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.

Microalgae emit volatile organic compounds (VOCs) that can profoundly impact climate by leading to new particle formation and influencing clouds. Among these VOCs, dimethyl-sulphide (DMS) is of particular interest due to its key role in atmospheric processes. Despite its importance, many detailed processes linking microalgae and sea-atmosphere interactions remain poorly understood. We investigated the response of a freshwater and saltwater microalgal species of haptophytes known to produce DMS, to air entrainment and bubble-bursting mechanisms relevant for wave-breaking over the ocean. We show that bubbling resulted in the successful aerosolisation of microalgae and concurrent emission of DMS. In contrast, only background levels of DMS were detected when bubbling ceased, suggesting a critical role of bubbles in the sea-air exchange of DMS under the studied conditions. DMS mixing ratios were not correlated with the emitted particle concentrations and decreased over time, while particle concentrations remained stable. Bubbling also significantly reduced the viability of aquatic microalgae. Approximately half of the aerosolised microalgae were viable upon emission, but were not able to grow during subsequent cultivation recovery. Thus, the potential for microalgae to disperse to new environments via aerosolization is low, while their climate impact through the release of DMS remains substantial.

Aerosol-cloud interactions (ACI) are a major source of uncertainty in climate science, critically affecting our ability to project near-term climate evolution and assess societal risks. These interactions influence effective radiative forcing, cloud dynamics, and precipitation patterns, yet remain insufficiently constrained due to limitations in observations, modeling, and process understanding. This uncertainty hampers robust policy advice across multiple domains-from estimating remaining carbon budgets and climate sensitivity, to anticipating regional extreme events and evaluating climate interventions such as solar radiation modification. In many cases, the influence of ACI is either underappreciated or excluded from decision-making frameworks due to its complexity and lack of quantification. This perspective outlines a path forward to overcome these barriers by leveraging emerging opportunities in satellite remote sensing, ground-based and airborne observations, high-resolution climate modeling, and machine learning. We identify key areas where rapid progress is feasible, including improved retrievals of cloud microphysical properties, better representation of natural aerosols in a warming world, and enhanced integration of observational and modeling communities. Even as anthropogenic aerosol and its impacts on clouds is reducing owing to emissions controls, addressing ACI uncertainties remains essential for refining climate projections, supporting effective mitigation and adaptation strategies, and delivering actionable science to policymakers in a rapidly changing climate system.

Coastal aerosols are formed through the complex mixing between marine air masses and continental emissions, which originate from both natural and anthropogenic sources. The properties of coastal aerosols are decisive for their interaction with sunlight and their influence on clouds, as well as the potential health implications for the population in these areas. In this study, the aerosol properties and sources at Aarhus Bay, Denmark, were investigated by combining in situ aerosol light scattering and absorption with size distribution measurements and footprint analysis by FLEXPART. Our analysis demonstrates a considerable contribution of anthropogenic aerosols from both fossil fuel combustion and biomass burning, as well as periods with highly scattering aerosols. Furthermore, good agreement was found between in situ and modelled black-carbon data. Combining in situ measurements and FLEXPART analysis further evidenced a major impact of local emissions, as well as a few long-range transport intrusions.

Atmospheric ice nucleating particles (INPs) can affect cloud radiative properties and lifetimes and thus Earth's climate. Such particles may be emitted into the atmosphere from seawater via wave breaking processes. Here, we perform an exploratory investigation on the ice nucleating properties of seawater sampled on four days over a year (February, April, June, and November) from a coastal site near Aarhus in Denmark. We use a cold stage instrument (droplet size: 1 mu L) to probe immersion mode freezing events. We find that bulk seawater contains INPs with T50 values around -20 degrees C independent of the month of sampling and INP concentrations ranging from 6 x 10(3) to 5 x 10(6 )INP L-1 in a temperature range of -12 to -34 degrees C across all four samples. All samples displayed sensitivity to filtration (0.02 mu m), as indicated by a decrease in INP concentration (lowering of freezing temperature). The filtered April and June samples froze at higher temperatures than the filtered November and February samples, which could indicate a variation in the population of INPs (>0.02 mu m) over the year. Sea surface microlayer samples did not show enrichment of INPs compared to bulk seawater. Our results are discussed in the context of INP activity of seawater from other locations. While further studies are needed to understand the nature and potential seasonality of seawater INPs, we confirm the presence of INPs in coastal Baltic seawater that may contribute to atmospheric INP concentrations.

