Orals: Thu, 7 May, 08:30–10:15 | Room 1.85/86
New particle formation (NPF) is the predominant source of atmospheric aerosols globally in terms of particle number concentration [1]. NPF is a multi-step process, consisting of the nucleation of low-volatility vapors from the gas-phase, and subsequent growth of the initial molecular clusters through condensation. Nucleation is, in many environments, primarily driven by inorganic acids, such as sulfuric acid over land or iodic acid over the ocean. Organic acids, such as pinic acid, an oxidation product of the highly abundant alpha-pinene, are also contributing to the mass of fully grown particles. It is, however, not yet clear at what stage of NPF organic compounds become important and whether organic compounds can nucleate even without inorganic substances being present [2]. This knowledge gap arises partly from the fact that mass spectrometry provides only compositional data and lacks structural insights into potential bindings of initial-stage clusters.
Here, we show that matrix-isolation Fourier transform infrared spectroscopy (MI-FTIR) can be used to study the initial steps of nucleation. Monomers and dimers of pinic acid are studied in cryogenic argon matrices to characterize their bindings based on the vibrational spectrum. In such cryogenic matrices, the infrared spectra are greatly simplified compared to gas-phase FTIR measurements due to the suppression of the rotational bands, making small dimer bands clearly visible. In addition, we investigate the free energies and harmonic vibrational frequencies of pinic acid monomers and dimers using molecular dynamics simulations and quantum chemical calculations, aiming to assess how different molecular alignments during nucleation influence the IR spectrum. By comparing the experimental MI-FTIR data with these density functional theory (DFT) calculations, it is shown that the measured pinic acid dimer spectra best fit the calculated ones for dimers with only one OH-bridge having formed during dimerization. The existence of such singly bonded clusters, which are not predicted as the lowest free energy conformer by DFT, could be ideal for subsequent growth due to up to three unbound OH-groups available for further oligomerization. We also find that pinic acid forms dimers much more easily than other alpha-pinene oxidation products, such as pinonic acid.
This study on the nucleation of pinic acid shows that MI-FTIR is a versatile method to study the structure of precursor molecules of NPF and the very first stages of nucleation. The toolbox of MI-FTIR in conjunction with DFT calculations can readily be implemented for other organic NPF-precursors, such as MBTCA, and their nucleation, leading to valuable insights into the structure of initial clusters.
References:
[1]: Stolzenburg, D. et al. Atmospheric nanoparticle growth. Rev. Mod. Phys. 95, 045002 (2023).
[2]: Elm, J., et al.: Quantum Chemical Modeling of Organic Enhanced Atmospheric Nucleation: A Critical Review. WIREs Comp. Molec. Sc. 13, e1662 (2023).
How to cite: Enders, V., Dinu, D. F., Pedersen, A. N., Elm, J., Grothe, H., Podewitz, M., Stolze, J., and Stolzenburg, D.: Nucleation of pinic acid: Using matrix-isolation Fourier transform infrared spectroscopy to get a first glance at processes on the molecular level, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9612, https://doi.org/10.5194/egusphere-egu26-9612, 2026.
Peroxy radicals, RO2, are key species in the atmosphere. They are formed from a reaction of OH radi-cals with hydrocarbons:
RH + OH + O2 -> RO2 + H2O
In polluted environments, RO2 radicals react predominantly with NO, leading to formation of NO2, and eventually through photolysis of NO2 to formation of O3.
At low NOx concentrations such as in the marine boundary layer or the background troposphere, the life-time of RO2 radicals increases and other reaction pathways such as self- and cross reaction with other RO2 or with HO2 radicals become competitive.
To study the reactivity of peroxy radicals, UV absorption spectroscopy has been employed in the past: this technique gives good sensitivity for peroxy radicals, but poor selectivity as these radicals have broad absorption features in the UV. We have established a technique allowing to follow peroxy radicals with a better selectivity compared to UV, but with still good sensitivity by coupling laser photolysis to cw-Cavity Ring Down Spectroscopy in the near IR. Two identical cw-CRDS paths are installed in a recently constructed temperature-controlled photolysis reactor in a small angle with respect to the Excimer photolysis beam, leading to an overlap of around 35 cm between the photolyzed volume and the detection volume. A third detection path for UV absorption measurements is installed in a slightly larger angle, leading to an overlap of around 20 cm between photolysis and absorption volume.
Here, we will present the first results obtained in the new reactor: the reaction between RO2 radicals and NO2. This reaction leads in an equilibrium reaction to the formation of RO2NO2 species. If the lifetime of these RO2NO2 are long enough, they will be transported and become a NOx source in remote environments. Therefore, determination of rate- and equilibrium constants of such reactions is important. In this work, two RO2 radicals have been generated simultaneously by 248nm laser photolysis of acetone, leading to roughly 1/3 CH3C(O)O2 radicals and 2/3 CH3O2 radicals. Time-resolved decays have then been observed for both radicals in the presence of different NO2 concentrations. The detection of both RO2 radicals is done simultaneously by high sensitivity cw-CRDS. RO2 concentrations can be decreased to a level where self-reaction becomes negligible at still excellent S/N ratio, making the measurement of RO2 + NO2 reaction straightforward. NO2 is quantified by UV-multipass absorption spectroscopy at 532nm in the photolysis reactor, and concentrations are compared with the calculated ones from the use of calibrated flowmeters.
