicted by a three-dimensional chemical transport model. The approach used is the same as that used for the PMF analysis of measurements, but in this case the true source contributions are known and the PMF method results can be evaluated.PMCAMx-UF is a three-dimensional chemical transport model that simulates aerosol number and mass distributions by explicitly resolving key atmospheric processes, including advection, deposition, gas-phase chemistry, nucleation and coagulation. The model is applied over Europe at 36 x 36 km resolution, with increasing resolution over Athens where a 1 x 1 km grid is used. The aerosol number distribution is described using 42 sections. The contribution of the various sources to aerosol number according to PMCAMx-UF is quantified using the approach of Posner and Pandis (2015) for a summer and a winter month. Positive Matrix Factorization (PMF) was also used to apportion sources of particle number size by decomposing the PMCAMx-UF simulated size distributions into factors and calculating their time-resolved contributions.PMF seriously underestimated the contribution of new particle formation to particles larger than 10 nm in Athens during the summer, N10. PMF estimated that 25% of N10 was due to new particle formation, while this process was actually responsible for 62% of the N10. At the same time, PMF overestimates the contribution of traffic-related sources (57% compared to 13%). During winter, PMF does a reasonable job quantifying the role of new particle formation (17% versus the correct 22%) but still overestimates the role of traffic (71% compared to 34%). Posner, L. N., & Pandis, S. N. (2015). Sources of ultrafine particles in the Eastern United States. Atmospheric Environment, 111, 103–112. https://doi.org/10.1016/j.atmosenv.2015.03.033

There is a body of evidence on the risk to human health posed by the exposure to desert dust. However, results from epidemiological studies from different regions are inconsistent. Among possible causes of inconsistency is the scarcity of daily desert dust contributions to PM10 and PM2.5 levels recorded in populated areas in order to conduct rigorous epidemiological studies, but also the lack of robust and harmonized methodologies to deliver airborne desert-dust concentrations. This study evaluated the performance of the main statistical methods currently used by European countries to estimate the net load of desert dust on PM10 and PM2.5 levels during the occurrence of desert dust outbreaks. To this end, long-term data series (2010-2023) on PM10 and PM2.5 levels and composition obtained in a regional background (Montseny) and an urban background (Barcelona) monitoring site, in Northeast Spain, were used. The results identify the most appropriate method for determining the regional daily PM10 background concentration, excluding dust-days. This involves applying a moving 50th percentile with a 30-day time window to data from nearby regional background environments. Such PM10 background is essential for calculating the daily net dust contribution (PM10 net-dust). However, applying this procedure to data from urban or industrial environments causes an overestimation of PM10 net-dust values, which intensifies with higher local PM10 levels. On the other hand, in the case of low PM10 (net-dust) values(<3 mu g m(-3)) the relative errors are so high that it is not advisable to use these estimates of natural PM contributions to assess compliance with air quality standards. For PM2.5, however, the mineral dust content is much lower than for PM10. Consequently, applying the same methodology results in significantly greater relative errors in the PM2.5 net-dust estimates. In this case, it is also advisable to use PM2.5 data series obtained in regional background environments. However, if nearby regional background data is not available, the method can be applied directly to the evaluated urban or industrial datasets, but excluding traffic and industrial hotspots for the calculation of the PMnet-dust. In conclusion, accurately quantifying PMnet-dust is a complex issue, and it is necessary to continue improving current methods and developing new methods that allow for the most accurate estimation possible of daily desert dust contributions, especially for the low concentration ranges and for finer PM size fractions. The methodologies reported here are applicable to all regions affected by desert dust.

Mineral dust particles emitted from dry, uncovered soil can be transported over vast distances, thereby influencing climate and environment. Its impacts are highly size-dependent, yet large particles with diameters d(p)>10 mu m remain understudied due to their low number concentrations and instrumental limitations. Accurately characterizing the particle size distribution (PSD) at emission is crucial for understanding dust transport and climate interactions. Here we characterize the dust PSD at an emission source during the Jordan Wind Erosion and Dust Investigation (J-WADI) campaign, conducted in Wadi Rum, Jordan, in September 2022, focusing on super-coarse (10 < d(p )<= 62.5 m) and giant (d(p)>62.5 mu m) particles. This study is the first to continuously cover the full range of diameters from d(p)=0.4 to 200 mu m at an emission source by using a suite of aerosol spectrometers with overlapping size ranges. This overlap enabled a systematic intercomparison and validation across instruments, improving PSD reliability. Results show significant PSD variability over the course of the campaign. During periods with friction velocities (u(*)) above 0.22 m s(-1) (or similar to 3.3 m s(-1) threshold 4 m wind speed), the approximate threshold for local dust emission by saltation, both dust concentrations and the contributions of super-coarse and giant particles typically increased with increasing u(*), especially under neutral to unstable atmospheric stability conditions. These large particles accounted for about 90 % of the total mass concentration during the campaign. A prominent mass concentration peak was observed near d(p)=60 mu m in geometric diameter. While particle concentrations for d(p)<10 m showed good agreement among most instruments, discrepancies appeared for larger d(p) due to reduced instrument sensitivity at the size range boundaries and sampling inefficiencies. Despite these challenges, physical samples collected using a flat-plate sampler largely confirmed the PSDs derived from the aerosol spectrometers. These findings help to advance our understanding of the dust PSD and the abundance of super-coarse and giant particle at emission sources.