It is important to investigate formation, composition and properties of secondary organic aerosol (SOA) from monoterpenes in order to develop an accurate understanding of their atmospheric chemistry, impact on the aerosol budget and the effects of climate change. Δ3-Carene is one of the monoterpenes emitted in highest amounts in the boreal forest, yet only few studies have investigated the atmospheric chemistry and aerosol formation of Δ3-carene.In this work, we have investigated aerosol formation and composition of SOA from ozonolysis of Δ3-carene at different concentration levels in the AURA atmospheric simulation chamber at Aarhus University, Denmark. At low concentrations of Δ3-carene (about 10 ppb), SOA formation shows minimal temperature dependence under dry conditions. This contrasts with results from studies of Δ3-carene at higher concentrations (about 50 ppb) and studies of the structurally quite similar monoterpene a-pinene. Furthermore, we observed increased particle nucleation at higher relative humidity (about 80% RH, 10°C). Chemical analysis of the SOA found a series of carboxylic acids, in line with previous studies, with different concentration profiles over time, depending on experiment temperature. In experiments with ozonolysis of mixtures of Δ3-carene and a-pinene, we were able to identify a mixed dimer composed of molecular units from each of the precursors.

Biogenic ice-nucleating particles (INPs) can significantly impact mixed-phase clouds by enhancing precipitation and reducing albedo. As Arctic sea ice diminishes, the exposure of open ocean may increase aerosolization rates of marine bioaerosols and INPs. We investigated INP concentrations and microbial communities in ambient marine air, sea bulk water (SBW), and sea surface microlayer (SML) along a transect from the Davis Strait to Baffin Bay. INP concentrations in SBW increased with latitude, regardless of the extent of terrestrial freshwater input. We further identified correlations between INP levels and abundances of specific microbial taxa, including Formosa, Lewinella, Micromonas, and Dino-Group-I-Clade-5, suggesting potential ice nucleation activity of these taxa. Air samples exhibited distinct microbiomes compared to seawater, indicating terrestrial contributions, but at the highest observed wind speeds (7-8 m/s), substantial contributions of the seawater microbiome were detected in the air. Elevated atmospheric INP concentrations at higher latitudes correlated with seawater INP levels, which was supported by laboratory sea spray experiments showing that INPs in SBW influenced aerosol INP levels. Our findings highlight the Arctic Ocean as a significant source of biogenic atmospheric INPs and enhance our understanding of marine microbes as contributors to biogenic INPs. By identification of potential ice nucleation active microbial taxa and examination of aerosolization processes, this study provides a framework for future research on Arctic marine-derived INPs and their atmospheric impact.

This study investigates how changes in temperature affect the secondary organic aerosol (SOA) phase state. SOA was formed by α-pinene ozonolysis in an atmospheric simulation chamber at temperatures (T0) in the range of 257-283 K at RH0 < 20%. After more than 14 h of SOA aging, one or more heating and cooling ramps were performed. Upon heating, we observe that the onset of evaporative SOA shrinkage is delayed by up to ∼20 K relative to T0. Our observations are supported by aerosol dynamics and kinetic multilayer model simulations, relating observed changes to an effectively reversible temperature- and SOA-composition-dependent phase transition from solid glassy to semisolid. We demonstrate that the SOA content of highly oxygenated organic molecules (HOMs) increases with T0 and at lower α-pinene concentrations. Higher HOM SOA content results in more viscous SOA with a higher glass transition temperature (Tg). The model was used to quantify how Tg varies with T0 and the amount of α-pinene being oxidized. Because the SOA phase state is influenced by the conditions under which it forms, and affects SOA lifetime, reactivity, water uptake, and potentially ice nucleating properties, the results presented herein may have wide implications for the design of future SOA experiments, air quality, and climate.

