Exposure to ambient air pollution is a major risk factor for human health yet, the physiological effects of particulate matter (PM) remain poorly understood. Oxidative stress due to excess formation of reactive oxygen species (ROS) is a leading hypothesis for the molecular mechanism behind the adverse health effects of PM. Thus, measurements of ROS production and antioxidant depletion are widely used to assess the oxidative potential (OP) of PM. Here we introduce a chemical kinetic model of oxidative potential (KM-OP) to elucidate and quantify the effects of PM on the production of ROS and the consumption of ascorbic acid (AA) and dithiothreitol (DTT). The chemical mechanism of the model is based on literature rate coefficients and a large compilation of laboratory data on the effects of transition metal ions, quinones, and organic aerosol (OA). We apply the model to field measurement data of PM composition and OP from three European cities (Grenoble, Paris, London), obtaining good correlations (R2>0.75) and low model bias (<15 %) for 4 out of 6 data sets. Previous studies found that PM may inflict damage to biomolecules in the lungs mainly via the production of hydroxyl (⚫OH) radicals. The antioxidant-based OP assays investigated in this study show a good correlation with modeled ⚫OH production. We identify OA as the strongest contributor to antioxidant-based OP assays, with minor contributions from Cu and Fe ions. Cu dominates the production of hydrogen peroxide (H2O2), but does not substantially affect ⚫OH production. Our model and results provide a basis for further investigation and comparison of different metrics of the potential toxicity of PM.

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.

Abstract Heavy-duty vehicles (including non-road mobile machinery, eg excavators), aircrafts and ships are contributing significantly to emissions of green-house gases and health-relevant air pollutants, such as fine airborne particulate matter (PM2.5) as well as emerging pollutants. For decarbonization of the sector, several new fuels, ranging from hydrogen via ammonia to synthetic eFuels are considered. An important question is how these potential new fuels will influence emissions of air pollutants and climate relevant compounds. LowC will address if these new fuels for high-power engines have an impact on the emissions of air pollutants and climate-drivers, considering also upstream emissions and secondary pollutants formed under different atmospheric conditions (daytime photochemical aging or night-time atmospheric radical chemistry). LowC will apply a series of state-of-the-art technologies to generate and characterize the emissions and assess the effects in lung cell models. The toxicological testing will be applied in a tiered manner (screening and in-depth verification), in line with visions of Toxicity Testing in the 21st Century. Emission data, including regulated and emerging pollutants, will feed into integrated assessment and atmospheric transport models, currently used to underpin EU policy and the Zero Pollution Action Plan, to assess impacts on the EU environmental policy objectives. Finally, LowC will evaluate health and environmental impacts and provide guidance and recommendations to ensure that solutions to reduce CO2 emissions and prevent climate change are safe and sustainable. The work is supported by the European Union’s Horizon Europe research and innovation program under Grant Agreement No. 101192913.

Eddy-covariance measurements allow us to directly monitor the vertical turbulent CO2 flux at a specific point in the urban atmosphere. Under some assumptions such as stationarity and sufficient turbulence, this flux corresponds to the net emissions in a variable footprint area. Combined with a footprint model and a biospheric CO2 flux model, this method has a high potential for validating and optimizing urban emission inventories. However, the reliability of EC measurements depends on a careful site selection, data processing and quality control. Often, sensor heights below z=50 m a.g.l. are chosen to mitigate issues associated with horizontal heterogeneity, storage flux, and horizontal and vertical advection. The storage flux describes the temporal change of the CO2 amount in the control volume between the surface and sensor height. Tall-tower sites (z>50 m a.g.l.) would be beneficial to capture emissions from a larger part of the city but require careful consideration of these issues. While a few studies have reported plausible EC measurements for urban tall-tower sites, little is known about the impact of the storage flux and advection terms. In the ICOS-Cities project, tall-tower EC systems and networks of mid-cost and low-cost CO2 concentration sensors were installed in three cities. Here, we aim to better quantify the storage flux and identify periods with horizontal advection by leveraging data from the sensor networks in Zurich, Switzerland, and Munich, Germany, and thus improve the reliability of the observed net CO2 emissions. The low-cost sensors were deployed in the urban canopy layer while the mid-cost sensors were mostly located at the rooftop level and collocated with wind and temperature sensors. We estimate the storage flux by dividing the control volume into three to four layers and averaging data from different sensors in the same layer. The storage flux is then added to the turbulent flux to estimate net surface emissions. To filter out periods in which this estimate is biased by horizontal advection, we consider horizontal CO2 gradients determined using mid-cost sensors at rooftop sites. This approach is compared to the often-used filtering with a friction velocity threshold.As expected, the storage flux is most important on days with a pronounced diurnal cycle in atmospheric stability. It reduces the net CO2 emission estimates in the morning hours after sunrise and generally increases these estimates at night. From 1.5 to 5 h after sunrise, this effect amounts on average to -7.3 and -8.0 µmol m-2 s-1 in Zurich and Munich, respectively, while in the first 3.5 hours after sunset, it amounts to +4.7 and +3.0 µmol m-2 s-1 (46% and 24% of the turbulent flux) in Zurich and Munich, respectively. On days with a small diurnal cycle in stability, the storage flux plays a smaller role, especially in winter. We will also present insights in the frequency of horizontal advection and favorable conditions for it. Finally, we will discuss the plausibility of median diurnal cycles of the derived net CO2 emissions, considering the directional dependence on land cover and associated sources and sinks.