How to cite: Fittschen, C., Fang, B., Xia, Y., Chen, I.-Y., Wu, Y.-X., Batut, S., Lahccen, A., Zhao, W., Tang, X., Luo, P.-L., and Pillier, L.: The reactivity of peroxy radicals, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11258, https://doi.org/10.5194/egusphere-egu26-11258, 2026.
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.
How to cite: Geretti, V., Baker, Y., Bannan, T., Voliotis, A., He, Q., Hohaus, T., Kang, S., Priestley, M., Tsiligiannis, E., Wang, H., Wu, R., Zanders, A., Zorn, S. R., McFiggans, G., Wu, C., Mentel, T. F., and Hallquist, M.: Atmospheric HO2/RO2 Ratios Weaken NOₓ Suppression of α-Pinene SOA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22171, https://doi.org/10.5194/egusphere-egu26-22171, 2026.
The formation of highly oxygenated organic molecules (HOM) during the OH-initiated oxidation of naphthalene remains poorly understood, despite experimental evidence for efficient aerosol precursor formation (Molteni et al., 2018; Garmash et al., 2020). As the simplest polycyclic aromatic hydrocarbon (PAH) and a major anthropogenic volatile organic compound in urban atmospheres, naphthalene is ubiquitous and readily oxidized under ambient conditions. However, current molecular-level descriptions of its oxidation predict autoxidation rates that are too slow to explain the observed HOM abundances, indicating missing or overlooked chemical pathways (Zhang et al., 2012; Shiroudi et al., 2015; Lannuque et al., 2024).
Ozone is one of the most abundant atmospheric oxidants, yet it is generally assumed to play a negligible role in the gas-phase oxidation of PAHs and their contribution to secondary organic aerosol (SOA) formation. Here, we show that this assumption does not hold for naphthalene oxidation, and that ozone can strongly influence the early stages of its autoxidation chemistry.
We investigated the hydroxyl radical (OH)–initiated oxidation of naphthalene using a flow reactor coupled to a nitrate-based chemical ionization mass spectrometer (NO₃⁻-CIMS), with reaction times ranging from 0.7 to 1.8 s. The presence of ozone led to a pronounced enhancement in product signal intensities, particularly for monomeric species (C₁₀H₉O5-10). At the shortest reaction time (0.7 s), a distinct suite of oxygenated monomers was observed only in the presence of ozone, indicating rapid ozone-assisted chemistry. Experiments using isotopically labelled ozone (¹⁸O₃) demonstrate that ozone directly influences the early stages of OH-initiated naphthalene oxidation. High-level quantum chemical calculations support mechanistic pathways in which ozone alters the fate of key radical intermediates, enabling efficient HOM formation. Moreover, the experiments on OH initiated oxidation of 1-naphthol, 2-naphthol, biphenyl, and anthracene show that this behavior is strongly structure-dependent, highlighting the broader relevance of ozone-assisted chemistry for PAHs.
Finally, global modeling indicates that ozone-driven pathways can increase anthropogenic SOA formation from naphthalene by up to 7% on a global scale. Together, these results reveal an unrecognized role of ozone in PAH oxidation and provide a mechanistic framework that helps to resolve the discrepancies between laboratory observations and current molecular-level understanding of PAH derived SOA formation.
References:
Molteni, U. et al (2018) Atmos. Chem. Phys. 18, 1909-1921.
Garmash, O. et al (2020) Atmos. Chem. Phys. 20, 515-537.
Zhang, Z. et al (2012) Phys. Chem. Chem. Phys. 14, 2645 - 2650.
Shiroudi, A. et al (2015) Phys. Chem. Chem. Phys. 17, 13719-13732.
Lannuque, A. et al (2024) Atmos. Chem. Phys. 24, 8589–8606.
How to cite: Kumar, A., Bezaatpour, M., Ojala, A., Seal, P., Barua, S., Marjanen, P., Garmash, O., Rönkkö, T., Iyer, S., and Rissanen, M.: Fast formation of aerosol precursors in polycyclic aromatic hydrocarbon oxidation: Evidence for ozone-assisted chemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20621, https://doi.org/10.5194/egusphere-egu26-20621, 2026.
Pyruvic acid (PA) is an atmospherically relevant organic compound that belongs to the family of keto acids, molecules that contain both a carbonyl and a carboxylic moiety and whose reactivity is suggested to contribute to the formation of secondary organic aerosols (SOAs) in the atmosphere [1]. The photochemistry of PA has received a great deal of attention due to it being its primary atmospheric sink, with stark differences observed when PA is in the gas phase or in an aqueous environment. In the gas phase, PA photochemistry is primarily driven by singlet states [2], although experimental evidence suggests that triplet states may still contribute to the formation of specific photoproducts [3]. Conversely, triplet pathways appear to dominate the photochemical reactivity of the molecule in aqueous phase [4], giving rise to different photoproducts. Despite the advances in the understanding of the photochemistry of PA, the underlying chemical mechanisms governing the photochemistry of the system remain unclear.