Monoterpenes, comprising 15% of global biogenic volatile organic compound emissions, play a pivotal role in atmospheric chemistry. ∆3-carene, the second most prevalent monoterpene, has been identified as a significant source of secondary organic aerosol (SOA) upon oxidation, potentially surpassing α-pinene under similar conditions. Despite its importance, research has predominantly focused on α-pinene , leaving gaps in our understanding of ∆3-carene's oxidation pathways, particularly its capacity to form highly oxygenated organic molecules (HOM).To address this knowledge gap, we conducted an investigation into HOM formation during the ozonolysis of ∆3-carene using atmospheric simulation chambers. Employing a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with nitrate as the reagent ion (NO3-CIMS), we measured HOM resulting from ∆3-carene ozonolysis. Additionally, we explored the impact of temperature and relative humidity on HOM composition and distribution across various conditions (0, 10, and 20 ºC, and humidity levels below 15% and around 80%).Our analysis revealed diverse HOM monomers and dimers from ∆3-carene ozonolysis. Predominant HOM monomers included C10H14,16O9 and C9H12,14O9, while the largest dimers comprised C19H30O6,10,11 and C20H32O7,9,11. Significantly, HOM monomers with 9 or more oxygen atoms and all dimers irreversibly condensed onto particles, while those with 6-8 oxygen atoms behaved as semi-volatile organic species, maintaining notable gas-phase concentrations. Intriguingly, ∆3-carene ozonolysis produced higher HOM concentrations than α-pinene, suggesting distinct formation pathways for these two monoterpenes. Furthermore, we observed a substantial decrease in HOM concentrations at lower temperatures, consistent with previous studies on α-pinene ozonolysis. Despite similar main HOM species at temperatures of 20, 10, and 0 ℃, the ratio of HOM dimers to monomers increased from 0.78 to 1.51 as temperatures decreased. This temperature-dependent variation underscores the complexity of ∆3-carene's atmospheric processing, revealing nuanced behaviors of HOM under different environmental conditions.In conclusion, this study provides valuable insights into the HOM formation pathways of ∆3-carene, shedding light on its unique atmospheric chemistry. The observed differences in HOM concentrations and temperature-dependent behaviors highlight the need for a more comprehensive understanding of various monoterpenes, moving beyond the well-studied α-pinene. These findings contribute to the broader knowledge of biogenic volatile organic compounds and their impact on atmospheric processes.

Atmospheric aerosols contribute to the largest uncertainty in estimates of the Earth’s global energy balance. Their interactions with sunlight and their ability to affect cloud formation leads to both direct and indirect influence of radiative forcing. The substantial uncertainties associated with aerosol climate effects stem amongst others from the complexity of their sources, composition, and properties. Aerosols in coastal areas present a challenging mix of inorganic and organic particles from diverse sources, making measurements and characterization of their properties in these regions essential. This study presents measurements of the optical properties of ambient aerosols on Askö, Sweden, from October 2024 to January 2025. Askö, an island and nature reserve located approximately 80 km south of Stockholm, experiences low levels of local pollution, making it an ideal location for studying marine aerosols. Its location in the Trosa Archipelago, facing the Baltic Sea, also makes it well-suited for investigating the impact of long-range transport from Central and Eastern Europe.Instruments were placed in a container situated on top of a floating platform near the island. Scattering coefficients, measured with a nephelometer, and absorption coefficients, measured with an aethalometer, were used to calculate Scattering and Absorption Ångström Exponents. The Ångström matrix was used to characterize aerosol types found in the area at different times. The optical data set is further complemented by local meteorological data, particle size distributions, and back trajectory analysis. This combination of data will give valuable insights into aerosol sources at this remote location, the degree of aerosol ageing, and the identification of prevailing emissions sources such as local emissions versus long-range transport of air masses.

