Secondary formation of OA in polluted urban environments involves diverse precursors and complex pathways. When the severity and frequency of heatwaves are expected to rise, little is about the heatwave impacts on the OA formation in such polluted environments. Here we deployed a long time-of-flight aerosol mass spectrometer (LTOF-AMS) to measure the real-time chemical composition of submicron aerosols along with advanced time-of-flight chemical ionization mass spectrometers (TOF-CIMS) to detect gaseous and particulate oxidation products in Beijing in recent years. Six process-level secondary OA (SOA) factors are resolved and unique molecular tracers for each of the six processes are identified. The six SOA factors can be explained by intensified photochemical and heterogeneous reactions with higher volatile organic compounds emissions and oxidant level, increased aerosol surface, stronger aerosol acidity, and higher ammonia concentration etc. The six-factor SOA seperation provides a machanistic understanding of the net enhancement of SOA during heatwave events, which have been observed in many places worldwide in summer. We further applied machine learning methods to identify the key drivers of the SOA enhancement and used the simulated key parameters from the GEOS-Chem model to perturb the SOA formation during the heatwave episode under clean air actions and climate change scenarios. Our results suggest that the SOA enhancement due to heatwave will be increasingly important in the future. This study underscores the urgency of validating temperature responses of organic aerosol in chemical transport models to facilitate air quality management in a warming world. 

How to cite: Chen, Q., Zheng, Y., Koenig, T. K., Miao, R., Cheng, X., Zhang, Q., Wang, H., and Ge, Y.: Sources of organic aerosol in polluted urban environments and the heatwave impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17292, https://doi.org/10.5194/egusphere-egu26-17292, 2026.

Knowledge of the molecular characteristics of organic aerosols is essential for evaluating their atmospheric processes and associated environmental and health effects. However, little is known regarding the molecular characteristics of organic aerosols in coal resource-based cities. Herein, the molecular characteristics of water-soluble organic matter (WSOM) in wintertime PM2.5 during haze and heavy haze days in a typical coal resource-based city (Taiyuan, China) were analyzed using Fourier-transform ion cyclotron resonance mass spectrometry. A total of 5106 CcHhOoNnSs formulas were assigned, with m/z values predominantly concentrated in the range of 150–400 Da. The proportion of CHOS is higher than that in other cities, and a series of C7H6(CH2)0–8O5S formulas exhibited high intensities, most of which could be traced to coal combustion sources. The relative abundance of sulfur-containing organic molecules increased significantly on heavy haze days compared to haze days (30.4% vs. 25.0%) and was much higher than that observed in other cities. Additionally, CHO, CHON, and CHOS formulas consistently exhibited higher oxygen content on heavy haze days, likely due to atmospheric oxidation processes. Moreover, oxygen addition, methylation, and carboxylic acid reactions were identified as the primary possible pathways driving the transformation of primary organic aerosol into secondary organic aerosol under both haze and heavy haze conditions. Meanwhile, a total of 263 organophosphorus (OP) formulas were identified, which were predominantly distributed within the 180–550 Da range. 41.9% of assigned OP formulas contain −OPO3 or −RPO3 groups, and most OP formulas were oxidation-available with high environmental stability. Correlation analyses indicated that urban atmospheric OP may be emitted from biological sources. These results highlight the significant and distinct roles of both sulfur- and phosphorus-containing compounds in the complex atmospheric chemistry of coal resource-based urban environments.

How to cite: Guo, Z., Zhang, C., Liu, F., Zhang, G., Wang, J., and Bi, X.: Molecular characteristics of water-soluble organic aerosols in a coal resource-based city revealed by FT-ICR MS: a significant role of sulfur- and phosphorus-containing compounds, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7888, https://doi.org/10.5194/egusphere-egu26-7888, 2026.

Aromatic hydrocarbon secondary organic aerosols (SOAs) derived from anthropogenic sources are a typical class of secondary organic aerosols that significantly impact global climate and human health. The composition and physicochemical properties of atmospheric aerosols are notably influenced by varying oxidation conditions during the day and night. Currently, the molecular composition and formation mechanism of SOAs generated from benzene under diurnal oxidation conditions remain unclear. This study focuses on the molecular composition and chemical formation mechanism of secondary organic aerosols (SOAs) derived from benzene. We utilized the SAPHIR smog chamber to simulate diurnal oxidation experiments of benzene. The molecular composition evolution of benzene SOAs was characterized in real-time online using an Extractive Electrospray Ionization Chemical Ionization Mass Spectrometer (EESI-CIMS). We compared the differences in SOA composition under different oxidation conditions during the day and night, identified key characteristic products, and ultimately proposed relevant mechanisms for SOA formation. This research not only enhances our understanding of the chemical formation mechanisms of SOAs but also provides a scientific basis for air pollution control and climate change assessment.

