Highly oxygenated organic molecules (HOM) are formed via autoxidation during ⚫OH-initiated oxidation of α-pinene. We investigated the relative contributions of OH-addition and hydrogen (H)-abstraction to HOM formation from α-pinene photooxidation under varying nitrogen oxide (NO) conditions. HOM molecules were detected by a nitrate chemical ionization mass spectrometer (CIMS). In the absence of NO, C10H17Ox⚫ peroxy radicals and related termination products (e.g. C10H18Ox) dominated the HOM spectrum, accounting for > 70 % of total HOM. The presence of NO substantially altered HOM products, particularly by rapid formation of C10H15Ox⚫-related HOM, like C10H15NO8. The ratio of C10H15NOx to C10H17NOx increased from 0.34 to 0.84 as the RO2⚫ loss rate via reaction with NO increased from 0.18 to 1.06 s−1. Under high-NO conditions, C10H15Ox⚫-related HOM contributed up to 34 % to total HOM from α-pinene oxidation systems. The H-abstraction channel proved to be the source of C10H15Ox⚫-related HOM. Fuzzy c-means clustering indicated that C10H15Ox⚫-related HOM exhibited the fastest formation rate among the identified HOM groups, consistent with first-generation products. Comparison with pinonaldehyde oxidation, obtained by normalizing HOM yields to pinonaldehyde turnover, suggests that pinonaldehyde contributed ∼ 5 % of HOM in α-pinene systems, excluding secondary oxidation as the dominant source. Detection of C10H15NO4 under high-NO conditions by propylamine-CIMS indicates the formation of C10H15O3⚫ peroxy radicals, formed by alkoxy radical decomposition and six-membered ring opening in the H-abstraction channel. This study highlights the role of the H-abstraction pathway in ⚫OH-initiated α-pinene oxidation under NO-influenced conditions and provides new constraints on detailed HOM formation mechanisms.

Biogenic Volatile Organic Compound (BVOC) emissions account for more than 70% of VOC global emissions and play a significant role in atmospheric chemistry due to their high reactivity. These compounds are naturally emitted by trees as they are involved in ecological processes such as plant communication and defense against biotic and abiotic stressors. However, climate change has increased those stress factors for trees, altering both magnitude and composition of BVOC emissions. Once oxidized in the atmosphere, BVOCs contribute to the production of Secondary Organic Aerosols (SOA) which are involved in cloud formation, mitigating the Earth’s radiative balance and impacting air quality. Investigating how emission rates and compositions are impacted by such stressors is essential in understanding how atmospheric chemistry is affected.So far, most studies focused on coniferous trees and isolated stress factors, leaving broad-leaved trees and combined stress impacts understudied, although common. To address the gap, this study sheds light on the BVOC emissions from two common deciduous tree species in Europe: English oak (Quercus Robur), an isoprene emitter, and European beech (Fagus Sylvatica), a monoterpene emitter. They were successively exposed to herbivory feeding of gypsy moth larvae (Lymantria Dispar Dispar) then both herbivory and heat stress (up to ~40°C).To quantify BVOC fluxes, the emissions were sampled from a climate-controlled plant-chamber (SAPHIR-PLUS) by PTR-TOF-MS coupled to a fastGC. Temperature ramp experiments were conducted under each condition to study the temperature sensitivity of terpenoid emissions in response to the different stressors.Here, we present results from an intensive two-month campaign with focus on the main primary BVOC emissions. Preliminary results indicate that herbivory feeding increased monoterpenes emissions, but heat stress induced a stronger burst in emissions for both tree species, highlighting their role in plant defense response.