In the past decades, China has witnessed high air pollution associated with rapid economic development, although regulatory efforts have alleviated the situation since 2013. Haze events characterized by high particulate matter (PM) levels in China are not only of enormous magnitude but also represent a distinct chemical regime. Once driven by direct emissions, these high-PM episodes are now more affected by secondary aerosol, especially secondary organic aerosol (SOA). This Review synthesizes the state of the science of SOA formation in urban China, specifically (i) how the dominance of anthropogenic precursors affects SOA formation, (ii) what are the prevailing SOA formation mechanisms, and (iii) how important are the multipollutant and multiphase processes in SOA formation and evolution. We also highlight essential directions for future studies.

It is important to investigate formation, composition and properties of secondary organic aerosol (SOA) from monoterpenes in order to develop an accurate understanding of their atmospheric chemistry, impact on the aerosol budget and the effects of climate change. Δ3-Carene is one of the monoterpenes emitted in highest amounts in the boreal forest, yet only few studies have investigated the atmospheric chemistry and aerosol formation of Δ3-carene. In this work, we have investigated aerosol formation and composition of SOA from ozonolysis of Δ3-carene at different concentration levels in the AURA atmospheric simulation chamber at Aarhus University, Denmark. At low concentrations of Δ3-carene (about 10 ppb), SOA formation shows minimal temperature dependence under dry conditions. This contrasts with results from studies of Δ3-carene at higher concentrations (about 50 ppb) and studies of the structurally quite similar monoterpene a-pinene. Furthermore, we observed increased particle nucleation at higher relative humidity (about 80% RH, 10°C). Chemical analysis of the SOA found a series of carboxylic acids, in line with previous studies, with different concentration profiles over time, depending on experiment temperature. In experiments with ozonolysis of mixtures of Δ3-carene and a-pinene, we were able to identify a mixed dimer composed of molecular units from each of the precursors.

We describe three developments at the AURA atmospheric simulation chamber (made of Teflon and with a volume of similar to 5 m3) aimed at an improved understanding of the physical conditions of the chamber to facilitate a better basis for comparisons between experimental data and results from numerical models. First, we demonstrate how the volume of the AURA chamber can be monitored by observing the position of a chamber wall using fixed laser distance sensors. The absolute volume calibration is obtained through a measurement of the relative humidity in the chamber during a controlled dilution experiment. Second, through a direct measurement, we characterize the occurrence, magnitude (similar to 0 - 80 kV/m), and decay time (similar to 10 - 20 h) of static electric fields inside the AURA chamber after charging. Further, we confirm directly that the AURA chamber can be significantly discharged and kept in a steady mode of charging with the addition of ion fans to the enclosure where the chamber is suspended. Third, we improve the air mixing capabilities at the AURA chamber by adding two mixing fans that allows efficient mixing of the chamber air within a few minutes. We characterize the effect of the electric field in the chamber and the rate of air mixing by direct measurements of the particle loss rate of injected polydisperse ammonium sulfate particles.

Micro- and nanoplastic particles have been detected in most environmental compartments. The presence of microplastics in the remote marine atmosphere and close to large lakes suggests bubble mediated water-air transfer as a source of airborne microplastics, however, quantitative estimates of plastic emission from surface waters remain uncertain. In this work, we elucidate the emission of submicron polystyrene nanospheres by bubble bursting in a laboratory setting from low salinity waters (salinity 0-1.0 g kg-1), polystyrene particle diameter (103, 147 and 269 nm), aqueous particle number concentrations in the range 4 x 107-2 x 109 cm-3, and bubble formation rate (0.88-3.35 L min-1 of air). Production of polystyrene aerosols was demonstrated using a scanning mobility particle sizer and confirmed by analysis of filter samples using pyrolysis gas chromatography coupled to mass spectrometry. We show that production of polystyrene aerosol particles scales linearly with the number concentration of plastic particles in the water. Our results suggest that small amounts (0.01 g kg-1) of salt increase polystyrene particle production. To the best of our knowledge this is the first study of bubble mediated water-air transfer of plastic particles as small as 100 nm. Bubbling air through water containing mono-disperse polystyrene nanoparticles is shown to result in transfer of nanoplastic particles to air.