How to cite: Luo, H., Shen, H., Wu, R., He, Q., Zorn, S. R., Fuchs, H., Mentel, T. F., and Zhao, D.: Molecular Composition and Formation Mechanism of Benzene SOA under Diurnal Oxidation Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4155, https://doi.org/10.5194/egusphere-egu26-4155, 2026.

New particle formation (NPF) proceeds efficiently in polluted urban areas, where oxygenated organic compounds are crucial for early particle growth. Their contribution to NPF depends on volatility distribution, described by the volatility basis set (VBS). Urban volatile organic compound (VOC) emissions have shifted significantly due to regulations and changing consumer habits. Volatile chemical products (VCPs) from cleaning agents, personal care products, and coatings, now rival or exceed combustion sources as primary urban VOC emitters. Though many VCP constituents are secondary organic aerosol (SOA) precursors, their oxidation pathways and NPF contributions remain poorly understood. Recent research highlights that assessing NPF potential requires knowing the full volatility range, including moderately oxygenated molecules (MOMs).

We investigate the full oxidation chain of selected VCP emissions, from VOC precursors to MOMs to highly oxygenated molecules (HOMs) and resulting particle yields. Oxidation occurs in a newly developed flow reactor designed to minimize wall losses and enable steady-state conditions. We employ multi-pressure chemical ionization mass spectrometry with an ultra-high-resolution Orbitrap mass spectrometer. By switching between low-pressure (<1 mbar) ionization for VOC detection using the internal fluoranthene (C₁₆H₁₀) ion source and two different atmospheric-pressure ionization schemes for MOMs (uronium, CH₅N₂O⁺) and HOMs (nitrate, NO₃^-), we capture the VBS across the full volatility range. New particle yields are quantified using a scanning mobility particle sizer (SMPS) and their chemical composition is evaluated using nanoelectromechanical sensors with Fourier transformation infrared spectroscopy (NEMS-FTIR).

We access household cleaning products as potential VCPs emitters. Method performance and consistency are evaluated using limonene oxidation as a reference system for NPF-relevant oxidation chemistry. The obtained VBS distributions can be compared to limonene ozonolysis, under the assumption that charging efficiencies might be well-related to compound volatility. This comparison enables an estimation of the relative NPF potential of different VCP emitters. We find significant variety in the NPF potential among cleaning products of different suppliers. While lemon-scented products resemble limonene (a major ingredient of these products) spectra, we can clearly demonstrate that the complex mixtures present in the cleaning products can enhance the NPF potential of certain products. Altogether, our results demonstrate that multi-pressure chemical ionization provides a powerful approach to link molecular composition, volatility, and NPF potential for emerging urban organic sources. Beyond advancing mechanistic understanding of urban aerosol formation, this framework enables identification of key VOC precursors in e.g. source apportionment approaches. In addition, our first results from different cleaning products show that through product reformulation new opportunities arise to mitigate air quality impacts associated with VCP emissions.

How to cite: Verma, R., Tischberger, M., Ellmauer, M., Grothe, H., and stolzenburg, D.: Volatile chemical products as potential emerging drivers of urban new particle formation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14377, https://doi.org/10.5194/egusphere-egu26-14377, 2026.

Nanoparticle growth is a critical process determining whether newly formed nanoparticles can survive to cloud condensation nuclei (CCN) and haze sizes, thereby influencing climate and air quality. The growth is primarily driven by the condensation of low-volatility organic vapors. Therefore, comprehensive and accurate understanding of organic-driven particle growth processes is crucial for accurately assessing their environmental, climatic, and health impacts, and for developing targeted mitigation strategies. Three-dimensional models are essential tools for elucidating regional-scale particle evolution mechanisms. However, existing 3D atmospheric models fail to characterize the formation and condensation of low-volatility organics driving particle growth, which hinders accurate simulation and mechanistic understanding of growth processes.

Here, we develop an advanced 3D numerical modeling framework for organic gas-phase oxidation and particle growth by implementing the integrated two-dimensional volatility basis set (I2D-VBS) and a kinetic gas-particle partitioning model in WRF-Chem. This model accurately simulates organic oxidation products across the full volatility range and their condensation-driven nanoparticle growth processes. The model effectively reproduces process-level particle growth observed at different sites and seasons across China, reducing growth rate errors from orders of magnitude to reasonable ranges and significantly enhancing simulations of particle number size distributions.

