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 (NO_x), 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 NO_x. While NO_x 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 NO_x-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 (C₁₀H₉O_5-10). 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.

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 (E_abs) 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 E_abs 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 E_abs 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.