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.

This paper presents the findings from a collaborative interlaboratory comparison exercise designed to assess oxidative potential (OP) measurements conducted by 20 laboratories worldwide. This study represents an innovative effort as the first exercise specifically aimed at harmonising this type of OP assay, setting a new benchmark in the field. Over the last decade, there has been a noticeable increase in OP studies, with numerous research groups investigating the effects of exposure to air pollution particles through the evaluation of OP levels. However, the absence of standardised methods for OP measurements has resulted in variability in results across different groups, rendering meaningful comparisons challenging. To address this issue, this study engages in an international effort to compare OP measurements using a simplified method (with a dithiothreitol (DTT) assay). Here, we quantify the OP in liquid samples to focus on the protocol measurement itself, while future international OP interlaboratory comparisons (ILCs) should aim to assess the whole chain process, including the sample extraction. We analyse the similarities and discrepancies observed in the results, identifying the critical parameters (such as the instrument used, the use of a simplified protocol, the delivery and analysis time) that could influence OP measurements and provide recommendations for future studies and interlaboratory comparisons even if other crucial aspects, such as sampling PM methods, sample storage, extraction methods and conditions, and the evaluation of other OP assays, still need to be standardised. This collaborative approach enhances the robustness of the OP DTT assay and paves the way for future studies to build on a unified framework. This pioneering work concludes that interlaboratory comparisons provide essential insights into the OP metric and are crucial to move toward the harmonisation of OP measurements.

The toxicity of particulate matter (PM) is highly related to the concentration of particle-bound reactive oxygen species (ROS). Chemical properties, including dissolved metals and the sources of PM, influence ROS production and ROS oxidative potential. Here, the photochemical aging of a secondary organic aerosol proxy (citric acid, CA) with metal complexes (iron–citrate, FeIII(Cit)) is assessed toward the production of particle-bound ROS with an online instrument (OPROSI). We studied the photochemically induced redox chemistry in iron/copper–citrate particles experimentally with an aerosol flow tube (AFT), mimicking atmospheric UV aging. Experiments were performed at different relative humidity (RH) levels, leading to variation in the physicochemical properties of the particles, e.g., viscosity. We found that UV-aged CA aerosol containing 10 mol % FeIII generated ROS concentrations on the order of 0.1 nmol H2O2 eq.µg-1, indicating the photochemically driven formation of peroxides. An increase in RH leads to only a slight but overall lower concentration of ROS, possibly due to a loss of volatile HO2 and H2O2 in the gas phase in the less viscous particles. The RH effect is enhanced in absence of oxygen. Compared to the FeIII(Cit)/CA particles, the iron/copper–citrate samples show a uniformly decreased ROS level. Interestingly, in the high-RH nitrogen experiment with copper, we found an enhanced drop in the ROS concentration down to 0.02 nmolH2O2eq.µg-1 compared to all other irradiation experiments. We suggest that copper may suppress radical redox reactions, and when particles are more viscous, ROS are still produced with photochemistry, but the levels are more sensitive to the presence of copper than under humid or lower-viscosity conditions.