Recently, we have focused on studying the light-induced reactivity of PA using computational methods, in direct collaboration with experimental spectroscopists, to rationalise the phase-dependency of the photochemistry of this molecule. The computational approaches employed include static explorations of the ground and excited states potential energy surfaces of PA, which involve the determination of critical points and connected pathways between them, conformational analyses to establish the most relevant conformers of the molecule in gas and aqueous phases, and the obtention of absorption properties and vibrationally resolved photoelectron spectra.
In this contribution, we will discuss the main computational analyses carried out in a recent study in which we combined anion photoelectron spectroscopy and computational photochemistry to accurately determine the energy gap between the lowest singlet and triplet excited states of the molecule in the gas phase, a key quantity for understanding the photochemistry of the molecule that has remained elusive until now [5]. Furthermore, we will comment on the effect of aqueous solvation on the excited-state properties of the system. The results presented here will contribute to having a better understanding of why the light-induced reactivity of PA changes significantly from the gas phase to an aqueous environment and, ultimately, will help to assess the role and fate of this molecule in the atmosphere.
References:
[1] Rapf, R. J.; Perkins, R. J.; Carpenter, B. K.; Vaida, V. J. Phys. Chem. A 2017, 121 (22), 4272–4282.
[2] Hutton, L.; Curchod, B. F. E. ChemPhotoChem. 2022, 6 (11), e202200151.
[3] Sauer, L. J.; Davis, H. F. J. Phys. Chem. Lett. 2025, 16 (15), 3721– 3726.
[4] Griffith, E. C.; Carpenter, B. K.; Shoemaker, R. K.; Vaida, V. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (29), 11714– 11719.
[5] Burrow, E. M.; Carmona-García, J.; Clarke, C. J.; Curchod, B. F. E.; Verlet, J. R. R. J. Am. Chem. Soc. 2025, 147 (41), 36987-36991.
How to cite: Carmona-García, J. and Curchod, B. F. E.: Shedding light on the atmospheric photochemistry of pyruvic acid with theory and experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15039, https://doi.org/10.5194/egusphere-egu26-15039, 2026.
Understanding the chemistry in our atmosphere requires, at the fundamental level, a mechanistic understanding of the various processes taking place. Many steps in the large reaction networks involve photochemical reactions and dissociation plays a particularly important role. From a theoretical standpoint, modelling such processes is challenging due to the highly non-equilibrium nature of the problem. In this presentation, it will be demonstrated how accurate quantum chemistry methods can be used to characterise excited states of key molecules (e.g. methanol), and how quantum dynamical simulations are then able to fully describe the dissociation pathways accessible in a given range of wavelengths. Quantitative branching ratios can be automatically obtained for each channel, along with their timescales, which offers valuable information for atmospheric modellers. Furthermore, a newly developed procedure is also discussed, which allows these same quantum dynamics simulations to be performed in an explicit, atomistic environment. Applying these techiques to atmospherically relevant systems is sure to yield valuable insights and to reduce error bars on many of the parameters used in large scale models.
How to cite: Cigrang, L. and Worth, G.: Quantum dynamical modelling of photochemistry in gas-phase and complex environments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3025, https://doi.org/10.5194/egusphere-egu26-3025, 2026.
ABSTRACT
Organosulfur compounds (OSs), including organosulfates (R–OSO₃⁻) and sulfonates (R–SO₃⁻), are important constituents of atmospheric aerosols and cloud droplets, playing a significant role in Earth’s energy balance and climate dynamics. OSs are considered dominant contributors to particulate-phase organosulfur, accounting for approximately 5–30% of the organic mass in PM10, and are formed through complex chemical pathways involving biogenic volatile organic compounds and sulfate under acidic conditions. Inspired by recent experimental studies using LP-LPA [1], LC–MS [2,3], and DART [4] techniques, this work presents a comprehensive theoretical investigation of the conformational preferences, chemical reactivity, and atmospheric transformation pathways of methyltetrol sulfates (2-MTS and 3-MTS), together with several smaller organosulfates and sulfonates.
In particular, the reaction of these OSs with hydroxyl radical OH was examined by means of computational chemistry methods. DFT optimization of all stationary points at the M06-2X/6-311++G(d,p) level was used, followed by single-point calculations using CBS/DLPNO-CCSD(T1) with implicit solvation through the SMD model [5]. Rate constants at 298 K were obtained from transition-state theory, enabling direct comparison with experiments [1-4]. We also explored reaction channels leading to fragmentation, functionalization, conversion to non-organosulfate products, and degradation to inorganic sulfate.
Keywords: Organosulfur, atmospheric chemistry, quantum chemistry, hydroxyl radicals, kinetics.
References
[1] Lai, D.; Schaefer, T.; Zhang, Y.; Li, Y. J.; Xing, S.; Herrmann, H.; Chan, M. N. ACS ES&T Air, 2024.
[2] Gweme, D. T.; Styler, S. A. J. Phys. Chem. A, 2024, 128, 9462–9475.
[3] Lam, H. K.; Kwong, K. C.; Poon, H. Y.; Davies, J. F.; Zhang, Z.; Gold, A.; Surratt, J. D.; Chan, M. N. Atmos. Chem. Phys., 2019, 19, 2433–2440.