Based on the improved model, we conduct a comprehensive analysis of organic-driven particle growth in China and identify the primary organic sources driving particle growth. Results show that particle growth rates in China are predominantly contributed by oxidation products of intermediate/semi-volatile organic compounds (I/SVOCs), accounting for >65% in winter and >59% in summer across key regions including Beijing-Tianjin-Hebei, Yangtze River Delta, and Pearl River Delta. Anthropogenic VOCs (AVOCs) rank second in contribution, though the contribution from biogenic VOCs (BVOCs) may exceed that from AVOCs in some southeastern regions during summer. Among precursor categories, aromatics and aliphatics are the most important, followed by oxygenated aromatics. Finally, we further elucidate the impacts of organic condensation-driven growth on particle and CCN number concentrations. This study fills critical knowledge gaps regarding particle growth mechanisms and their environmental impacts in China.

How to cite: Li, Z., Zhao, B., He, Y., Shen, J., Yin, D., and Wang, S.: Elucidating Mechanisms of Organic-Driven Nanoparticle Growth in China through Advanced Modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19850, https://doi.org/10.5194/egusphere-egu26-19850, 2026.

The southern African region, particularly the Atlantic coast of Namibia, is an ideal natural laboratory influenced by contrasting aerosol sources that represent endmembers of aerosol formation processes, including aged marine aerosols, mineral dust resuspension, and long-range transported biomass-burning aerosols (BBA), with only moderate local anthropogenic influence. This setting enables direct linkage between aerosol chemical composition and optical properties under near-pristine conditions, thereby improving the representation and projection of aerosol–radiation and aerosol–cloud interactions and associated climate feedbacks in close proximity to the highly sensitive southeastern Atlantic stratocumulus deck, one of the major regulators of planetary albedo.

Although organic matter (OM) typically accounts for only a minor fraction of aerosol mass, it demonstrates a disproportional contribution to optical properties, hygroscopicity and cloud-formation, especially in a dust-dominated environment. Here, we present a comprehensive characterization of OM with respect to its sources, molecular composition, aging processes, and relationships with aerosol optical properties. Organic aerosol (OA) was extracted from daily particulate matter (PM₁₀) samples collected during a month-long field campaign at Gobabeb, Namibia, and analyzed using high-resolution mass spectrometry (HRMS), complemented by inorganic ion analysis, organic and elemental carbon quantification, and multivariate statistical methods. These approaches were used to distinguish three dominant aerosol regimes: dust-dominated, BBA-influenced, and marine-dominated periods.

Organic molecules associated with BBA events exhibited elevated O/C ratios and double-bond equivalent (DBE) values, consistent with enhanced light absorption in the UV–visible range. These molecular features show strong correlations with bulk aerosol extinction and scattering coefficients, highlighting the optical relevance of OM despite its limited mass contribution. To better constrain the contribution of dust-derived OM, laboratory resuspension experiments were conducted using local soils. Comparison of OM extracted from parent soils and resuspended aerosols revealed substantial compositional differences, indicating selective transfer of specific organic components into the aerosol phase. This selectivity allowed identification of soil-derived OM fractions that systematically contribute to atmospheric aerosols and their optical properties.

Finally, we applied a novel formula-difference approach to the HRMS data to resolve aerosol aging processes across the different aerosol regimes. By comparing molecular transformation patterns, we explored period-specific aging pathways reflected in characteristic gains and losses of functional groups and molecular connectivity. These chemical fingerprints indicate periods with higher influence of oxidation, condensation and aromatisation of OM, which provides additional insights into the fate of organic matter in mixed aerosol systems and its role in modifying aerosol optical properties during atmospheric aging.

How to cite: Zherebker, A., Baldo, C., Formenti, P., and Giorio, C. and the Aerofog team: Organic matter in dust-dominated aerosols over Namibia influenced by biomass burning and marine emissions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10531, https://doi.org/10.5194/egusphere-egu26-10531, 2026.

Per and polyfluoroalkyl substances (PFAS), a class of toxic compounds often referred to as “forever chemicals”, are increasingly detected in the atmosphere. Aerosolisation from contaminated aqueous reservoirs has been proposed as a pathway for atmospheric PFAS, drawing analogy to sea-spray processes and supported by their elevated concentrations reported near sewage treatment facilities (Kizhakkethil et al., 2025). However, aerosolisation and particle formation in anthropogenically impacted waters differ fundamentally from marine systems, and the physico chemical controls governing PFAS aerosolisation outside the marine context remain poorly understood.

The aim of this work was to investigate the effect of PFAS molecular properties, including carbon chain length and functional groups, on aerosolisation from contaminated aqueous solutions. Experiments were conducted in the Chamber for Aerosol Modelling and Bio-aerosol Research (ChAMBRe), Italy. Twenty five PFAS, covering short, medium and long chain perfluoroalkyl carboxylic acids, perfluoroalkane sulfonates, fluorotelomer sulfonates and emerging alternatives representative of wastewater impacted environments were investigated. The role of bioaerosol seed particles commonly present in such environments was also assessed, as they could act as sinks or carriers for highly surface active PFAS and thereby influence their aerosol phase distribution.