Exposure to airborne particulate matter (PM) has been attributed to millions of deaths annually. However, the PM components responsible for observed health effects remain unclear. Oxidative potential (OP) has gained increasing attention as a key property that may explain PM toxicity. Using online measurement methods that impinge particles for OP quantification within seconds, we reveal that 60 to 99% of reactive oxygen species (ROS) and OP in secondary organic aerosol and combustion-generated PM have a lifetime of minutes to hours and that the ROS activity of ambient PM decays substantially before offline analysis. This implies that current offline measurement methods substantially underestimate the true OP of PM. We demonstrate that short-lived OP components activate different toxicity pathways upon direct deposition onto reconstituted human bronchial epithelia. Therefore, we suggest that future air pollution and health studies should include online OP quantification, allowing more accurate assessments of links between OP and health effects.

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.

Urbanization impacts Earth’s systems by increasing CO2 emissions, creating urban heat islands (UHI), and affecting climate, energy use, and public health. Understanding urban microclimates and energy and CO2 balances is crucial for designing effective climate change adaptation and mitigation strategies. This paper presents preliminary results from the UrbaNature project, focusing on vegetation’s role in urban microclimates. The goal is to develop a 3D microscale ecophysiological model for CO2, water, and energy exchanges between plants and the urban atmosphere, including a 3D radiation exchange model for urban canyons with tree canopies. Using high-resolution digital surface models (DSMs), land cover, and leaf area index (LAI), we construct a 3D urban landscape of isometric voxels categorized as buildings, trees, terrain, or empty. This landscape supports radiation exchange simulations via a ray tracing algorithm, computing parameters like sky view factor (SVF), and light transmission coefficient, stored in look-up tables (LUTs). Meteorological data from an urban tower in Basel, Switzerland, is used to evaluate the model’s accuracy by comparing simulation results with ground-level station data. Preliminary results show that the model effectively replicates observed shortwave radiation patterns, highlighting its potential to enhance urban climate research and understanding of urban energy balance.

Collection of secondary organic aerosol (SOA) onto filters is often used in combination with LC-MS for detailed chemical characterization. Studies have shown that a large fraction of laboratory-generated SOA can be attributed to high molecular weight oligomeric compounds. Often there is a significant delay between sample collection and analysis (e.g., during automated filter collection) which may lead to changes in the chemical profile of the samples. In this study β-pinene SOA was generated by O3 and OH oxidation and collected onto filters, which were either extracted in a mixture of water and acetonitrile or left on the filter and stored at room temperature, to investigate changes in the overall chemical composition profile over time up to one month. The samples were analyzed by UHPLC-ESI-MS in negative polarity mode. An untargeted analysis led to several thousand detected compounds and principal component analysis indicated significantly different compositional changes between the samples stored on filters or as extracts. In order to understand these differences, further focus was put on previously identified carboxylic acids, dimer esters and other oligomers. The concentration of several hundred compounds in the monomer mass range increased in extracts over the 4-week time span, whereas the opposite was observed for samples stored on filters, where the concentration of a large number of monomers decreased. For dimers in samples stored on filters, a large number of compounds increased in concentration, while extracts show the opposite behavior. A possible explanation for these trends could be the decomposition through hydrolysis of dimer esters and other components in extracts and the formation of oligomers on filters. This indicates the continuous chemical aging of the SOA particles deposited and stored on filters. To test this hypothesis, we nebulized a solution of carboxylic acid standards in excess onto filters where β-pinene SOA was deposited and monitored the temporal evolution of dimer esters which have been reported to form from diaterpenylic acid and carboxylic acids. A stronger increase of the dimers (and a stronger decrease of the monomer precursor products) was observed for these “spiked” samples in comparison to the “non-spiked” SOA filter controls. This study not only highlights that the persistent changes observed on filters are due to previously overlooked on-filter reactions, which can lead to misinterpretation of the detailed chemical composition of samples collected on filters for offline analyses. These on-filter reactions also mimic particle phase non-oxidative aging of SOA over the entire lifetime of SOA particles in the atmosphere of days or weeks.