[4] Chen, Y.; Zhang, Y.; Lambe, A. T.; Xu, R.; Lei, Z.; Olson, N. E.; Zhang, Z.; Szalkowski, T.; Cui, T.; Vizuete, W. Environ. Sci. Technol. Lett., 2020, 7, 460–468
[5] Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B, 2009.
How to cite: Chouaib, Z., Duflot, D., and Toubin, C.: Theoretical Investigation of the Reactivity of Organosulfur Compounds with OH Radical, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9454, https://doi.org/10.5194/egusphere-egu26-9454, 2026.
Oxygenated organic molecules (OOMs), formed in the atmosphere by oxidation of volatile organic compounds, are expected to take part in new particle formation (NPF). To determine their contribution to NPF, it is necessary to sample global minima of OOM clusters. However, the complexity of potential energy surfaces and the requirement of expensive of quantum calculations makes modelling of OOM cluster formation extremely time consuming. We have previously addressed these bottlenecks by assuming that the minimum cluster energy is likely to found by maximizing the hydrogen bonds between the monomers. Thus, we initially perform a constrained sampling to force random hydrogen bond formation. Additional local minima are found by utilizing metadynamics simulations.
We further improve upon cluster sampling by replacing the costly DFT methods with significantly faster UMA and Orb-v3 neural network potentials (NNP). The pretrained models allow us optimize clusters geometries and predict cluster binding energies at near quantum chemical accuracy. We study the efficacy of the NNPs by generating dimer clusters of selected C10 sized OOMs. We find that the ability of OOMs to bind strongly is often hindered by the tendency of monomers to form intramolecular hydrogen bonds. Additionally, we show that C20 sized alpha-pinene accretion production may form cluster without the involvement of inorganic acids or ions, and their clustering ability with sulfuric acid is comparable to that of ammonia.
While our approach is more efficient, the sampling become less likely find the true global minima as cluster complexity increases. To further reduce the number of structures to needed optimize, we use the previously generated OOM cluster data to train a graph neutral network (GNN) model to predict energies of the configurations from graph-based descriptions. GNNs allow us to very quickly find a subset of hydrogen bond pairings most likely to optimize towards a new global energy minima though the prediction accuracy is significantly reduced compared to NNPs. Our goal is to train a general model which may also extrapolate to molecules and clusters not included in the training set.
How to cite: Kähärä, J., Haitsiukevich, K., Vehkamäki, H., and Kurtén, T.: Sampling of clusters of oxygenated organic molecules enhanced with machine learning models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13803, https://doi.org/10.5194/egusphere-egu26-13803, 2026.
Black carbon (BC) soot aerosol, produced during the incomplete combustion of fossil fuels, biofuels, and biomasses, is a major light-absorbing species and a key climate forcer. Despite its importance, BC remains challenging to represent in models due to persistent uncertainties in its spectral optical properties. In particular, the formation of non-absorbing coatings on fractal BC soot is a ubiquitous atmospheric process that enhances absorption, yet the magnitude of this enhancement (Eabs) remains highly uncertain and poorly represented in current models.
In order to advance on this topic, a set of experiments were performed using the 4.2 m3 CESAM simulation chamber on BC-soot aerosol generated from a propane diffusion flame. Experiments were conceived to systematically investigate the impact of coating formation and further ageing on soot spectral optical properties. Two chemical systems inducing the formation of a coating by a second scattering aerosol phase produced via the photo-oxidation of SO2 and the ozonolysis of α-pinene were considered.
The resulting dataset quantifies the magnitude and variability of Eabs under varying conditions, highlighting its dependence on soot morphology, soot–coating structure, and particle-to-particle heterogeneous mixing state. We show that the relative importance of these factors evolves with the dynamics of coating formation and ageing. Importantly, the Eabs cannot be reliably predicted using a fixed value or simple core–shell optical models, as commonly assumed in climate simulations.
How to cite: Di Biagio, C., Heuser, J., Yon, J., Cazaunau, M., Bergé, A., Pangui, E., Zanatta, M., Renzi, L., Marinoni, A., Yu, C., Chevaillier, S., Ferry, D., Laj, P., Maillé, M., Formenti, P., Picquet-Varrault, B., and Doussin, J.-F.: Evolution of the spectral optical properties of black carbon soot due to coating and ageing: insights from simulation chamber experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11876, https://doi.org/10.5194/egusphere-egu26-11876, 2026.
Cloud chamber experiments provide a controlled environment for investigating the microphysical processes that govern cloud formation, as well as elucidating the physical mechanisms underlying artificial weather modification. The Dual-Tank Mixed Cloud Chamber (2.7 m³ + 9 m³) allows precise control of temperature (−40-40°C), pressure (30-1110 hPa), and humidity. It is equipped with a comprehensive suite of instruments capable of monitoring the entire particle size distribution and activation, from aerosols to fog droplets. Quantitative seeding experiments for warm cloud (12°C)and cold clouds (-4°C) were performed to systematically examine their formation characteristics and microphysical responses to different seeding agents. In the warm-cloud experiments, the introduction of 10 g of a hygroscopic catalyst induced rapid nucleation of small droplets (3-10 μm), while simultaneously increase the development of larger droplets (≥ 20 μm) through enhanced hygroscopic growth. Under the -4°C condition in the cold-cloud experiments, the introduction of 0.5 g of AgI increased the submicron aerosol number concentration to 2 × 10⁴ cm⁻³, promoting rapid ice crystal formation that subsequently triggered the freezing and precipitation of liquid droplets.
