Boreal forests are a major source of biogenic volatile organic compounds (BVOCs), which undergo atmospheric oxidation and contribute to the formation of secondary organic aerosol (SOA) and cloud condensation nuclei. Clear-cutting, a common forest management practice involving the uniform removal of most or all trees within a designated area, can substantially alter biosphere–atmosphere interactions. In Sweden, approximately 2% of the managed forest area is harvested annually.Here we present results from continuous observations conducted from 2020 to the present at the Norunda ACTRIS and ICOS research station in the Swedish boreal forest, where a clear-cutting event occurred in 2022 surrounding the main measurement tower. This event provided a unique opportunity to investigate the short- and long-term impacts of forest clear-cutting on atmospheric composition.Our results show that clear-cutting significantly altered BVOC concentrations. While enhanced emissions of terpenes were expected, we also observed unexpectedly elevated concentrations of aromatic compounds, indicating that stressed boreal forests may represent an important source of aromatics. Source apportionment analysis reveals the emergence of new VOC sources during and after cutting, highlighting a more complex response of VOC emissions to forest management than previously recognized. Post-cutting factors further suggest a persistent, long-term influence on atmospheric composition. In addition, a chemical box model is used to simulate VOC oxidation processes under different clear-cutting scenarios, providing further insight into the underlying chemical mechanisms.

Civil aviation and airports have been shown to be important sources of both Ultrafine particles (UFPs) and Volatile Organic Compounds (VOCs) in urban areas1. UFPs are a major air quality concern because their small diameter (< 100 nm) allows them to reach the lungs’ alveolar regions causing adverse health effects. The aviation emission profile from the USA’s Environmental Protection Agency includes 15 hazardous VOCs, such as benzene and numerous carcinogenic Polycyclic Aromatic Hydrocarbons (PAHs)2. To assess the impact of UFPs and VOCs emissions from aviation on nearby air quality, two intensive one-month measurement campaigns of gaseous and particulate matter were performed in November 2022 and August 2024, 1 km downwind of Zürich Airport. The results indicate that high UFP number concentrations up to 300 000 cm⁻³ originate solely from aircraft operations, as shown by the similar diurnal profiles between air traffic movements and UFPs concentrations in Fig. 1a. These emissions are either advected downwind of the airport or mixed downward during aircraft landing overpasses. Using Positive Matrix Factorisation (PMF) on the VOCUS Proton Transfer Reaction Mass Spectrometer (PTR-MS) data, a factor containing naphthalene species and several alkanes with m/z > 100 (Fig. 1 c) has been attributed to VOCs aviation-related emissions. This is further supported by the co-increase of its time series with UFPs temporal evolution (Fig 1.b). However, when the site is not downwind and under the influence of landing overpasses, only UFPs concentrations increased, rather than the VOCs aviation-related factor (Fig. 1a), highlighting landing overpasses as a major source of UFPs but not of VOCs. This contrast likely results from lower engine thrust during taxiing at the airport than during landing overpass, which produces more VOCs due to reduced combustion efficiency3. At this stage, we cannot exclude a contribution of VOC emissions from engine refuelling. Future work will investigate the formation and evolution of VOCs in aviation plumes and their potential role in UFPs formation and growth. The widespread presence of UFPs and the co- emission of VOCs poses health concerns for communities near airports that regulators should address.Figure 1: 10-minutes averaged a) Diurnal cycle of air traffic at Zürich airport, UFPs number concentration ntotalPM , and VOCs aviation emissions when the measurement site was downwind of Zürich airport during the fall 2022 measurement campaign and b) 10-days time series of the same variables. C) Factor profile of the VOCs aviation emissions determined by a source apportionment on the VOCUS PTR-MS data.This work was supported by the Swiss Federal Office of Civil Aviation (SFLV 2020-080). We acknowledge the support from ZHAW, EMPA, Frithjof Siegerist (SRTechnics), and the City of Kloten. (1) Masiol, M.; Harrison, R. M. Aircraft Engine Exhaust Emissions and Other Airport-Related Contributions to Ambient Air Pollution: A Review. Atmos. Environ. 2014, 95, 409–455. https://doi.org/10.1016/j.atmosenv.2014.05.070.(2) US EPA, O. Organic Gas Speciation Profile for Aircraft. https://www.epa.gov/regulations-emissions-vehicles-and-engines/organic-gas-speciation-profile-aircraft (accessed 2026-01-12).(3) Anderson, B. E.; Chen, G.; Blake, D. R. Hydrocarbon Emissions from a Modern Commercial Airliner. Atmos. Environ. 2006, 40 (19), 3601–3612. https://doi.org/10.1016/j.atmosenv.2005.09.072.