Forest ecosystems are increasingly stressed through heatwaves, drought periods, and other factors such as ozone pollution or insect infestations. These stressors have a profound impact on the emissions of biogenic volatile organic compounds (BVOC) from trees, which in turn influence aerosol formation and atmospheric oxidation cycles and thus feedback on the atmospheric cleansing capacity and climate change itself. While previous studies have investigated the impacts of specific stressors on BVOC emissions, analyses of combined stress effects are rare, even though the stressors seldomly occur in isolation. This study investigates the impact of heat and (nighttime) ozone stress, both individually and in combination, on BVOC emissions from two ecologically significant temperate tree species: European beech (Fagus sylvatica L.) and English oak (Quercus robur L.). In a climate-controlled chamber, both tree species were subjected to heat stress (38 +/- 3.3 degrees C) and ozone stress (similar to 120 ppb), separately and in combination. BVOC emission rates were measured using proton transfer reaction time-of-flight mass spectrometry, and the results were compared across pre-stress, heat, ozone, and combined heat-ozone conditions.Heat stress elicited the strongest emission increases of isoprene, monoterpene, and green leaf volatiles in both species, while ozone suppressed the emissions of most BVOCs. Combined stress led to non-additive responses different from those in single-stress scenarios. Both machine learning and positive matrix factorization analyses were performed to identify key VOC fingerprint markers that may be applied to identify stress-impacted emissions from field data, and both methods showed good agreement. The OH reactivity of the emissions, which serves as a measure for their atmospheric chemistry and ozone formation impacts, was consistently highest under heat stress for both species. However, nighttime ozone stress led to reduced OH reactivity of emissions (by 10 %-18 %).Our results underscore that the study of realistic combinations of stressors is crucial to understand future BVOC emissions and indicate that BVOC emissions could alter atmospheric chemistry and feedback with air quality and climate as heatwaves and pollutant-induced stress become more frequent due to climate change.

Atmospheric simulation chambers are one of the best available tools to study atmospheric processes, as they enable experiments under conditions that are both reproducible and well-controlled. 16 unique simulation chamber facilities are part of the distributed pan-European Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Their research focuses on fundamental gas-phase reaction kinetics, complex reaction mechanisms, aerosol formation and cloud chemistry, as well as other aspects of atmospheric processes. They use both simplified and complex air mixtures in their research. Results of chamber experiments enable the discovery of unknown chemical mechanisms and the determination of physicochemical parameters of atmospheric constituents. Simulation chambers are ideal for testing instruments and quality assurance of their data. The variability of their research capability is reflected in differences in the size (ranging from approximately 1-270 m3), the wall material, and the type of instrumentation used to measure physical parameters, gas-phase species, physicochemical properties of aerosol particles as well as cloud droplets and ice crystals. Most chambers in ACTRIS are indoors and use artificial light sources to initiate photochemical reactions while some chambers are located outside so that natural sunlight can be used. During experiments, steady state conditions may be achieved, the evolution of initial conditions may be observed, or expansion and mixing techniques may induce cloud formation. In this paper, the ACTRIS simulation chambers are described along with the quality control measures for carrying out experiments and reporting data. An overview of how users from the research community and industry can gain access to the ACTRIS simulation chambers and associated data centre is presented. Recent developments in the application of ACTRIS simulation chambers for answering current and future atmospheric research questions are discussed.

Predicting plant responses to changing climate, particularly to heat extremes and elevated near-ground ozone, is a key obstacle in robustly quantifying future biogenic volatile organic emissions (BVOCs) and understanding the climate feedback loop. Scaling BVOC emissions from leaf to landscape level and identifying stress-specific emission fingerprints require controlled-chamber experiments with sequential stress exposure that realistically mimic natural events.In a climate-controlled chamber, periodic stress exposures were applied to forest (Fagus sylvatica L., Quercus robur L.) and urban tree species (Castanea sativa Mill., Tilia cordata Mill.) during summer 2024 and 2025. The forest species were exposed to heat (~40°C) and nocturnal ozone stress (100-120 ppb), while urban species experienced heat stress (~40°C) and a 72h simultaneous ozone exposure (100-120 ppb). BVOC emission fluxes were measured using proton-transfer reaction time-of-flight mass spectrometry and compared across pre-stress, heat, and combined heat–ozone conditions.Heat stress strongly increased BVOC emissions, with urban tree species showing 2–8-fold increases in isoprene and ~3-fold increases in monoterpenes, along with elevated sesquiterpenes and green leaf volatiles. Forest species showed more selective heat-induced emissions, primarily in monoterpenes and green leaf volatiles. In contrast, combined ozone–heat stress following ozone exposure suppressed most BVOC emissions by 30–60%, largely independent of species, despite differing ozone treatments. The concurrent increase of methyl salicylate, a stress-alarm compound, emissions under combined stress compared to heat alone showed a non-additive physiological response. Heat stress consistently yielded the highest OH reactivity of BVOCs across all species and decreased by 10–30% following ozone-mixed heat exposure. A cross-investigation using machine learning and positive matrix factorization identified stress- and species-specific VOC fingerprints, with a good agreement.These multi-stress experiments provide mechanistic insight into stress-induced BVOC emissions and could improve parameterizations of BVOC emissions in Earth system and air-quality models under increasing pollution and heat events.