Aerosol mass size distributions revealed a strong dependence on molecular structure, indicating compound-specific particle-phase behaviour. The presence of biological particles did not systematically alter PFAS size-resolved distributions, suggesting that the studied PFAS exhibited limited interaction with bioaerosols and remained predominantly in the submicron size range under the investigated conditions, which may favour their atmospheric persistence and long-range transport.

Overall, these findings indicate that primary aerosol formation from contaminated aqueous systems represents a chemically selective pathway for introducing PFAS into the organic aerosol, with size-resolved characteristics governed primarily by molecular properties and aerosol formation processes.

Reference: Kizhakkethil, J. P., Shi, Z., Bogush, A., and Kourtchev, I.: Measurement report: Per- and polyfluoroalkyl substances (PFAS) in particulate matter (PM10) from activated sludge aeration, Atmos. Chem. Phys., 25, 5947–5958, https://doi.org/10.5194/acp-25-5947-2025, 2025.

How to cite: Kourtchev, I., Coupe, S., Pandamkulangara Kizhakkethil, J., Gatta, E., Massabò, D., Prati, P., Vernocchi, V., and Mazzei, F.: Effect of per and polyfluoroalkyl substances (PFAS) molecular properties on aerosolisation and size resolved distributions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1761, https://doi.org/10.5194/egusphere-egu26-1761, 2026.

Tropical forests are essential ecosystems for the global aerosol population [1]. The mechanisms behind new particle formation (NPF) in the Amazon have long remained elusive, with traditional “banana” events being rarely observed. Recent studies show that forest emissions of volatile organic compounds (VOCs) that are subsequently oxidized produce particles that can serve as Cloud Condensation Nuclei (CCN), which are critical for the tropical hydrological cycle. Several studies showed that new particle formation can occur at high altitudes (12-14 Km) [2]. Nanoparticles are also produced by different mechanisms at the canopy level, following oxidation by VOCs and downdrafts [3]. Another study showed that Quiet New Particle Formation also occurs in Amazonia and can be responsible for a significant fraction of the aerosol population [4]. These studies show a wide diversity of processes that produce nanoparticles, which adds to the population of primary biological particle emissions [1]. Ground-based long-term measurements were conducted at the Amazon Tall Tower Experiment (ATTO), integrating over 10 years of wet-season size-distribution measurements. Intensive aircraft campaigns conducted during the CAFÉ-Brazil experiment have identified the mechanisms underlying high-altitude particle production. We have also studied the role that deep convection and strong precipitation events play in modulating the particle population at the forest canopy level.

Of particular interest is the strong interaction between the plant metabolism and the climate they control, since aerosol particles influence the radiation balance, carbon cycling, and precipitation patterns. These natural wet-season processes compete with the dry-season biomass burning emissions, which strongly alter the particle population.

In this presentation, we will discuss the complex picture of particle production and development in Amazonia. This study sheds light on a previously unknown process of nucleation and growth occurring frequently in the Amazonian BL, distinct from the known intense particle bursts and growth associated with downdrafts.

[1] P. Artaxo, et al. Tellus Series B 24.1 (2022): 24–163.

[2] J. Curtius et al., Nature, 636 (2024) 124–130.

[3] L. A. T. Machado, et al., Atmos. Chem. Phys., 21.23 (2021) 18065–18086.

[4] B. B. Meller, et al., EGUsphere, 2025-4581 (2025).

How to cite: Artaxo, P., Meller, B., Rizzo, L., Machado, L., Valiati, R., and Pöhlker, C.: New particle formation in the Amazonian atmosphere and the role of organic compounds, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5958, https://doi.org/10.5194/egusphere-egu26-5958, 2026.

Atmospheric organic aerosol (OA) contains a mixture of molecules from various sources, both primary and secondary, and with different levels of atmospheric aging and propensity for particle-phase reactions. This highlights the need for detailed characterization of OA composition to better understand its sources, atmospheric transformation, and resulting physicochemical properties. This characterization is also preferably conducted in real-time, to capture both sudden changes of airmasses as well as fast reactions within the particles. We performed an intercomparison at our field station in Hyytiälä in the Finnish boreal forest where we deployed four different mass spectrometers able to measure aerosol composition in real-time. The instruments included an Aerosol Chemical Speciation Monitor (ACSM) as the reference instrument as well as three chemical ionization mass spectrometers (CIMS): a Vaporization Inlet for Aerosols coupled with a nitrate CIMS (VIA-NO₃-CIMS), an Extractive Electrospray Ionization TOF (EESI-TOF), and a Filter Inlet for Gases and AEROsols coupled with an iodide CIMS (FIGAERO-I-CIMS). We also performed a follow-up chamber study to complement some missing comparisons due to instrumental problems during the field campaign.