Aircraft emissions are a significant source of particulate matter (PM) and ultrafine particles (UFP) during takeoff, landing, taxiing, and idling, degrading air quality near airports. With air traffic projected to grow by 4.2% annually, doubling pre-pandemic levels by 2040 (IATA, 2023a), the environmental and health implications are profound. Increased PM and UFP has been linked to respiratory and cardiovascular diseases and airports contribute to primary and secondary PM, affecting urban and regional air quality, with studies showing impacts extending up to 18 km downwind from major airports like LAX (Hudda et al., 2012).The Aviation Plume PROPeRtIes AT Point of Exposure (APPROPRIATE) project investigates aircraft emissions at Zürich Airport, Switzerland’s largest. The project integrates laboratory and test cell measurements with field campaigns to bridge critical knowledge gaps in understanding the influence of aviation on local/regional air quality and human health. As part of this initiative, an intensive, month-long measurement campaign was conducted in the fall of 2022, approximately 1 kilometer east of the airport (downwind side), where a specialized container equipped with state-of-the-art instrumentation was deployed. Key measurements included LTOF-AMS (organic and inorganic composition), EESI-LTOF (molecular-level organic aerosol composition), and VOCUS-PTRMS (organic gases) to sample the complex emissions generated during aircraft operations.LTOF-AMS source apportionment PMF results from the 2022 campaign, resolved nine factors: two OOA factors, one COA, one HOA, one NOA, two BBOA factors, one organic nitrogen–rich factor, and one event-related factor, providing an overview of the dominant PM₂.₅ components and source influences. Several of these factors show signatures consistent with airport-related emissions, indicating a substantial impact of airport activities on local air quality. Within the organic aerosol fraction, fragments associated with aircraft lubrication oil are observed. Complementary measurements from EESI, VOCUS, and other instruments will be used and distinguish airport emissions from other anthropogenic and biogenic sources.ReferencesIATA (2023a), Global Outlook for Air Transport.Neelakshi Hudda, Scott A. Fruin, Environmental Science & Technology 2016 50 (7), 3362-3370Zhenhong Yu, Scott C. Herndon, Luke D. Ziemba, Michael T. Timko, David S. Liscinsky, Bruce E. Anderson, and Richard C. Miake-Lye, Environmental Science & Technology 2012 46 (17), 9630-9637

Keywords: Mass Spectrometry, Real-Time, Trace Elements, Source Apportionment, MobileDetermination of the elemental composition of airborne nanoparticles and micro-particles is essential to understand the source(s) of these particles and also to predict potential health effects.1 The most common approach to measure the metal content of air is to collect samples on filters and then analyze digests by ICP-MS; however, this strategy offers poor time resolution (e.g. days) and only provides bulk element composition information. To understand the spatiotemporal characteristics of the emission of metal-containing aerosols, which is key to assessing exposure, real-time analysis strategies are essential. Here, we report on the development of a microwave induced plasma time-of-flight mass spectrometer (mipTOF) used for the direct analysis of metal-containing airborne particles.The mipTOF is a field-deployable trace-element mass spectrometer. It uses a nitrogen-sustained high-power plasma (MICAP, Radom Instruments)2, 3 to quantitatively vaporize and atomize aerosols with sizes from the ultrafine to PM10. Singly charged atomic ions are generated in the plasma with high efficiency (up to 99%), and then extracted into the mass spectrometer, where they are sorted according to mass-to-charge ratio and recorded. Ambient air is sampled into the plasma via a concentric pneumatic nebulizer set up as a Venturi pump5 at flowrates from 100-200 cm3/min. With the mipTOF, concentration LODs range from 10 ng/m3 (potassium) to 0.05 ng/m3 (lead) with a time resolution of 10 seconds. The high-sensitivity, high-speed metal-aerosol measurements possible with mipTOF enable new research into real-time spatiotemporal analysis of metals in air. We will report on the use of the mipTOF in mobile lab measurements in Switzerland and Massachusetts, USA. In these measurements, we identified several unique sources of airborne metals, including emissions from automotive brake wear, trains, metal-plating industries, cement manufacturers, and light aircraft. In addition to presenting data from these campaigns, we will discuss aspects of instrument design and operation, including power and size requirements, calibration strategies, and instrumental figures of merit.References:(1) Daellenbach, K. R. et al. Nature 2020, 587 (7834), 414-419.(2) Jevtic, J.; Menon, A.; Pikelja, V. PCT/US14/24306, 2015.(3) Schild, M. et al. Analytical Chemistry 2018, 90 (22), 13443-13450.(4) Nishiguchi, K.; Utani, K.; Fujimori, E. J. Anal. Atom. Spec. 2008, 23 (8), 1125-1129.