Abstract. Oxidative potential (OP) of atmospheric particulate matter (PM) is a metric of increasing scientific interest because it potentially links chemical particle properties to particle health effects. OP has been recently introduced as a recommended monitoring metric in the European Air Quality Directive. However, inconsistent protocols in the existing literature make it difficult to compare results across studies. Following a 2023 inter-laboratory comparison that focused on PM OP measured using the dithiothreitol (DTT) assay, this paper presents the findings and lessons learnt from a second inter-laboratory study focused on the ascorbic acid assay (OP-AA). In this study, twenty-six laboratories worldwide quantified OP of four PM filter samples and of one chemical compound to evaluate the entire analytical chain, including the extraction step, using a simplified OP-AA protocol. While most laboratories produced repeatable internal results when applying the simplified protocol, significant discrepancies between participants highlight the need for each laboratory to carefully evaluate deviations from the simplified OP-AA protocol. Over half of the 26 participants achieved satisfactory results, suggesting that the protocol is suitable for large-scale implementation. Beyond assessing performance, this work investigates technical, analytical, and mathematical refinements to measurement protocols. Building on the first DTT assay study, this second inter-laboratory comparison represents a significant step toward harmonizing OP assays, and provides specific recommendations to ensure consistent future measurements, ready to be applied in the new air quality directive EU 2024/2881.

Accurate calibration of dust concentration measuring instruments is essential for ensuring the reliability of industrial and environmental monitoring. This study systematically investigates the uniformity and stability of an advanced high-concentration dust calibration device through comprehensive verification experiments, including factory inspection, point-by-point concentration testing, long-term fixed-point measurement, and the evaluation for a wide-range concentration. The results demonstrate that the established device can achieve a spatial uniformity deviation of ±4.4% and a stability standard deviation of ±1.6%, both within the ±5% threshold specified by JJG 846-2015. Across a concentration range of 100-1000 mg/m3, although slightly higher repeatability values were observed at 500 and 600 mg/m3, most concentration levels remaining below the 5% reference criterion. These findings indicate that the calibration system can provide a homogeneous and stable dust-laden environment suitable for accurate and traceable calibration of dust concentration monitors. The research offers valuable insights for improving the metrological reliability of dust measurement technologies and contributes to the standardization of calibration practices in environmental and industrial applications.

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.

A low emission zone (LEZ) was implemented in the Barcelona metropolitan area in 2020, concurrently with the onset of the COVID-19 pandemic. We used a synthetic control method to estimate the impacts of the LEZ on NO2, PM2.5 and PM10 concentrations in Barcelona, accounting for the effects of COVID-19 and other underlying trends, in addition to meteorology and desert dust outbreaks. Results showed that the LEZ reduced annual mean NO2 concentrations by 7.6 & micro;g/m3 (95% CI: 6.7, 8.5 & micro;g/m3) at traffic stations, and by 4.5 & micro;g/m3 (95% CI: 3.7, 5.3 & micro;g/ m3) at urban background stations. The LEZ was less effective in reducing PM2.5 and PM10 concentrations. These results indicate that the LEZ in Barcelona was effective in reducing NO2 concentrations, while the effects on PM were more uncertain, probably due to the major secondary PM origin.