CIMS has become a key tool for probing gas-phase composition, and studies have demonstrated how different reagent ions are able to detect distinct molecule types. In addition, using different methods for transferring aerosol-phase molecules into the gas-phase for detection by CIMS will most likely also result in differences in detected molecules. This study aimed to evaluate differences in instrument sensitivity for different types of OA and assess the fraction of OA that could be measured with these state-of-the-art methods deployed together.

The campaign in Hyytiälä (Sept 2-25, 2024) provided several interesting results. The foremost finding was a very high correlation between that the organics measured by the ACSM and the VIA–NO₃-CIMS (R² = 0.90) and FIGAERO-I-CIMS (R² = 0.88). Consequently, also the VIA and the FIGAERO correlated extremely well, which was unexpected given that iodide and nitrate CIMS instruments tend to show very few common signals in typical gas-phase measurements. Sulfate measured by the VIA–NO₃-CIMS agreed almost perfectly with ACSM measurements (R² = 0.97), further validating that the instrument was working well throughout the campaign. Due to technical issues, however, the EESI-TOF did not provide enough data during the Hyytiälä campaign, and therefore we instead compared the VIA and EESI instruments with an AMS during a later chamber campaign using different types of aerosol precursors. This data is currently being analyzed in more detail, but at least for monoterpene-derived OA, also the EESI shows good correlation with the VIA, though with a higher sensitivity for less-oxygenated molecules while the VIA had higher sensitivity for the most oxygenated compounds. I will present more in-depth comparison results at the conference, including key differences between the methods, but our comparison indicates that all three aerosol CIMS instruments are able to detect a large fraction of the OA, at least in regions dominated by biogenic secondary OA.

How to cite: Ehn, M., Mickwitz, V., Li, Y., Qi, L., Zhang, J., Alton, M., Canagaratna, M., Graeffe, F., Nissinen, A., Pusfitasari, E., Schobesberger, S., and Zhao, J.: Comparison of mass spectrometric approaches for real-time characterization of organic aerosol, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16379, https://doi.org/10.5194/egusphere-egu26-16379, 2026.

New particle formation occurs broadly via two processes. First, small clusters can be stabilized by a reaction between constituents, such as proton transfer in acid-base clusters like sulfuric acid and ammonia. Second, constituents can simply be sufficiently sticky to remain clustered until more constituents arrive to make the cluster grow. This is classical nucleation and it depends on high vapor saturation ratios (supersaturation). It can occur for argon under the right conditions, but in the atmosphere nucleation involving organics is most interesting. Organics add the feature that they are an incredibly rich mixture of constituents, generally highly oxygenated, each with many oxygenated functional groups. We can describe this rich mixture in terms of volatility using the volatility basis set (VBS), and it has been established that nucleation appears to be second order with respect to the concentration of the least volatile class in the VBS, the so-called Ultra Low Volatility Organic Compounds (ULVOCs). Here we present an analysis of the overall volatility distribution to determine the fraction of ULVOCs that govern nucleation in both neutral and ion-induced nucleation. This depends on overall saturation ratios as well as the overall volatility distribution. The framework successfully describes the temperature and concentration dependence of both neutral and ion-induced nucleation for the canonical alpha-pinene + ozone system measured at the CERN CLOUD experiment between 223 and 298 K. Ultimately there are two competing effects: volatility drops as temperature drops, increasing saturation ratios, but the peroxy radical autoxidation chemistry that creates the highly functionalized ULVOCs accelerates as temperature increases, increasing saturation ratios in the opposite sense. Both theory and observations show a minimum in nucleation rates between 263 and 278 K, with higher rates to either side.

How to cite: Donahue, N. M.: Nucleation and the Volatility Basis Set, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21813, https://doi.org/10.5194/egusphere-egu26-21813, 2026.