How to cite: Luo, J. and Wu, H.: Design of a Dual-Tank Mixed Cloud Chamber and Quantitative Seeding Experiments for Warm and Cold Clouds, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4934, https://doi.org/10.5194/egusphere-egu26-4934, 2026.
Experiments in atmospheric simulation chambers enable the parameters of reaction kinetics to be determined and chemical mechanisms to be tested. However, observations are also affected by processes within the chambers that must be considered when interpreting experiments. This work compares results from experiments conducted in two chambers: the outdoor chamber SAPHIR at Forschungszentrum Jülich in Germany and the indoor chamber IASC at University College Cork in Ireland. The experiments were conducted on the same hydrocarbon oxidation processes. Box model calculations that included the chemical mechanisms of the species under study and the chemical and physical processes related to the chambers demonstrate good agreement between the modelled and measured time series of the observed species. This shows that the effects of the chambers were accurately characterised. The results could be used as a template for quality assurance and quality control protocols for simulation chambers, particularly within the ACTRIS research infrastructure.
How to cite: Fuchs, H., Wills, P., Campos-Pineda, M., Saito, S., Brik, A. B., Roman, C., Chandran, S., Wenger, J., Novelli, A., Färber, M., Gu, Y., Kharee, P., Tillmann, R., Wilson, D., Wegener, R., Adam, M. G., Bohn, B., and Ruth, A. A.: Comparison of the results of hydrocarbon oxidation experiments in an indoor and an outdoor atmospheric simulation chamber, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5980, https://doi.org/10.5194/egusphere-egu26-5980, 2026.
Plants that are prone to environmental oxidative and thermal stress exhibit high emission rates of biogenic volatile organic compounds. In this context sesquiterpenes have received an increased interest during the past decade, however, the organic compounds formed from their atmospheric degradation have not been thoroughly investigated, and their contribution to secondary organic aerosol (SOA) remains poorly characterized. Moreover, the nocturnal chemistry of sesquiterpenes has received virtually no attention.
The farnesenes are acyclic sesquiterpenes emitted by plants and agricultural crops and have applications in the biofuel, pharmaceutical, and food industry, and more recently in biotechnology (Sandoval et al., 2014). The (E)-β-farnesene has been studied in reactions with OH radicals and ozone (Kourtchev et al., 2009; 2012) however, the gas-phase chemistry of nitrate radical (NO3) initiated oxidation of (E)-β-farnesene has not yet been investigated. In particular, the study of nighttime SOA formation from (E)-β-farnesene oxidation is important, because nocturnal chemistry generates preconditions to daytime ozone formation and secondary organic aerosol growth.
Investigations on the gas-phase kinetic and atmospheric chemical degradation of (E)-β-farnesene with NO3 were performed in the 27 m3 Irish Atmospheric Simulation Chamber IASC (Cork, Ireland) at 295±2 K and 1000±3 mbar of atmospheric pressure. Positive-benzene and negative-iodide mode Chemical Ionization Mass Spectrometry (CIMS) analysis was used to evaluate the chemical composition of the gas mixture. SOA were sampled on PTFE filters (1 µm, 25 mm) and subsequently analyzed via the FIGAERO N2-thermo-desorption inlet connected to the CIMS instrument. A Scanning Mobility Particle Sizer (SMPS) was employed to measure particle number, size distributions and formation yields.
The reactivity of (E)-β-farnesene toward NO3 radicals was investigated by a relative rate method with 2-methoxyphenol and 2,5-dimethylfuran as reference compounds. (E)-β-farnesene was found to have a short atmospheric lifetime due to its fast reactions with NO₃ radicals, which efficiently remove it from the atmosphere under nighttime conditions.
The formation of gas-phase organic nitrates and peroxynitrates and highly oxygenated products were identified during the NO3-initiated oxidation of (E)-β-farnesene. The aerosol composition was also investigated in this study. Compounds observed in both gas and particle phases provide a direct link between gas-phase chemistry and aerosol composition since their volatility decreases through functionalization or accretion reactions. Atmospheric chemical mechanisms for the formation of these oxidation products, both in the gas and particle phase, will complement daytime OH and ozone (E)-β-farnesene oxidation. The degradation of (E)-β-farnesene on the reaction pathways will also be discussed.
Acknowledgement: This research is part of a Transnational access project that is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRAIA-2020-1, ATMO-ACCESS Grant Agreement number: 101008004. Funding is also provided by Taighde Éireann - Research Ireland (Grant numbers 21/FFP-A/8973 and GOIPD/2025/1260).
References:
Kourtchev, I., Bejan, I., Sodeau, J. R., & Wenger, J. C. (2009) Atmos. Environ., 43, 3182–3190.
Kourtchev, I., Bejan, I., Sodeau, J. R., & Wenger, J. C. (2012) Atmos. Environ. 46, 338–345.