Climate models project a further increase in the average global temperature for the following decades, with Alpine regions (and their ecosystems) expected to be over-proportionally more affected. Biogenic volatile organic compounds (BVOCs) comprise the largest, most highly complex, and diverse fraction of the volatile organic compounds (VOCs) emitted into the atmosphere (1). By emitting BVOCs, plants communicate, fight herbivores, and attract pollinators (2). It is well known that biotic stressors (e.g., insects feeding on plants) lead to changes in plants' BVOC emissions: certain compounds can be promoted, and others reduced. Atmospheric oxidation of BVOCs affects the concentration of methane, carbon monoxide, and tropospheric ozone, leading to the formation of Secondary Organic Aerosol (SOA). Atmospheric aerosol load is crucial in defining the radiative balance and negatively impacts air-quality standards (3). Stress-induced changes in plant emissions may thus lead to changes in atmospheric chemistry and SOA properties (e.g., ref. 4). The impact of prolonged changes in abiotic factors and abiotic stress (e.g., heat and drought) on plants' BVOC composition and emissions quantities, and how this may impact atmospheric chemistry and SOA properties, need to be better understood. Within the experimental project "Acclimation and environmental memory” (AccliMemo), we study BVOC composition and quantities at basal conditions and under prolonged heat and drought. To this purpose, Scots pine (Pinus Sylvestris) seedlings were grown from seeds collected from selected mother trees from the long-term irrigation experiment Pfynwald. Those mother trees experienced different long-term water availability. This also allows us to examine the consequence of transgenerational memory on BVOC emissions (5). Our conference contribution will give insight into our findings from plant chamber experiments and address i) gas-phase BVOC samples collected on sorbent tubes and analyzed by Thermal Desorption GC-MS and ii) gas-phase BVOC measurements collected in-situ using a PTR-ToF-MS. These data provide a well-resolved picture of terpene compositions and diurnal trends in emission levels. The BVOC analysis in the gas phase is complemented by a detailed analysis of the secondary metabolites in needle samples. Secondary metabolites are extracted in organic solvents and analyzed by liquid injection GC-FID/MS. Bibliography (1) Sindelarova, K., Granier, C., Bouarar, I., Guenther, A., Tilmes, S., Stavrakou, T., Müller, J.-F., Kuhn, U., Stefani, P., and Knorr, W.: Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years, Atmospheric Chem. Phys., 14, 9317–9341, https://doi.org/10.5194/acp-14-9317-2014, 2014.(2) Niinemets, Ü. and Monson, R. K. (Eds.): Biology, Controls and Models of Tree Volatile Organic Compound Emissions, Springer Netherlands, Dordrecht, https://doi.org/10.1007/978-94-007-6606-8, 2013.(3) Seinfeld, John H. and Pandis, Spyros N.: Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 3rd Edition., Wiley, 1152 pp., 2016.(4) Smith, N. R., et al.: Viscosity and liquid–liquid phase separation in healthy and stressed plant SOA, Environ. Sci. Atmospheres, 1, 140–153, https://doi.org/10.1039/D0EA00020E, 2021.(5) Bose, A. K., et al.: Memory of environmental conditions across generations affects the acclimation potential of scots pine, Plant Cell Environ., 43, 1288–1299, https://doi.org/10.1111/pce.13729, 2020.Funding: Swiss National Science Foundation, Project Numbers 189109, 199317, and, 194390.

Organic peroxides are recognized as major contributors to the toxicity and adverse health effects of secondary organic aerosols (SOA). However, their molecular composition and chemical properties in SOA remain largely unexplored. Here we develop a novel analytical strategy for the molecular characterization of organic peroxides in α-pinene SOA, combining iodometry kinetic experiments with liquid chromatography-high-resolution mass spectrometry. Using non-targeted analysis, we identify over 300 organic peroxides in α-pinene SOA, showing a wide range of reactivities with iodide, spanning 4 orders of magnitude. The structures of 12 organic peroxides derived from stabilized Criegee intermediates are further proposed and discussed. Our findings present a reliable, standard-free methodology for identifying previously uncharacterized organic peroxides in SOA at the molecular level, offering new avenues for investigating formation pathways and health impacts of this large, diverse, yet rarely explored compound class. Additionally, we propose peroxide-iodide reactivity as a novel metric for future predictions of oxidative potential─a key indicator of toxicity─and for elucidating the structure of individual organic peroxides in SOA.