Particulate matter (PM) pollution at urban traffic sites reflects a complex mixture of exhaust and non-exhaust traffic emissions together with contributions from other urban sources such as biomass burning, cooking, secondary aerosol formation, and natural inputs. Understanding the temporal variability and seasonal evolution of these sources is essential for designing effective mitigation strategies, yet it is often constrained by traditional offline source apportionment methods. Real-time source apportionment (RT-SA) offers the ability to continuously resolve PM sources and track changes in their chemical composition at high time resolution.This study presents results from the MI-TRAP 2025 measurement campaign conducted at a single urban traffic site in Athens, Greece, within the framework of the EU Horizon Europe MI-TRAP project. The site was strongly influenced by road traffic emissions while simultaneously capturing the full spectrum of PM sources typically present in an urban traffic environment, including residential biomass burning, cooking activities, secondary aerosol, and natural sources.An integrated ACSM–Xact–Aethalometer (AXA) system was deployed in combination with the SoFi RT software to perform real-time PM source apportionment. The system provides simultaneous measurements of organic aerosol composition, elemental concentrations, and black carbon, enabling detailed characterization of both primary and secondary PM sources. Traffic-related emissions dominated the PM mass, while secondary components accounted for a significant percentage of the total PM.A rolling-window RT-SA approach was applied to capture temporal and seasonal changes in source profiles and contributions. This analysis revealed distinct seasonal variability in the chemical composition of several sources, particularly in biomass burning. Changes in elemental markers and organic aerosol signatures reflected shifts in fuel use, atmospheric processing, and driving conditions between seasons. The rolling-window methodology proved essential for resolving these evolving source characteristics, which would be obscured in a single static source apportionment model.The SoFi RT framework enabled continuous and near-instantaneous source apportionment with automated data processing. Comparison between real-time results and an optimized offline approach showed good agreement, confirming the robustness of the real-time methodology. Overall, the MI-TRAP Athens campaign demonstrates the capability of real-time source apportionment combined with rolling-window analysis to provide new insights into the seasonal dynamics and chemical evolution of PM sources at urban traffic sites.

Open biomass burning across different regions of the world is a major source of gaseous and fine-mode particulate species emitted into the atmosphere. Post-monsoon crop residue fires of North-West (N-W) India continues to have significant contribution to global burned area estimates (GloCAB product – Hall et al., 2024) and air quality impact in downwind urban cities of Indo-Gangetic Plain (IGP) including megacity Delhi. Yet, detailed in-situ observations of the fire smoke’s evolution and emission characteristics near the source are lacking and largely remain uncertain. We conducted STUB-BURN (Stubble Burning emissions study) measurements characterizing speciated fine particulate matter and related gaseous species via a mobile research platform in rural Punjab from 27 Oct to 18 Nov 2023. We observed rapid oxidation of OA with more than half dominated by oxygenated form of OA even in near-source field sampling conditions. By combining organics, metals, and black carbon (BC) in source apportionment technique, roughly ~ 50% is attributed to ongoing crop residue burning. Further, varying contribution of primary and aged OA factors were found in identified nine individual plume events. However, the dilution corrected enhancement ratio of OA w.r.t CO shows no net increase or decrease in mass enhancement with increasing O:C values as an indicator of ageing. Emission factors (EFs) of 17 species are calculated and their variability with global averages used in global fire emission estimates for this region are highlighted. Broadly, obtained EFs under open field scale combustion conditions for major species are up to three-fold lower than average estimates from widely used fire emission inventories. Overall, this study reinforces the need to account for fire characteristics that govern subsequent emissions to represent regional contributions more accurately within global emission estimates.