Organic aerosol (OA) is a major component of tropospheric submicron aerosols, influencing air pollution, human health, and climate change. Primary and secondary OA (POA and SOA) exhibit distinct physicochemical properties that lead to different health and climate impacts. Current chemical transport models (CTMs), however, have difficulties not only in capturing OA concentrations, especially in polluted regions, but also in reproducing the fraction of POA and SOA. Here, we develop an OA simulation framework for full-volatility-range organic precursors, with a particular focus on improving OA formation from semivolatile and low-volatility organic compounds (S/LVOC) and intermediate-volatility organic compounds (IVOC), based on the atmospheric chemical transport model GEOS-Chem. Cooperating with a newly developed bottom-up global anthropogenic emission inventory, MEIC-global-FVOC, the improved OA scheme shows a good model performance when evaluated against a comprehensive dataset of worldwide measurements for OC, POA, and SOA, driven by increased POA formation from S/LVOC and SOA formation from IVOC. The model indicates that several populated regions in Asia, Africa, America, and Europe suffer from high OA exposure with annual mean over 5 μg m^-3, highlighting the importance of controlling OA pollution. In East Asia, South Asia, the northern part of Africa, and Europe, anthropogenic SOA and POA are the largest two contributors to OA pollution, suggesting the need for reducing residential combustion that contributes over half of anthropogenic S/LVOC and IVOC emissions. For other regions, most of OA is from natural sources, which may be easily affected by extreme events (e.g., wildfires) and the warming climate. The estimated global OA burden in 2018 is 2.50 Tg, with a fraction of 75% from SOA. The SOA burden is higher than previous estimates, resulting from increased formation of S/LVOC and IVOC, highlighting that the role of SOA should be given more attention in assessing aerosol climate impact. The estimation of OA burden is sensitive to pyrogenic emission estimates and wet deposition parameterization, which need more constraints in future studies.

How to cite: Miao, R., Xu, R., Huang, S., Xiao, S., Wang, H., Zheng, Y., Liu, S., Li, J., Geng, G., Shrivastava, M., Haddad, I. E., Karydis, V. A., Tsimpidi, A. P., Zhang, Q., and Chen, Q.: Global budgets of atmospheric primary and secondary organic aerosols based on the simulation for full-volatility-range organic precursors, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11007, https://doi.org/10.5194/egusphere-egu26-11007, 2026.

Recent studies have shown that evaporation rates of secondary organic aerosol (SOA) particles may be slower than expected (Vaden et al. 2011; Berkemeier et al. 2020) and that growth rates of ambient SOA nanoparticles show surprisingly little dependency on condensable vapors in the gas phase (Kulmala et al., 2022). A large fraction of SOA may exist in oligomerized form, which might alter their condensation and evaporation. Additionally, SOA can be highly viscous, which leads to kinetic limitations in evaporation, slowing of particle-phase chemistry, and non-equilibrium partitioning. The effects of composition, oligomerization, and slow diffusion are inherently coupled, as high concentrations of low-volatility compounds or products of accretion reactions can cause high viscosity.

We use a kinetic multi-layer model to estimate the kinetic limitations affecting SOA formation and fate in laboratory experiments and the ambient atmosphere. The model explicitly considers gas- and particle-phase chemistry, kinetic gas-particle partitioning, and composition-dependent bulk diffusivity. We re-analyze data from laboratory chamber experiments with mixtures of terpenes as SOA precursors (Berkemeier et al. 2020) as well as published field and laboratory chamber data of nanoparticle growth (Stolzenburg et al., 2025) to find pronounced effects of multiphase chemistry and particle phase state under these conditions. Especially the partitioning of semi and low-volatile organic compounds (SVOC/LVOC) is strongly affected by these processes in the model, while the partitioning of extremely- and ultra-low volatility organic compounds (ELVOC/ULVOC) is less affected. We discuss the possible effect of growth limitation through bulk accommodation in models that follow monolayer adsorption schemes versus models that allow the “burying” of surface-adsorbed molecules through multi-layer adsorption.

The model predicts that, during particle evaporation, particles may be radially heterogeneous with respect to composition and diffusivity: higher volatility chemical species evaporate more quickly than oligomers or lower volatility species, leaving behind a near-surface layer crust of more viscous material that presents a barrier for further evaporation. The results highlight gaps in our knowledge about the physical and chemical properties of SOA and their interactions.

References

Berkemeier, T., Takeuchi, M., Eris, G., Ng, N. L. Atmos. Chem. Phys. 20, 15513-15535 (2020).
Kulmala, M., Cai, R., Stolzenburg, D., et al. Environ. Sci.: Atmos. 2, 352-361 (2022).
Stolzenburg, D., Sarnela, N., Bianchi, F. et al. npj Clim Atmos Sci 8, 75 (2025).
Vaden, T. D., Imre, D., Beranek, J., et al. P. Natl. Sci. Acad. USA 108, 2190–2195 (2011).

How to cite: Berkemeier, T., Kang, H. G., Zhang, Z., Radecka, M., Takeuchi, M., Ng, N. L., and Pöschl, U.: Interplay of phase state and multiphase chemistry in nanoparticle growth and evaporation of secondary organic aerosol, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19660, https://doi.org/10.5194/egusphere-egu26-19660, 2026.