Secondary organic aerosol (SOA) form from the atmospheric oxidation of volatile organic compounds (VOC), and impacts both climate and human health. Chamber studies typically show significant decrease of SOA by nitrogen oxides (NOx), yet these experiments often neglect atmospherically relevant hydroperoxy radical (HO₂) levels. Here we investigate α-pinene photooxidation under low and high hydroperoxy-to-organic peroxy radical (RO₂) ratios with added NOx. While NOx reduces aerosol formation under both conditions, suppression is substantially weaker under high HO₂ conditions (33–55%) than under low HO₂ conditions (60–70%). Under high HO₂ conditions, enhanced formation of low-volatility monomers offsets enhanced fragmentation, yielding a more condensable product mixture despite higher bulk volatility. These results demonstrate that laboratory studies conducted under low under high HO₂ conditions likely underestimate secondary organic aerosol formation in NOx-influenced atmospheres.

Chemical ionization mass spectrometry (CIMS) offers high time-resolution measurements for diverse compounds, but atmospheric quantification remains challenging. Here, we combine a recently published method for determining the collision limit using a single reagent ion with a voltage scanning approach for assessing the relative sensitivities of diverse adduct ions. We used voltage scanning in a Multi-Reagent Chemical Ionization Mass Spectrometer (MR-CIMS) to assess ion-molecule adduct strength. The sensitivities to most detectable compounds were calculated based on this relationship using a collision-limit sensitivity of 13.87 +/- 0.69 ncps pptv-1 determined for alpha-pinene using the benzene channel. Following previously published work, the collision limit sensitivity of the other reagent ions used was assumed to be equal to that of the benzene channel and was further examined using the binding energy and measured sensitivity of nitrophenol in the bromide channel. Calibration of 13 molecules, including nitric acid, formic acid, and oxygenated VOCs, was performed to obtain a universal relationship between the sensitivities and the voltage at which the adduct signal halves (dV50). Quantification uncertainties stayed below 20 % for compounds with sensitivities above 5.69 and 5.30 ncps pptv-1 in bromide and iodide channels, respectively. High-level quantum chemical calculations indicated that the detected compounds predominantly form hydrogen-bonded clusters with bromide and iodide. In a large photochemical chamber, estimated sensitivities of more than 260 compounds were determined. Based on this, we achieved their quantification with multiple negative reagent ions. The quantification was validated by comparing nitrous acid concentrations measured using MR-CIMS with those obtained from a calibrated iterative cavity-enhanced differential optical absorption spectroscopy (ICAD) (R2 = 0.891, slope = 1.24). Six of the organic compounds were taken as examples to show the results of this method for a daytime oxidation chamber experiment. The measurement uncertainties for these pptv-level compounds ranged from 8.9 % to 39.0 %, depending on their sensitivities and concentrations. Further theoretical and experimental investigations showed that halogen compounds can form intermolecular halogen bonds with strength comparable to their intramolecular bonds, preventing this approach from being applied to determine their sensitivities. This work highlights that voltage scanning is a useful approach to the determination ion-molecule adduct sensitivity in CIMS.