Mediterranean forests emit biogenic volatile organic compounds (BVOCs) that significantly influence atmospheric chemistry and ecosystem functioning. Due to high solar radiation and the important anthropogenic emission of atmospheric pollutants in the Western Mediterranean Basin, photochemical activity is enhanced, favouring the formation of ozone and secondary organic aerosols. We measured ambient mixing ratios of volatile organic compounds (VOCs) using Proton Transfer Reaction Mass Spectrometry (PTR-MS) at Montseny Natural Park, a Mediterranean forest 60 km from Barcelona, during summers 2021-2023. Surprisingly, during heat wave conditions, a toluene early morning peak of 0.23 (interquartile range 0.18) ppbv was observed. These early morning peaks were associated with prolonged high temperatures (approximately 5 degrees C higher as compared to non-peak days) and persistently higher vapour pressure deficit (approximately 877 Pa higher as compared to non-peak days), suggesting a potential link to vegetation drought stress. Cross-validation of measurements with Gas Chromatography-Mass Spectrometry (GC-MS) enhanced data reliability. Analysis of the meteorology and the variability of other atmospheric pollutants at the site, allowed to attribute these peaks to a local biogenic origin. These findings suggest that early morning toluene peaks could serve as an indicator of Mediterranean forest vulnerability to climate-induced stressors, with potentially more frequent and intense peaks occurring in the future.

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.

This study explores the vertical and horizontal recirculation mechanisms of tropospheric ozone (O3) in Barcelona utilizing WRF-CAMx simulations. We analyzed extreme O3 episodes that occurred in 2015, 2018, and 2019. These episodes significantly impacted public health in one of Spain's most populated urban areas. Key contributing factors include prior O3 accumulation, the weekend effect, Tramontana winds, and multiregional air mass transport. Our findings identified a recirculation mechanism based on the trajectory of polluted air masses. O3-enriched air masses affected the city in the early morning hours after being transported from higher altitudes. These insights enhance the understanding of O3 dynamics and may aid in predicting future pollution episodes in Barcelona and similar complex terrains.

Coal-bearing strata in the Ningdong Coalfield (western Ordos Basin) contain elevated concentrations of critical metals; however, the processes governing their enrichment remain incompletely understood. This study integrates zircon U–Pb geochronology, LA-ICP-MS elemental mapping, sequential chemical extraction, and mineralogical and bulk geochemical analyses to constrain the provenance, host phases, and enrichment mechanisms of Li, REY, and Ga in Late Carboniferous–Early Permian coals from the Ningdong Coalfield. The No. 5 and No. 9 coals are medium-ash bituminous coals significantly enriched in Li and rare earth elements and yttrium (REY) relative to world coal averages. Zircon U–Pb ages reveal distinct provenance for the two coal seams: the No. 5 coal records a dominant, proximal volcanic source linked to the Central Asian Orogenic Belt (CAOB) at ca. 298 Ma, whereas the No. 9 coal reflects mixed contributions from both the CAOB and the Alxa Block. Tonsteins and dispersed pyroclastic materials, interpreted as altered alkaline to peralkaline volcanic ash, constitute the principal source of critical elements. Titanium-bearing vermicular kaolinite formed through in situ alteration of volcanic ash exhibits the highest Li concentrations among the host phases, whereas REY are mainly concentrated in authigenic goyazite within phosphorus-rich horizons. Coal benches adjacent to tonsteins and partings display higher concentrations of Li, REY, and Ga than the associated non-coal lithologies, indicating leaching and redistribution of these elements by acidic diagenetic fluids. The weighted mean Li2O concentrations in coal ash reach 861 μg/g and 871 μg/g for the No. 5 and No. 9 coals, respectively, exceeding the proposed recovery threshold (800 μg/g); rare earth oxides (REO) contents locally attain ~5200 μg/g, substantially surpassing the cut-off grade (1000 μg/g); and Ga in the No. 9S coal ash averages 157 μg/g, exceeding the economic cut-off (100 μg/g). These results demonstrate that coupled alkaline volcanic ash input and acidic leaching exert the primary control on critical-element enrichment in the Ningdong Coalfield, suggesting that critical-element mineralization is regionally pervasive rather than locally confined.