In today's rapidly evolving society, the sources of atmospheric particulate matter (PM) emissions are shifting significantly. Stringent regulations on vehicle tailpipe emissions, in combination with a lack of control of non-exhaust vehicular emissions, have led to an increase in the relative contribution of non-exhaust PM in Europe. This study analyzes the spatial distribution, temporal trends, and impacts of brake wear PM pollution across Europe by modeling copper (Cu) concentrations at a high spatial resolution of ∼250 m which is a key tracer of brake-wear emissions. We integrated coarse-resolution brake-wear Cu from CAMx chemical transport model and high-resolution land use data into a random forest (RF) model to predict Cu concentrations at ∼250 m over whole of continental Europe. The RF model was trained using an unprecedented dataset of over 50,000 daily Cu measurements from 152 sites. It corrected CAMx underestimation and downscaled Cu to a higher spatial resolution. In validation, the model showed robust spatial and temporal prediction with good Pearson's correlation coefficients of 0.6 and 0.7, respectively. We generated 10 years (2010-2019) of daily Cu concentrations over Europe, revealing spatial patterns aligned with urbanization and road networks, with peaks in cities and lower values in rural areas. Temporal trends reveal that Cu concentrations generally peak on weekdays and in winter. Despite a decline in PM across Europe over decades, Cu concentrations showed no decrease in many cities from 2010 to 2019. Cu levels are strongly correlated with population density with more than 12 million Europeans exposed to levels exceeding 40 ng/m3, equivalent to around 1 μg/m3 of total PM10 from brake wear. Our findings highlight the need for expanded metal measurement for non-exhaust tracers for a better understanding of the health relevance of PM composition including Cu, and more effective regulations of non-exhaust PM emissions as included in EURO 7 vehicles.

INTRODUCTION:Enhancing paediatric asthma diagnosis is crucial. Molecular analysis of exhaled breath is a rapidly evolving field aimed at harnessing established and innovative technologies for clinical applications. This study evaluates the feasibility of using online proton-transfer-reaction mass spectrometry (PTR-MS) to identify distinctive breath signatures in children with allergic asthma. METHODS:Exhaled breath samples of 81 children (41 with allergic asthma and 40 healthy controls) were analysed using the Vocus CI-TOF mass spectrometer (Tofwerk AG, Switzerland), with mass spectra acquired in H3O+ and NH4 + ionisation modes. Significant mass-to-charge (m/z) features were extracted using the Wilcoxon rank-sum test. Molecular identification was conducted using two-dimensional gas chromatography time-of-flight mass spectrometry (GCxGC-Q-TOF). RESULTS:Statistical analysis revealed 89 significant m/z features associated with paediatric allergic asthma, 66 in H3O+ mode and 23 in NH4 + mode. Supervised machine learning achieved an average accuracy of 74.7% in distinguishing between the groups. GCxGC-QTOF analysis identified a subset of significant features, including four previously reported asthma predictors in breath analysis studies. 16 novel asthma predictor candidates were additionally detected, including 7 likely endogenous, 4 unknowns and 3 exogenous. The main group of breath metabolites was structurally related fatty acids, methyl esters and aldehydes, including four known biomarkers of lipid peroxidation. CONCLUSION:Our findings demonstrate the suitability of PTR-MS for real-time breath analysis in paediatric populations. Moreover, the identification of distinct breath signatures exclusive to allergic asthma in children suggests the potential of leveraging such technology for non-invasive diagnostic applications.

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.

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.

The chemical composition of atmospheric aerosols is frequently investigated offline via mass spectrometry techniques. The established offline aerosol mass spectrometry (Off-AMS) uses water for extracting airborne particulate filter samples, efficiently detecting water-soluble constituents of organic aerosols (OA), which are prevalent in secondary OA (SOA), oxygenated primary OA (POA), and aged POA. Consequently, sources with substantial water-insoluble fraction may be undetectable via Off-AMS or be subject to increased uncertainties due to the absence of useful markers (e.g., polycyclic aromatic and aliphatic hydrocarbons). This potentially compromises the investigation of primary and less-aged OA from diverse anthropogenic sources (e.g., traffic, coal combustion, waste burning, tire wear, among others). Here, we present a new analytical method that combines Off-AMS with organic solvent-based sample extraction (termed: SOff-AMS) to extract and quantify both aged and fresh aerosols simultaneously. Ultrahigh-purity methanol and high-purity acetone were used, alongside water as a reference, and the extracts were reaerosolized to be analyzed via a high-resolution time-of-flight aerosol mass spectrometer (ToF-AMS). Multiseason airborne particulate matter (PM) filter samples collected in urban and rural environments were used in these tests. The organic solvents extracted substantially higher fractions of organic carbon, which for winter samples ranged from 45 to 85% of the total organic carbon in comparison to 12-40% in water alone. The AMS spectra of samples extracted in organic solvents showed significantly increased contributions from OA fragments that are known tracers of fresh and aged emissions. These included small and polycyclic aromatic hydrocarbons and oxygenated and reduced nitrogen-containing fragments that were enhanced over a broad range of factors (1.2-50). A comparison with an online quadrupole aerosol chemical speciation mass spectrometer (Q-ACSM) in Krakow showed highly similar spectra, demonstrating that SOff-AMS-based offline measurements can provide very similar information as the online data. Future SOff-AMS-based source apportionment could identify air pollution sources more comprehensively regardless of sampling locations, particle sizes, and seasonal conditions, especially in complex urban areas with both primary and secondary source contributors.