Δ3-carene is a prominent monoterpene in the atmosphere, contributing significantly to secondary organic aerosol (SOA) formation. However, knowledge about Δ3-carene oxidation pathways, particularly regarding their ability to form highly oxygenated organic molecules (HOMs), is still limited. In this study, we present HOM measurements during Δ3-carene ozonolysis under various conditions in two simulation chambers. We identified numerous HOMs (monomers: C7−10H10−18O6−14; dimers: C17−20H24−34O6−18) using a chemical ionization mass spectrometer (CIMS). Δ3-carene ozonolysis yielded higher HOM concentrations than α-pinene, with a distinct distribution, indicating differences in formation pathways. All HOM signals decreased considerably at lower temperatures, reducing the estimated molar HOM yield from ∼ 3 % at 20 °C to ∼ 0.5 % at 0 °C. Interestingly, the temperature change altered the HOM distribution, increasing the observed dimer-to-monomer ratios from roughly 0.8 at 20 °C to 1.5 at 0 °C. HOM monomers with six or seven O atoms condensed more efficiently onto particles at colder temperatures, while monomers with nine or more O atoms and all dimers condensed irreversibly even at 20 °C. Using the gas- and particle-phase chemistry kinetic multilayer model ADCHAM, we were also able to reproduce the experimentally observed HOM composition, yields, and temperature dependence.

Monoterpenes, comprising 15% of global biogenic volatile organic compound emissions, play a pivotal role in atmospheric chemistry. ∆3-carene, the second most prevalent monoterpene, has been identified as a significant source of secondary organic aerosol (SOA) upon oxidation, potentially surpassing α-pinene under similar conditions. Despite its importance, research has predominantly focused on α-pinene , leaving gaps in our understanding of ∆3-carene's oxidation pathways, particularly its capacity to form highly oxygenated organic molecules (HOM). To address this knowledge gap, we conducted an investigation into HOM formation during the ozonolysis of ∆3-carene using atmospheric simulation chambers. Employing a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with nitrate as the reagent ion (NO3-CIMS), we measured HOM resulting from ∆3-carene ozonolysis. Additionally, we explored the impact of temperature and relative humidity on HOM composition and distribution across various conditions (0, 10, and 20 ºC, and humidity levels below 15% and around 80%). Our analysis revealed diverse HOM monomers and dimers from ∆3-carene ozonolysis. Predominant HOM monomers included C10H14,16O9 and C9H12,14O9, while the largest dimers comprised C19H30O6,10,11 and C20H32O7,9,11. Significantly, HOM monomers with 9 or more oxygen atoms and all dimers irreversibly condensed onto particles, while those with 6-8 oxygen atoms behaved as semi-volatile organic species, maintaining notable gas-phase concentrations. Intriguingly, ∆3-carene ozonolysis produced higher HOM concentrations than α-pinene, suggesting distinct formation pathways for these two monoterpenes. Furthermore, we observed a substantial decrease in HOM concentrations at lower temperatures, consistent with previous studies on α-pinene ozonolysis. Despite similar main HOM species at temperatures of 20, 10, and 0 ℃, the ratio of HOM dimers to monomers increased from 0.78 to 1.51 as temperatures decreased. This temperature-dependent variation underscores the complexity of ∆3-carene's atmospheric processing, revealing nuanced behaviors of HOM under different environmental conditions. In conclusion, this study provides valuable insights into the HOM formation pathways of ∆3-carene, shedding light on its unique atmospheric chemistry. The observed differences in HOM concentrations and temperature-dependent behaviors highlight the need for a more comprehensive understanding of various monoterpenes, moving beyond the well-studied α-pinene. These findings contribute to the broader knowledge of biogenic volatile organic compounds and their impact on atmospheric processes.