Sandoval, C. M., Ayson, M., Moss, N., Lieu, B., Jackson, P., Gaucher, S. P., Horning, T., Dahl, R. H., Denery,
How to cite: Bejan, I., Roman, C., O'Sullivan, N., Movila, L., Ben Brink, A., Campos Pineda, M., Galloway, E., Ruth, A., and Wenger, J.: Nocturnal Radical Chemistry of (E)-β-Farnesene in the Atmosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20055, https://doi.org/10.5194/egusphere-egu26-20055, 2026.
Health effects associated with particulate matter (PM) exposure are closely linked to the ability of particles to induce the formation of reactive oxygen species (ROS) and trigger oxidative stress. Accordingly, the oxidative potential (OP) of PM is considered a more health-relevant toxicity metric than mass concentration alone. However, in densely populated and ecologically sensitive areas in the northwestern Mediterranean, the main sources contributing to OP remain poorly constrained, particularly regarding differences between urban and rural environments.
This sutdy systematically evaluated the OP of total suspended particulate matter (TSP) at an urban coastal site (Endoume) and a rural coastal site (Banyuls) in the region. OP was quantified using the dithiothreitol (DTT) assay and evaluated the contributions of primary emission sources and secondary formation to OP. Chemical tracers, dual carbon isotopes (¹³C & ¹⁴C), and positive matrix factorization (PMF) were used to apportion the main local sources.
The results reveal pronounced differences in both magnitude and source contribution to OP between urban and rural coastal aerosols. The annual mean organic-carbon-normalized DTT activity (DTTm) at Endoume was 20.0 ± 9.1 pmol min⁻¹ μg⁻¹ and 17.0 ± 5.3 pmol min⁻¹ μg⁻¹ at Banyuls (Mann–Whitney U test, p = 0.06). The annual mean volume-normalized OP (DTTv) was comparable at both sites (≈ 0.04 ± 0.02 pmol min⁻¹ m⁻³, Mann–Whitney U test, p < 0.05).
At Endoume, DTTv showed strong positive correlations with traffic- and fossil-fuel-combustion-related (FF) tracer metals (Pb, Cu, Zn) and elemental carbon (ρ ≈ 0.60–0.66, p < 0.01), together with pronounced seasonal variability. In spring, OP was primarily controlled by traffic and industrial emission (ρ > 0.75); in summer, ship emission emerged as the dominant driver (V–DTTv: ρ > 0.9, p < 0.01); while in autumn and winter, the contribution from biomass burning (BB) increased substantially (DTTv–nssK⁺: ρ = 0.74, p < 0.01). In contrast, the OP at Banyuls was dominated by traffic emission in spring (Zn–DTTv: ρ > 0.7, p < 0.01), whereas BB and ship emission jointly influenced OP in summer (V–Ni: ρ ≈ 0.7, p < 0.01). Additionally, dust and sea salt contributed significantly to OP at both sites, with a more pronounced influence at Banyuls (nss-Ca²⁺–DTTv: ρ ≈ 0.85, p < 0.01). Carbon isotope analysis showed that autumn samples at both sites exhibited lower OCNF and DTTv values, indicating that the influence of FF on OP may be more pronounced.
PMF results further show that at Banyuls, traffic emission and BB together accounted for approximately 25% of OPv, with natural dust contributing about 14%, whereas at Endoume, industrial emissions (25%), BB (20%), and traffic emission (19%) were the major contributors to OPv. For OPm, industrial emission dominated at Endoume, while natural sources such as sea salt and dust were the primary contributors at Banyuls; secondary formation processes contributed substantially to OPm at both sites. Overall, this study demonstrates strong spatial and seasonal source dependence of PM oxidative toxicity in the northwestern Mediterranean coastal region, providing important constraints for health-oriented air pollution assessments.
How to cite: Wei, M., Molina, C., Violaki, K., Bard, E., Kerhervé, P., Bridoux, M., Panagiotopoulos, C., and Nenes, A.: Source-driven variability of particulate matter oxidative potential at urban and rural coastal sites in the northwestern Mediterranean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20162, https://doi.org/10.5194/egusphere-egu26-20162, 2026.
How to cite: Han, J.: Bottom-up Coarse-Graining of Fluid Particles via Time-Evolving Fuzzy Clustering: A Pressure-Correction from Membership Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16326, https://doi.org/10.5194/egusphere-egu26-16326, 2026.
Recent advances in machine learning interatomic potentials have enabled the simulation of cluster formation from precursor vapor at a high level of theory. However, performing these simulations requires verifying that the models accurately describe cluster formation dynamics, particularly collision processes. In this work, we study the performance of two distinct machine learning (ML) architectures, AIMNet2 and PaiNN, against GFN1-xTB and ωB97X-3c reference data for atmospherically relevant collision systems (H2SO4–H2SO4, H2SO4–HSO4-, and H2SO4–NH(CH3)2).
We evaluate the models' ability to reproduce one-dimensional potentials of mean force (PMFs) and collision probabilities. Both models achieve excellent agreement with reference PMFs, yielding RMSEs at least an order of magnitude lower than chemical accuracy (1 kcal mol-1). Notably, PaiNN achieves lower errors in the binding region.