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.

Air pollution, particularly from particulate matter (PM), poses serious public health and environmental risks, especially in urban areas. To address this, accurate source apportionment (SA) of PM is essential for effective air quality management. Traditional SA approaches often rely on offline data collection, limiting timely responses to pollution events. SA applied on data from online techniques, especially with high temporal resolution, is advantageous over offline techniques, enabling the study of the diurnal variability of emission sources and also the study of specific events. Recent technological advancements now enable real-time SA, allowing continuous, detailed analysis of pollution sources. This study presents the first application of the ACSM-Xact-Aethalometer (AXA) setup integrated with SoFi RT software for real-time source apportionment of PM in Athens, Greece. The AXA setup integrates chemical, elemental, and black carbon (BC) data streams, covering a broad spectrum of PM components and capturing a comprehensive representation of PM sources in an urban environment. SoFi RT handles data from the AXA instruments as separate inputs within a single matrix, placing them in distinct diagonal blocks. Each main instrument's data (ACSM, Xact) is processed independently, with the model applying instrument-specific constraints and generating separate source factors, effectively performing two parallel source apportionments in a single run of the ME-2 solver. Equivalent sources identified across the two instruments are then combined post-analysis to provide a unified interpretation of source contributions. The apportionment of BC to BCsf and BClf (solid fuel and liquid fuel) can be performed in either of the main instrument experiments and does not require dedicated processing. The results demonstrate that traffic-related emissions are the largest contributors to PM, with significant contributions from secondary species such as sulfate, nitrate, ammonium, and secondary organic aerosols, which together accounted for approximately 57 % of the PM mass. Primary sources such as biomass burning and cooking contributed around 10 % each, with natural sources like dust and sea salt comprising the remainder. The SoFi RT software is employed for continuous SA, offering automated analysis of PM sources in near real time (minutes after the measurements). Our findings demonstrate that this setup effectively identifies major pollution sources. This work underscores the AXA system's potential for advancing urban air quality monitoring and informs targeted interventions to reduce PM pollution.

Atmospheric fine particulate matter (PM2.5) has been acknowledged to exert adverse health effects through reactive oxygen species (ROS) initiated oxidative stress. Oxidative potential (OP) is a chemical parameter that reflects the abilities of PM to generate ROS and deplete antioxidants as well. Dithiothreitol (DTT) method was used in this study to measure the OP activity (described as OPDTT). Some redox metals have been demonstrated to possess strong OPDTT activities. However, it is challenging to estimate the contribution of each metal in PM2.5 to OPDTT by chemical method. In this study, the quantitative contribution and ranking of 22 kinds of metals/metalloids to OPDTT were proposed by means of machine leaning models based on hundreds of Beijing PM2.5 samples. Among several combined models, the combined multiple linear regression (modified version) and weighted least square model emerged as the most effective. It was revealed that not only well-known metals such as Mn and Cu were important, but also some less-recognized metals like Tl made significant contributions. The modeled results presented in this paper encompass the contributions of most metals to OPDTT and are conducive to a comprehensive understanding of the health effects of PM2.5.

Organic aerosols (OA) are a main component of PM2.5 (20-90%), which contains thousands of compounds. Thus, the formation processes and sources of OA remain poorly understood. In this study, the seasonal and diurnal variabilities of submicron PM chemical composition (both inorganic and organic aerosols) and OA sources were characterized by aerosol mass spectrometry (AMS) and an aethalometer at a rural background site in the Czech Republic (National Atmospheric Observatory Kosetice - NAOK) from January to October 2019. The effects of meteorological conditions and local, regional, and long-range atmospheric transport influences in Central Europe were also studied. The overall average submicron PM concentration was 9.26 +/- 5.88 mu g m-3. Using positive matrix factorization (PMF) analysis, we identified four OA factors for summer and five for all other seasons. Three factors were associated with primary sources of OA (POA): hydrocarbon-like OA (HOA), biomass-burning OA (BBOA), and OA from coal burning (CCOA, absent in summer), the CCOA factor enabling a better description of the residential heating effect on the background station in Central Europe. Two remaining factors represented oxygenated OA (OOA) sources: less oxidized OOA (LO-OOA) and more oxidized OOA (MO-OOA).Higher pollution episodes of submicron PM and all OA sources were predominantly associated with continental air masses. The effect of dispersion conditions, as assessed by the ventilation index (VI) and not yet been studied at a rural background site, proved to be a critical factor. The extension of the number of primary factors to include CCOA in PMF analysis, together with the reflection of the influence of seasonality, air mass origin and changes meteorology, especially dispersion conditions, has elucidated the origin and fate of OA in the atmosphere at type of European background stations.