Biogenic secondary organic aerosols (BSOA) are formed from oxidation of biogenic volatile organic compounds (BVOC). They are the main contributor to the global SOA fluxes with the values still remaining highly uncertain. The formation mechanism of BSOA has been studied extensively, however few studies up to date have been conducted at elevated relative humidity (85-95%) and in the presence of different aqueous inorganic seeds. These conditions are more realistic e.g., under night-time conditions or in coastal or tropical regions. As two typical and abundant BVOC, we focus on the SOA formation from isoprene and α-pinene. Experiments are conducted inside the AIDA aerosol and cloud simulation chamber under various humidities and the presence of different seed aerosols at atmospherically relevant conditions. During the campaign, NaCl, NH₄NO₃ or (NH₄)₂SO₄ seed aerosol particles are introduced into the chamber, followed by the oxidation of isoprene or α-pinene. Formation and aging of oxidation products are measured in the gas and condensed phase in the dark and with simulated solar radiation.

Our results show that the SOA mass production is enhanced under higher relative humidities. High-resolution aerosol mass spectrometry data show a higher oxidation state of SOA formed under higher humidities, suggesting further oxidation of SOA products within the condensed phase. Further molecular analysis on the particle phase oxidation products with FIGAERO-CIMS suggests that the increased oxidation state can mainly be explained by the production of HOMs and low-molecular weight dicarboxylic acids during the aging process, especially under illumination. Our observations indicate the critical influence of relative humidity and pre-existing seed aerosol composition on different secondary organic aerosol formation mechanisms, particularly through potential aqueous-phase reaction pathways.

How to cite: Fu, J., Kroese, W., Novák, L., Ou, H., Holzinger, R., Saathoff, H., and Dusek, U.: Formation and aging of biogenic secondary organic aerosol in aqueous aerosol particles containing reactive nitrogen , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21608, https://doi.org/10.5194/egusphere-egu26-21608, 2026.

An important property of the compounds comprising organic aerosol is volatility, typically described in terms of saturation vapor pressure or saturation concentration (measured in µg/m³). The volatility of aerosol constituents can be estimated based on their molecular formula using different parametrizations or measured experimentally by using a chemical ionization mass spectrometer (CIMS) coupled to a filter inlet for gases and aerosols (FIGAERO). In this technique, aerosol sample is collected semi-online and evaporated via gradually heated nitrogen flow desorbing organic constituents to be measured by CIMS. From the temperature at which detected chemical species reach their maximum signal, it is possible to determine the respective compositions’ volatility.

In 2024, the FIGAERO-CIMS was deployed at the CHANEL (household chemicals amplifying urban aerosol pollution) measurement campaign at the SAPHIR chamber at Jülich Research Centre, Germany. During the campaign, complex reactive mixtures representing urban air scenarios were injected into the chamber, and exposed to both day- and night-time oxidation via opening or closing the roof to natural sunlight. We developed a multi-peak fitting algorithm to fully fit each composition’s thermogram (signal vs. desorption temperature), resulting in multiple nominal saturation concentrations per detected composition. We interpret these as combinations of simple volatility-driven desorption and decomposition (typically at higher temperatures) of larger compounds, such as accretion products.

We tracked the chemical composition and volatility of secondary organic aerosol throughout its formation and subsequent aging in the chamber over several hours. The chemical composition measured by FIGAERO-CIMS was compared with other co-located online mass spectrometric techniques, e.g., CIMS following online aerosol evaporation by a heated sheath flow (WALL-E). Our initial results show how aerosol volatilities typically decreased with age, as more oxygen was incorporated. Further, night-time conditions resulted in both increased organonitrate formation and lower product volatility relative to day-time conditions.

How to cite: Nissinen, A., Buchholz, A., Pullinen, I., Pusfitasari, E. D., Liu, L., Perrier, S., Riva, M., Roska, M., Yang, B., Bates, K. H., Coggon, M. M., Fry, J. L., Pfannerstill, E. Y., Rohrer, F., Stockwell, C., Tillmann, R., Khare, P., Bell, D., Gkatzelis, G. I., and Schobesberger, S. and the SAPHIR CHANEL: Organic aerosol volatility and its drivers in realistic urban air replicas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16772, https://doi.org/10.5194/egusphere-egu26-16772, 2026.

Considerable progress has been made in recent years in our understanding of the diffusivity of various species in viscous  aqueous organic aerosol. However, little is known about ion diffusivity in such matrices. Here, we use experimental evaporation rates of volatile ammonium nitrate in a levitated, viscous proxy organic aerosol droplet to deduce the diffusivities of the nitrate and ammonium ions. We compare the ion diffusivities with those of semi-volatile maleic and malonic acid in the same proxy organic aerosol droplet. In addition, we measured viscosity of the proxies. Our finding indicates significantly slower diffusion of the ions compared to those of the organic acids, although the viscosity of the mixed solutions is comparable. Overall, the effective diffusivity of the ions seems to follow the Stokes-Einstein relationship, whereas the small organic acids diffuse faster than predicted. These findings have implications for the gas-particle portioning of ammonium nitrate which may be stronger limited by kinetic mass transfer than previously thought.