However, we observe significant differences in collision probabilities. While AIMNet2 accurately reproduces these probabilities, PaiNN fails to capture long-range interactions beyond its local cutoff (10 Å). For the charged H2SO4–HSO4- system, this leads to a complete loss of collision probability beyond 14 Å and an underestimation at shorter distances.
Our results demonstrate a clear trade-off: while PaiNN offers superior accuracy for equilibrium properties and binding energies, its local nature makes it unsuitable for collision kinetics in systems with strong long-range interactions. Conversely, AIMNet2's ability to model these long-range interactions makes it the necessary choice for simulating collisions in such systems.
How to cite: Neefjes, I., Kubecka, J., and Elm, J.: Machine learning interatomic potentials with accurate long-range interactions for molecular dynamics collision simulations of atmospherically-relevant molecules, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21549, https://doi.org/10.5194/egusphere-egu26-21549, 2026.
Ice and mixed phase clouds in the earth's atmosphere form predominantly through heterogeneous nucleation on seed particles, such as mineral dust and organics. Determining the atomistic ice-nucleation mechanism on these particles is challenging for experiments and simulations. When simulating ice nucleation using Molecular Dynamics (MD), one typically relies on classical empirical potentials (force fields) to describe interactions between atoms in the particle surface and water. However, due to the large number of different materials ice-nucleating particles can consist of, accurate classical empiric potentials are not available for all systems, leading to heavy computational costs for creating and testing new ones.
In recent years, foundation neural network potentials (NNPs), trained on large sets of quantum chemical data, aim to enable simulations of any system, thus circumventing the issue of creating new potentials. These NNPs would ideally combine accuracies of Density Functional Theory (DFT) with simulation speeds of classical MD. To determine the viability of using foundation models in MD simulations of heterogenous ice nucleation, we have benchmarked the ice-water equilibria of four NNPs: SO3LR (Kabylda et al. 2025), Orb v3 (Rhodes et al. 2025), Fennix-Bio1 (Plé et al. 2025) and ANI-2x (Devereux et al. 2020). We determined their melting points and, where not available in the literature, the water density isobars they exhibit in the temperature range 250-300 K. We have used the coexistence method: A system initially containing hexagonal ice and liquid water is simulated in the NPT ensemble, and the melting point is determined as the temperature at which the number of ice-like water molecules (counted using the classification algorithm LICH-TEST) does not change over time.
The SO3LR potential was the only one of the four displaying a melting point close to 273 K. Fennix-Bio1 underestimated the melting point by 20 K, while both Orb v3 and ANI-2x overestimated it by over 75 K. By comparing variants of the latter two models, we can infer that inclusion of dispersion interactions during either training or inference improves the water density isobar, which in turn leads to a more accurate melting point. In addition, we find that while the NNPs are in theory reactive models, no Grotthus-like proton transfers were observed in the simulations.
Kabylda et al.: Molecular Simulations with a Pretrained Neural Network and Universal Pairwise Force Fields, ChemRxiv, 2025.
Rhodes et al.: Orb-v3: atomistic simulation at scale, https://arxiv.org/abs/2504.06231, 2025.
Plé et al.: A Foundation Model for Accurate Atomistic Simulations in Drug Design, ChemRxiv, 2025.
Devereux et al.: Extending the Applicability of the ANI Deep Learning Molecular Potential to Sulfur and Halogens, Journal of Chemical Theory and Computation, 16, 4192–4202, 2020.
How to cite: Nilsson, R., Roudsari, G., Lbadaoui-Darvas, M., Reischl, B., and Ingram, S.: Using New Generation Neural Network Potentials to Benchmark Ice-Water Equilibria, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18505, https://doi.org/10.5194/egusphere-egu26-18505, 2026.
Micro- and nanoplastics (MNPs) have been detected in atmospheric deposition and cloud samples, suggesting that they may act as cloud condensation nuclei (CCN) and/or ice-nucleating particles (INPs), which is likely the main pathway through which atmospheric microplastics impact climate. Laboratory studies report immersion freezing temperatures for various microplastics between −15.1 °C (Ganguly 2019) and −23.2 °C (Seifried 2024), comparable to those of mineral dust, the dominant global source of atmospheric INPs. Immersion freezing IN activity also requires CCN activity as immersion freezing occurs in existing cloud droplets. Atmospheric aging may further enhance the CCN or IN activity of MNPs, consistent with laboratory evidence and observations showing that microplastics in cloud samples are predominantly aged, as indicated by increased hydroxyl functionalization identified by FTIR analysis (Wang 2023).
Despite growing evidence for their potential role in cloud microphysics, no parameterization currently exists to represent the CCN or IN activity of microplastics in cloud models, largely due to limited understanding of the underlying activation mechanisms. Here, we use hybrid Grand Canonical Monte Carlo–molecular dynamics simulations to investigate cloud droplet growth and activation on fresh and atmospherically aged crystalline polypropylene (PP) surfaces. Our results show that aged PP activates as CCN via an adsorption-driven mechanism, whereas fresh, non-oxidized PP does not activate under the simulated conditions. Activation on aged surfaces proceeds through (1) dropletwise adsorption of water nanoclusters at active sites, (2) cluster growth, and (3) coalescence into a continuous multilayer of water. Model calculations based on adsorption nucleation theory (Laaksonen 2015) indicate that activation occurs at slightly higher critical supersaturations than for mineral dust, while the critical radius is smaller than for illite, Saharan dust, or Arizona Test Dust. These findings provide mechanistic insight into CCN activation on aged microplastics and the model calculation provides a first approach to develop parameterizations of microplastics for cloud microphysics schemes.