BACKGROUND:Indoor air pollution is a major contributor to personal exposure to air pollution, yet accurately modeling indoor concentrations remains a major challenge in epidemiological studies. This issue is especially relevant since we spend a great portion of our time indoors. OBJECTIVE:To develop a two-stage model to estimate indoor nitrogen dioxide (NO2) concentrations. METHODS:The study was conducted in the Barcelona Life Study Cohort (2018-2021), a cohort of pregnant individuals in Barcelona, Spain. Indoor and outdoor NO2 concentrations were measured using passive NO2 samplers placed in participants' homes at weeks 12 and 32 of pregnancy. A total of 1695 indoor measurements and 1577 outdoor measurements were collected. In the first stage, we modeled the indoor-outdoor (I/O) NO2 ratio as a proxy for infiltration using a linear mixed-effects model with repeated measures using 1528 1-week integrated I/O NO2 measurements. In the second stage, we used a random forest algorithm to estimate weekly indoor NO2 concentrations covering the full pregnancy period, incorporating the predicted I/O ratios, a previously developed hybrid-model outdoor NO2 estimate, meteorological data, and home characteristics. RESULTS:The I/O NO2 ratio model showed moderate performance, with a cross-validated R2 of 0.27 under leave-one-subject-out (generalized model) validation and 0.70 under leave-one-observation-out validation (cohort performance). The indoor NO2 model achieved a 10-fold cross-validated R2 of 0.53 and RMSE of 1.28 µg/m3. Finally, we estimated indoor NO2 concentrations for each week of pregnancy using a quantile random forest (QRF) algorithm incorporating uncertainty estimation through prediction intervals. SIGNIFICANCE:The findings demonstrate that accounting for dynamic infiltration processes and individual-level behaviors improves the estimation of indoor NO2 exposure. IMPACT:Integrating indoor and outdoor NO2 measurements in a two-stage modeling framework and combining it with key indoor determinants improves indoor NO2 estimation.

Fine particulate matter (PM2.5) pollution poses a significant environmental risk to human health, ecosystems, and climate, both across Europe and globally. Ammonia (NH3), primarily emitted by agro-livestock activities, is a major precursor gas for secondary particulate formation; critically, however, it remains largely unregulated in European directives and is seldom monitored within conventional air quality networks. This study investigates the chemical composition, formation mechanisms, and potential sources of PM2.5 within a critical European agricultural region (Central Ebro Valley, CEV), recognized as a major atmospheric NH3 hotspot. NH3 concentration and PM2.5 chemical composition data were collected during two intensive campaigns spanning distinct winter and summer seasonal regimes. Results demonstrate that the study area is characterized by an NH3-saturated environment, which fundamentally controls PM2.5 formation dynamics and leads to severe regional pollution episodes. The high ammoniacal availability is identified as a key factor promoting the generation of high concentrations of Secondary Inorganic Aerosols (SIA), particularly during winter. Furthermore, the characteristic summer atmospheric dynamics, featuring high vertical dispersion driven by the intense development of the Planetary Boundary Layer (PBL), enhances the region’s role as an atmospheric NH3 reservoir for the wider Mediterranean Basin, establishing it as a major source area in Southern Europe. A preliminary source contribution analysis based on Positive Matrix Factorization (PMF) identified five factors with marked seasonality, confirming the unquestionable impact of agro-livestock NH3 emissions, biomass burning, mineral resuspension from arid land surfaces, and African dust intrusion during summer. This study underscores the critical importance of policy decisions targeting emission controls within the agricultural and livestock sectors to effectively mitigate PM2.5 levels locally and regionally.

Ultrafine particles (UFPs, <= 100 nm) are a major component of urban air pollution, influenced by source emission rates (of UFPs and precursors) and meteorological conditions, both of which vary over time. This study evaluates the effect of dispersion normalisation on UFP source apportionment and long-term trends in the urban background of Barcelona (NE Spain) over a 12-year period (2013-2024). Hourly particle number size distributions (PNSD) were normalised using the ventilation coefficient (VC), derived from boundary layer height and wind speed, and analysed with positive matrix factorisation (PMF) for cold (October-March) and warm (April-September) seasons. Seven factors were identified, with road traffic as the dominant contributor. Dispersion corrected (DC) traffic contributions averaged 57 % of particle number concentrations (PNCs), compared with 64 % in the uncorrected (UC) analysis, indicating overestimation of local sources when dispersion is not considered. Photonucleation and Nucleation growth showed higher DC contributions (37 % compared with 31 % for UC), consistent with midday occurrence under favourable mixing conditions. Trend analysis revealed significant decreases in PNCs (-4.4 % yr-1), strongest in the Aitken mode, likely linked to reduced traffic emissions following EURO 5/6 regulations, the introduction of diesel particulate filters (DPFs), and the implementation of Barcelona's low emission zone (LEZ). Traffic-related factors declined by-4.6 % yr-1, with the largest decrease in the diesel-related source. Nucleation growth and Regional-1 also declined, while Photonucleation and Regional-2 showed no clear trend. These findings highlight the value of dispersion normalisation in source apportionment and provide strong evidence of the effectiveness of emission control-policies in reducing traffic-related UFPs.