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.

Biomass burning organic aerosol (BBOA) particles are a major contributor to atmospheric particulate matter, with various effects on climate and public health. Quantifying these effects is limited by our understanding of the BBOA particles' evolving chemical composition during atmospheric aging, driven by their exposure to atmospheric oxidants. This study explores the role of ozone (O3) as an atmospheric oxidant in processing primary BBOA particles. We exposed particulate emissions from beech, spruce, and pine wood fires to O3 in an oxidative flow reactor, monitoring their chemical evolution using high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and extractive electrospray ionization time-of-flight mass spectrometry (EESI-ToF-MS). We found that the oxidative state of the particles increased with O3 exposure, as shown by the consistent, albeit minor, rise in O/C ratios. Analysis of the EESI-ToF-MS data revealed specific molecular groups containing 18 and 20 carbon atoms, likely mainly abietic, linoleic, and oleic acids, as highly reactive towards O3 and driving the increase in oxidative state. At higher relative humidity, increased oxidation and loss of reactive species indicate that enhanced O3 diffusion into particles allows the ozonolysis to progress further, highlighting humidity's role in overcoming diffusion barriers that limit ozonolysis in dry conditions. This study provides qualitative insights into the oxidative processing of primary BBOA particles in different phase states, presenting O3 as a selective oxidant. Further research could focus on quantifying the progression of the ozonolysis, in particular, the change in diffusion rates depending on relative humidity conditions or particle sizes.

Organic aerosols (OA) are a major component of Arctic aerosol mass and influence the region's radiation budget, yet their sources and physicochemical properties remain largely unknown. We investigate OA sources and climate-relevant characteristics over the central Arctic Ocean in spring and summer, by applying positive matrix factorization to aerosol mass spectrometry data from two ship-based expeditions (2018 and 2020), complemented by total and interstitial aerosol measurements during fog periods. Six distinct OA factors were identified: haze-related OA, Arctic oxygenated OA, two mixed-OA types resembling biomass burning and primary marine OA linked to warm-air intrusions, marine OA, and hydrocarbon-related OA. Seasonal transitions strongly shaped OA composition. Following polar sunrise, highly oxygenated OA, likely formed secondarily from photo-oxidized volatile organic compounds, became dominant. After the polar vortex collapsed in May, episodic spikes in marine OA from the marginal ice zone appeared, alongside a reduced influence from Eurasian anthropogenic sources. These transitions influenced OA oxidation state and related properties including volatility, acidity, and hygroscopicity, highlighting the role of the Arctic spring atmosphere as an active photochemical reactor. Overall, OA was highly oxidized, with particles activated in fog during summer showing even greater oxidation, suggesting that central Arctic OA can be highly cloud active.

Secondary organic aerosol (SOA), a major component of submicrometer particles, is critical to the climate and human health. SOA can form through nucleation of low-volatility organic compounds, following atmospheric oxidation, or by condensing these vapors onto existing particles. In either of these cases, the formation of SOA particles could be affected by atmospheric conditions (e.g., relative humidity (RH)) and particle liquid water content. This study examines the effects of RH on the formation and composition of SOA from dark α-pinene (C10H16) ozonolysis, as a canonical system, with or without ammonium sulfate (AS, (NH4)2SO4) seed particles across varying RH levels. Using online extractive electrospray ionization mass spectrometry, we identified monomers (C7-10) and dimers (C15-20) in the SOA with high chemical and temporal resolution. In both cases, high RH (>90%) promotes dimer formation in the particle phase, while they appear at the beginning of the experiment when (NH4)2SO4 seeds are present. The prompt increase in dimers in high RH seed containing experiments (60-65% dimers), which are absent at low RH (10%), suggests that intraparticle reactions are responsible for the dimer formation.