Ganguly, M.; Ariya, P.A. ACS Earth and Space Chemistry 2019, 3, 1729–1739.
Seifried, T.M.; Nikkho, S.; Morales Murillo, A.; Andrew, L.J.; Grant, E.R.; Bertram, A.K. Environmental Science & Technology 2024, 58, 15711–15721.
Wang, Y.; Okochi, H.; Tani, Y.; Hayami, H.; Minami, Y.; Katsumi, N.; Takeuchi, M.; Sorimachi, A.; Fujii, Y.; Kajino, M.; et al. Environmental Chemistry Letters 2023, pp. 1–8.
Laaksonen, A. The Journal of Physical Chemistry A 2015, 119, 3736–374.
How to cite: Lbadaoui-Darvas, M. and Nenes, A.: Adsorption-Driven Cloud Droplet Activation of Fresh and Aged Polypropylene Particles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17024, https://doi.org/10.5194/egusphere-egu26-17024, 2026.
Understanding how water adsorbs on metal oxide surfaces is essential for describing interfacial processes relevant to atmospheric chemistry, heterogeneous catalysis, and aerosol-cloud interactions. While recent experiments have shown pronounced adsorption-desorption hysteresis for water on nonporous oxides such as copper(II) oxide (CuO), the molecular mechanisms underlying this behavior remain unclear, particularly in the presence of chemisorption and surface hydroxylation. Molecular simulations provide a unique route to directly resolve these processes at the atomic scale.
In this work, we investigate water adsorption on the CuO(111) surface using a combined grand-canonical Monte Carlo (GCMC) and reactive molecular dynamics (MD) approach. GCMC simulations were performed at fixed temperature and water chemical potential to obtain adsorption isotherms directly comparable to experiment. Interatomic interactions were described using a ReaxFF reactive force field, allowing spontaneous water dissociation, proton transfer, and dynamic surface restructuring. Adsorption isotherms were constructed over a wide range of chemical potentials and converted to relative humidity using the simulated condensation chemical potential. The simulations reveal a multistage adsorption mechanism. At low chemical potentials, water adsorbs primarily via dissociative chemisorption, leading to progressive hydroxylation of the CuO surface. As chemical potential increases, additional water accumulates non-uniformly as hydrogen bonded clusters rather than as a continuous film. Near saturation, these clusters coalesce and trigger rapid multilayer growth. Reactive MD simulations show that chemisorbed species remain mobile and influence cluster stability, growth pathways, and desorption behavior. Simulated adsorption isotherms are in good agreement with experimental measurements and capture key features associated with adsorption-desorption hysteresis. By extracting adsorption parameters directly from the simulations, we assess the applicability of Frenkel-Halsey-Hill type multilayer adsorption models to reactive oxide surfaces, demonstrating that chemisorption must be explicitly accounted for in molecularly based adsorption frameworks.
How to cite: Roudsari, G., Lbadaoui Darvas, M., A. Piedehierro, A., Viisanen, Y., Laaksonen, A., and Nenes, A.: Molecular-Scale Simulation of Water Adsorption and Chemisorption on Copper Oxide Surfaces, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11435, https://doi.org/10.5194/egusphere-egu26-11435, 2026.
Ice and mixed-phase clouds can form at moderate supercooling on seed particles through heterogeneous ice nucleation. Feldspar particles constitute a significant fraction of mineral dust in the atmopshere and have been identified as good ice nucleating particles. However, they can exhibit different chemical composition and crystal structure, which affects their ice nucleation activity [1], and the details of the ice nucleation mechanism(s) remain unknown.
Here, we present atomic force microscopy images of ice crystals growing from the vapor phase at low temperature on the (001) surfaces of two types of feldspar crystals: the highly ordered microcline, and the disordered sanidine. In contrast to the prevailing view of active sites such as step edges or cracks being responsible for ice nucleation on feldspar [2], we observe ice growth at random positions on the bare terrace of feldspar microcline (001). For the closely related feldspar sanidine, ice nucleation is only observed at step edges, as previously reported.
Our observations underscore the exceptional ice nucleating ability of microcline as it demonstrates ice nucleation even in the absence of surface defects and raise important questions regarding the different ice nucleation mechanisms on these two feldspar mineral surfaces, which are investigated using atomistic simulations.
[1] Harrison, A. D., et al., Atmos. Chem. Phys., 16, 10927-10940, 2016.
[2] Kiselev, A., et al., Science, 355, 367-371, 2017.
How to cite: Reischl, B., Schneider, F., Nilsson, R., Bechstein, R., Koop, T., Kühnle, A., and Dickbreder, T.: Nanoscopic insights on ice nucleation on feldspar microcline and sanidine , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18933, https://doi.org/10.5194/egusphere-egu26-18933, 2026.