Particulate matter (PM) from marine traffic interacts with solar radiation and clouds, ultimately influencing Earth's radiative balance. Ships operated with conventional fossil fuel oils emit light-absorbing carbonaceous PM that offsets aerosol-driven cooling and can even exert a net positive radiative forcing, i.e. warming effect. Radiative properties of PM are possibly further altered by atmospheric aging processes, the effects of which are not fully understood. We present black carbon (BC) emission factors (EF) and optical properties of fresh and photochemically aged particle emissions from a marine engine, operated using low-sulfur heavy fuel oil (LS-HFO) and marine gas oil (MGO), complying with recent maritime sulfur regulations by the International Maritime Organization (IMO). The fresh particle emissions comprised mostly BC, with average BC EFs of 144 and 43.2 mg/kWh for LS-HFO and MGO, respectively. Light absorption was mostly attributed to BC in particles from both fuels, with absorption & Aring;ngstr & ouml;m exponent (AAE, 370 to 880 nm) values 0.9-1.0 (interquartile range), and 870 nm single scattering albedo (SSA) values 0.15-0.24 during the full cycles. Fresh LS-HFO emissions exhibited lower SSA values than those of high-sulfur fuels reported in literature, primarily associated with reduced sulfate emissions. Photochemical aging led to an absorption enhancement (Eabs) of 1.2-1.5 and an increase in SSA relative to fresh emissions, although SSA remained below 0.5, and the estimated direct radiative forcing effect stayed positive. Our results show that sulfur-compliant marine fuels can emit highly absorbing particles with an atmospheric warming potential, which is mostly maintained even after photochemical aging.

Oxidation of volatile organic compounds (VOCs) involving hydroxyl radicals (OH center dot) and nitrogen oxides (NOx), or nitrate radicals (NO3 center dot) forms organic nitrates that undergo gas-particle partitioning, changing the lifetime of nitrogen and their deposition on ecosystems. In urban areas, VOC composition is complex, with contributions from traffic, cooking, volatile chemical products (VCPs), and biogenic emissions. Secondary organic aerosol (SOA) formation from urban VOC mixtures was investigated using chamber experiments during the SAPHIR-CHANEL campaign under realistic VOC-NOx and oxidation conditions. The yield of total organic nitrates is higher for precursor mixtures with a higher percentage of unsaturated VOCs, such as those from traffic and cooking sources (11 %-21 %), compared to VCPs and complex urban emission replicas (2 %-7 %). Enhanced particle-phase partitioning is observed under nighttime oxidation (by NO3 center dot) versus daytime oxidation (by OH center dot). Particulate organic nitrates have a higher average molecular weight under nighttime conditions (330 +/- 80 gmol(-1)) than under daytime conditions (250 +/- 30 gmol(-1)) mainly due to a higher dimer fraction. Similarly, the mass fraction of the total organic aerosol that is organic nitrate is 2.6-4.5 times higher under nighttime than daytime conditions, likely due to higher molecular weight and lower temperatures. Although gas-phase organic nitrate composition varies substantially between precursor mixtures, bulk organic nitrate partitioning is generally similar to that of modeled oxidized monoterpene nitrates (10-4-10-2 m(3)mu g(-1) at 18-40 degrees C). These findings improve understanding of bulk organic nitrate sources and properties in complex urban environments, allowing better simulations of air quality and nitrate deposition.

Abstract. Numerical simulation of environmental chamber experiments is essential for improving models of atmospheric composition. This task often relies on box models, which are not integral to three-dimensional atmospheric chemical transport models. Here, we present an application of the Modular Earth Submodel System (MESSy) DWARF model for chamber simulation experiments. It was developed as a zero-dimensional chemical box model of atmospheric oxidation. Chamber-specific features—including the injection of trace gases, variable radiation conditions, gas- and aqueous-phase chemistry, wall emissions, and dilution—are represented using existing and adapted MESSy submodels. We describe the multi-phase kinetic framework, the estimation of aerosol liquid water content and a simplified treatment of wall losses. We tested the MESSy chamber DWARF setup with experiments from three environmental chambers: The SAPHIR and SAPHIR-STAR chambers at Forschungszentrum Jülich, and the BATCH chamber at the University of Bayreuth. These chambers cover diverse designs and experimental conditions. The model reproduces observed time series of radicals, nitrogen oxides and ozone during an experiment in SAPHIR in which 2-methyl-3-butene-2-ol was oxidised. In another test the model was used to check the consistency of the experimental boundary conditions in the BATCH chamber. Further chamber DWARF tests also show that the framework can predict organic aerosol mass concentration via kinetic partitioning between the gas phase and deliquescent particles in SAPHIR-STAR. Modeled organic mass concentration during this experiment where α-pinene was oxidised agrees well with observations, supporting the framework’s ability to simulate the chemical evolution in both the gaseous and aqueous phase. The MESSy chamber DWARF provides a useful tool for developing and evaluating kinetic models. Within this framework, we can test new laboratory findings directly for their atmospheric relevance. Thanks to the MESSy framework, we can use the same kinetic model for global simulations.