Microplastic is ubiquitous in the environment. Recently it was discovered that microplastic (MP, 1 mu m-5 mm) contamination is present in the atmosphere where it can be transported over long distances and introduced to remote pristine environments. Sources, concentration levels, and transportation pathways of MP are still associated with large uncertainties. The abundance of atmospheric MP increases with decreasing particle size, suggesting that nanoplastics (NP, < 1 mu m) could be of considerable atmospheric relevance. Only few analytical methods are available for detection of nanosized plastic particles. Thermoanalytical techniques are independent of particle size and are thus a powerful tool for MP and NP analysis. Here we develop a method for analysis of polystyrene on the nanogram scale using pyrolysis gas chromatography coupled to mass spectrometry. Pyrolysis was performed using a slow temperature ramp, and analytes were cryofocused prior to injection. The mass spectrometer was operated in selected ion monitoring (SIM) mode. A lower limit of detection of 1 +/- 1 ng and a lower limit of quantification of 2 +/- 2 ng were obtained (for the trimer peak). The method was validated with urban PM2.5 matrices of low (7 mu g per sample) and high (53 mu g per sample) aerosol mass loadings. The method performs well for low PM2.5 loadings, whereas high PM2.5 loadings seem to cause a matrix effect reducing the signal of polystyrene. This effect can be minimized by introducing a thermal desorption step prior to pyrolysis. The study provides a novel analysis method for qualitative and semi-quantitative analysis of PS on the nanogram scale in an aerosol matrix. Application of the method can be used to obtain concentration levels of polystyrene in atmospheric MP and NP. This is important in order to improve the understanding of the sources and sinks of MP and NP in the environment and thereby identify routes of exposure and uptake of this emerging contaminant.

Formation of oxidized products from Δ3-carene (C10H16) ozonolysis and their gas-to-particle partitioning at three temperatures (0, 10, and 20 °C) under dry conditions (<2% RH) and also at 10 °C under humid (78% RH) conditions were studied using a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) combined with a filter inlet for gases and aerosols (FIGAERO). The Δ3-carene ozonolysis products detected by the FIGAERO-ToF-CIMS were dominated by semivolatile organic compounds (SVOCs). The main effect of increasing temperature or RH on the product distribution was an increase in fragmentation of monomer compounds (from C10 to C7 compounds), potentially via alkoxy scission losing a C3 group. The equilibrium partitioning coefficient estimated according to equilibrium partitioning theory shows that the measured SVOC products distribute more into the SOA phase as the temperature decreases from 20 to 10 and 0 °C and for most products as the RH increases from <2 to 78%. The temperature dependency of the saturation vapor pressure (above an assumed liquid state), derived from the partitioning method, also allows for a direct way to obtain enthalpy of vaporization for the detected species without accessibility of authentic standards of the pure substances. This method can provide physical properties, beneficial for, e.g., atmospheric modeling, of complex multifunctional oxidation products.

Candle burning is a considerable contributor to indoor pollutants, while secondary organic aerosols (SOA) from monoterpene ozonolysis represent another type. However, knowledge of the interactions of different indoor pollutants is limited. We investigated physicochemical properties of SOA generated from typical indoor chemistry of the O3/alpha-pinene reaction with and without the presence of particles and gases from a burning candle. Ozonolysis of alpha-pinene in the presence of candle gaseous emissions yielded a considerably lower particle number, larger particle sizes, and lower particle oxygen-to-carbon ratio compared with experiments without candle emissions. More nitrogen-containing organic compounds were observed in the aerosol phase with candle emissions. Furthermore, concentrations of some typical particle-phase products from the O3/alpha-pinene reaction (i.e., terebic acid, cis-pinic acid, and 3-methyl-1,2,3-butanetricarboxylic acid) were less abundant in the presence of candle emissions. The predicted volatility of particulate organic compounds was higher in experiments with candle emissions. The study demonstrates that candle burning can affect the chemical and physical properties of particles formed from other sources (e.g., alpha-pinene ozonolysis) by affecting gas-phase chemistry and gas-particle partitioning.