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.

Generation and characterisation of metal nanoparticles (NP) gained attention in recent years due to their significant potential in applications as diverse as catalysis, electronics or energy storage. Despite the high interest in NPs, their characterization is challenging and detailed quantitative information on size, number concentration and morphologies are key to understand their properties. In this study we generated NPs from four metals, Au, Pt, Cu and Ni, via spark ablation in the aerosol phase, which allows to produce NPs as small as 1 nm in high quantities and purity. Particles were characterised with transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) as well as online aerosol particle size distribution measurement techniques. Particle modes for the four metals ranged between 3 nm and 4 nm right after generation. The number and size of particles generated correlated with thermal properties of the metals such as thermal or electrical conductivity. The four metal NPs were also coagulated with larger TiO2 NPs of about 120 nm size and the metal surface coverage of the TiO2 particles was characterised with electron microscopy and EDX spectroscopy.

The most recent European regulation, the Euro 6d emission standard, requires all gasoline direct injection (GDI) vehicles to use both a three-way catalyst (TWC) and a gasoline particle filter (GPF) as exhaust aftertreatment. These aftertreatment methods are aimed at reducing NOx and primary particle emissions. However, the formation of secondary organic aerosols (SOAs) from the volatile organic compound (VOC) emissions of a Euro 6d compliant GDI vehicle, factory equipped with a GPF is not yet investigated. Therefore, to explore the SOA formation and effects of the GPF, the exhaust of a Euro 6d compliant GDI vehicle was characterized at 4 different steady state speeds, idling (0 km h-1), 50, 80 and 100 km h-1. The exhaust was oxidised in a photochemical emission aging flow tube reactor (PEAR) by reactions with OH radicals equivalent of 2.2 days of atmospheric day time oxidation. It was found that the GPF completely removes primary particles larger than 10 nm, at all investigated vehicle speeds. However, significant SOA was formed after oxidation, with the highest SOA formation potential per kg fuel consumed at 50 km h-1. The main SOA precursors were determined to be toluene, xylene and trimethyl-benzene which were found to account for at least 50% of SOA formed at all driving speeds. Furthermore, high emissions of ammonia (NH3) could be observed in the exhaust under all driving conditions which resulted in the subsequent formation of ammonium nitrate (NH4NO3) after aging. The formation of NH4NO3 additionally facilitated the co-condensation of organic gas phase products after OH oxidation enhancing SOA mass even further. The exhaust of a Euro 6d compliant car with a three-way catalyst and GPF preinstalled was studied. The GPF removed almost all primary particles, however oxidation of emitted VOCs such as toluene and xylene led to significant SOA formation.

Ozonolysis of alkenes is known to produce reactive intermediates-stabilized Criegee intermediates (SCIs), and their subsequent bimolecular reactions with various carboxylic acids can form alpha-acyloxyalkyl hydroperoxides (AAHPs), which is considered a major class of organic peroxides in secondary organic aerosol (SOA). Despite their atmospheric and health importance, the molecular-level identification of organic peroxides in atmospheric aerosols is highly challenging, preventing further assessment of their environmental fate. Here, we synthesize 20 atmospherically relevant AAHPs through liquid-phase ozonolysis, in which two types of monoterpene-derived SCIs from either alpha-pinene or 3-carene are scavenged by 10 different carboxylic acids to form AAHPs with diverse structures. These AAHPs are identified individually by liquid chromatography coupled with high-resolution mass spectrometry. AAHPs were previously thought to decompose quickly in an aqueous environment such as cloud droplets, but we demonstrate here that AAHPs hydrolysis rates are highly compound-dependent with rate constants differing by 2 orders of magnitude. In contrast, the aqueous-phase formation rate constants between SCI and various carboxylic acids vary only within a factor of 2-3. Finally, we identified two of the 20 synthesized AAHPs in alpha-pinene SOA and two in 3-carene SOA, contributing similar to 0.3% to the total SOA mass. Our results improve the current molecular-level understanding of organic peroxides and are useful for a more accurate assessment of their environmental fate and health impact.