Tailpipe emissions from road traffic contribute substantially to the burden of fine inhalable particulate matter (PM2.5) and deteriorate air quality. Exhaust emission standards, forcing improvements in combustion and exhaust after-treatment technology, considerably decreases combustion-related PM2.5 emitted by modern cars. A549 cancerous alveolar and BEAS-2B normal bronchial epithelial cells were exposed at the air-liquid interface to the total aerosol or gas phase of either fresh or photochemically aged tailpipe emissions from a gasoline EURO 6d car equipped with a gasoline particulate filter. Diluted fresh emissions contained particle number concentrations comparable to low ambient air levels and induced no detectable cytotoxicity. Photochemical aging led to the formation of secondary aerosols and caused significant cytotoxicity. While the aged aerosol induced significant DNA damage, oxidative stress was more associated with volatile secondary species. Our results call for the consideration of the exhaust emission atmospheric transformation processes in future emission standards toward health effect-driven emission regulations.

The seasonal variations of aerosol sources and their atmospheric evolution are investigated using observations from the year-long JULIAC (Jülich Atmospheric Chemistry Project) campaign (January–November 2019) in Jülich, Germany. Non-refractory submicron aerosol components were continuously measured alongside oxidants (OH, O3, NO3), trace gases, and meteorological conditions. Organic aerosols (OA) dominated the aerosol composition throughout the year (39 %–58 %), with secondary formation being the major source. OA, including organic nitrate and organosulfate, peaked during a summer heatwave event due to enhanced daytime and nighttime secondary OA formation driven by elevated concentrations of atmospheric oxidants. Changes in the OA composition during the heatwave suggest a shift in the formation pathways, where isoprene may play an important role. Biomass-burning, mainly wildfires and anthropogenic activities (e.g., heating, industry), is the dominant primary OA source (45 %–83 %), which may grow in influence due to climate change and the expected energy transition. Air masses containing OA from regional transport from marine and wildfire sources are identified through source apportionment. Analysis and modeling prove this method to be more reliable than traditional tracer-based methods. Regional transport to this study site typically shows a cleansing effect on the aerosol concentration, except in winter. Furthermore, seasonal variations in the effects of regional transport are seen, where identical transport pathways led to different influences on aerosol properties, driven by seasonal differences in biogenic and anthropogenic emissions. This study enhances understanding of seasonal variation in submicron aerosol properties in response to their sources, atmospheric evolution, and transport.

Air pollution is one of the largest environmental health risks and one of the leading causes of adverse health outcomes and mortality worldwide. The possible importance of the oxidative potential (OP) as a metric to quantify particle toxicity in air pollution is increasingly being recognised. In this work, the OP and reactive oxygen species (ROS) activity of particles from fresh and aged petrol passenger car emissions and residential wood combustion (RWC) emissions were investigated using two novel instruments. Applying online instruments using an ascorbic acid (AA) and 2′,7′-dichlorodihydrofluorescein (DCFH) assay provides a much higher temporal resolution compared with traditional filter-based methods and allows for new insights into the highly dynamic changes in the OP and ROS activity of these sources. Due to the efficiency of the particulate filter in the Euro 6d car, almost no primary particles were emitted and, thus, no particle OP or ROS activity was detected in primary exhaust. However, a substantial and highly dynamic OP and ROSs were observed after photochemical ageing due to the formation of secondary particles. Increasing OP and ROS activity due to ageing was also observed when comparing fresh and aged RWC emissions. Overall, RWC emissions had higher OP and ROS signals compared with car emissions. This suggests that aged RWC emissions could be a major contributor to air pollution toxicity and may be an intrinsically more harmful emission source than car exhaust, although the formation potential for secondary particles from car emissions was still high. These measurements illustrate the strong differences and highly dynamic nature of toxicity-relevant particle properties from two air pollution sources and could contribute to more efficient air pollution mitigation policies.

The enforcement of global fuel sulfur content (FSC) regulations has significantly reduced SO2 and particulate matter (PM) emissions from ships. However, the impact of the International Maritime Organization's (IMO) sulfur reduction policy on gaseous hydrocarbon emissions, including volatile and intermediate volatility organic compounds (VOCs/IVOCs), remains underexplored. In this study, a 4-stroke single cylinder marine engine was operated using marine gas oil (MGO, FSC = 0.01%) and low-sulfur heavy fuel oil (LS-HFO, FSC = 0.5%) across various engine loads, ranging from 20 kW to a maximum of 80 kW. Emissions were photochemically aged in the oxidation flow reactor "PEAR," simulating an equivalent photochemical aging period from 1.4 +/- 0.2 to 4.6 +/- 0.8 days related to the OH exposure. Emission factors (EFs) of all targeted VOCs/IVOCs varied significantly, ranging from 20.0 +/- 2.5 to 180 +/- 20 mg kWh-1 and from 26.0 +/- 11.0 to 280 +/- 100 mg kWh-1 from a high (80 kW) to low engine load (20 kW) for MGO and LS-HFO, respectively. Monoaromatics dominated total fresh emissions for MGO (64%) and LS-HFO (76%), followed by alkanes. Naphthalene and alkylated naphthalene content declined more than monoaromatic and alkane content, thus changing the VOC/IVOC emission pattern after photochemical aging. Estimated SOA from targeted VOC/IVOC precursors accounted for 41% of the measured secondary organic aerosol (SOA) for MGO, while a lower contribution (34%) was observed for LS-HFO at 20 kW engine load, highlighting the role of unmeasured VOCs/IVOCs in SOA formation. Expanding the research on the effects of atmospheric aging on marine emissions will offer valuable insights into this underexplored area.

Ship traffic is known as an important contributor to air pollution. Regulations aimed at reducing sulfur oxide pollution by limiting the fuel sulfur content (FSC) may also decrease primary particulate matter (PM) emitted from ships. However, there is a knowledge gap regarding how the FSC affects secondary aerosol formation. The emissions from a research ship engine operated with either low sulfur heavy fuel oil (LS-HFO) (FSC = 0.5%) or marine gas oil (MGO) (FSC = 0.01%), were photochemically processed in the oxidation flow reactor “PEAR” to achieve an equivalent photochemical age between 0 and 9 days in the atmosphere. FSC was found to have no significant impact on secondary organic aerosol formation after 3 days of aging, at 1.7 ± 0.4 g/kg for MGO and 1.5 ± 0.4 g/kg for LS-HFO. Furthermore, the composition and oxidative pathways remained similar regardless of FSC. However, because of the higher secondary SO4 formation and primary aerosol emissions, LS-HFO had significantly higher total PM than MGO.

Aviation is an important source of urban air pollution, but the impacts of photochemical processing on the exhaust emissions remain insufficiently characterized. Here, the physical-chemical properties of fresh and photochemically aged emissions from a laboratory-scale jet engine burner operated with JP-8 kerosene were studied in detail with a range of online and offline methods. The fresh emissions contained high amounts of organic matter present predominantly in the gaseous phase. Photochemical aging in an oxidation flow reactor caused substantial formation of oxidized organic aerosol, increasing the particle mass approximately 300-fold. During aging, aromatic hydrocarbons and alkanes in the gas-phase decayed, while gas-phase oxidation products, such as small carbonyls and oxygenated aromatics, increased. The composition of organic matter became more complex by photochemical processing, with the average particulate carbon oxidation state increasingly growing throughout the addressed exposure range (equivalent to 0.2 to 7 d in the atmosphere) with a Delta H:C/Delta O:C slope of -0.54. Simultaneously, the near-UV wavelength absorption by the particles increased due to enhanced particulate mass. The imaginary refractory indices of organic particulate matter were 0.0071 and 0.00013 at the wavelength of 520 nm for the fresh and photochemically processed particles, respectively, indicating secondary production of weakly absorbing brown carbon. The direct radiative forcing by the exhaust particles was estimated by a Mie model, which revealed a prominent shift from a warming to cooling climate effect upon photochemical aging. The results highlight the importance of considering secondary aerosol formation when assessing the environmental impacts of aviation.