How to cite: Krieger, U., Klein, L., Luo, B., Bilde, M., and Peter, T.: Nitrate- and ammonium ion vs. dicarboxylic acid diffusivity in viscous organic aerosol particles: implications for gas-particle partitioning, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3908, https://doi.org/10.5194/egusphere-egu26-3908, 2026.

Biomass burning is a major contributor to the global burden of secondary organic aerosol (SOA), with significant impacts on air quality, public health, and the Earth’s radiation balance. Observations from laboratory experiments and field campaigns increasingly show that intermediate-volatility organic compounds (IVOCs) emitted from wood combustion dominate SOA formation, potentially accounting for more than half of biomass-burning SOA (1,2). However, IVOC emissions and their atmospheric evolution remain highly uncertain and are often substantially underestimated in global chemistry–climate models, limiting our ability to accurately simulate organic aerosol distributions and trends. In this study, we quantify the sensitivity of biomass-burning SOA formation to key assumptions related to IVOC emissions, oxidation chemistry, and volatility distributions. We implement recent literature-based developments into ORACLE, the organic aerosol chemistry submodule of the EMAC global chemistry–climate model. ORACLE represents primary and secondary organic aerosol using a volatility basis set (VBS) framework, accounting for gas–particle partitioning, chemical aging through multigenerational oxidation, and changes in volatility and molecular mass during atmospheric processing (3). This framework enables a process-based evaluation of how uncertainties in emissions and chemistry propagate to global SOA burdens.

We perform a comprehensive suite of global sensitivity simulations for the period 2012–2016, corresponding to the most recent fully published GFED biomass-burning emission inventory and providing broad observational coverage for model evaluation. The sensitivity experiments address three major sources of uncertainty. First, we investigate alternative IVOC emission scaling approaches, including scaling IVOC emissions relative to emitted organic carbon (OC), as commonly assumed in global models (4), and scaling relative to volatile organic compound (VOC) emissions (5,2). These methods reflect differing assumptions about the relationship between IVOCs and primary combustion emissions and lead to substantially different global IVOC source strengths. Second, we assess uncertainties in SOA formation efficiency and chemical processing. This includes exploring reported ranges in aerosol mass yields under different NO_x regimes (2), as recent experiments indicate that NO_x strongly modulates SOA formation from biomass-burning IVOCs. In addition, we examine the sensitivity of modelled SOA to uncertainties in the OH reaction rate of IVOCs, which controls their atmospheric lifetime and spatial distribution. Third, we evaluate the influence of alternative volatility distribution of the IVOC oxidation products across VBS bins. Previous studies propose contrasting assumptions regarding whether the dominant SOA yield is associated with lower- or higher-volatility oxidation products (4,5), which leads to implications for SOA formation, transport, and lifetime.

By systematically disentangling the influence of IVOC emissions, chemical processing, and volatility assumptions, this work aims to identify parameterizations that are both physically representative of diverse biomass-burning conditions and computationally feasible for global applications. The results provide new constraints on biomass-burning SOA formation and support ongoing efforts to improve organic aerosol representation in global chemistry–climate models, thereby reducing long-standing discrepancies between simulated and observed SOA burdens.

References

(1) Bruns et al., 2016; doi: 10.1038/srep27881

(2) Li et al., 2024; doi: 10.1093/nsr/nwae014

(3) Tsimpidi et al., 2016; doi: 10.5194/acp-16-8939-2016

(4) Ciarelli et al., 2017; doi: 10.5194/gmd-10-2303-2017

(5) Tilmes et al., 2019; doi: 10.1029/2019MS001827

How to cite: Scholz, S. M. C., Khare, P., Gkatzelis, G. I., Chen, Q., Karydis, V. A., and Tsimpidi, A. P.: Improving global simulations of biomass-burning SOA through IVOC-focused sensitivity studies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14197, https://doi.org/10.5194/egusphere-egu26-14197, 2026.

Wildfire smoke strongly affects air quality, human health, climate, and the Earth system. During atmospheric aging, wildfire aerosol particles undergo complex chemical and microphysical transformations that modify their optical properties, radiative effects, and cloud-forming ability. Of particular interest are organic surface coatings, which can enhance light absorption through lensing effects and increase particle hygroscopicity.

Here, we present single-particle mass spectrometry measurements from a boreal forest wildfire smoke experiment, resolving the coexistence of hydrophilic compounds and hydrophobic polycyclic aromatic hydrocarbons (brown carbon) within individual particles. We show that glyoxal and methylglyoxal are directly emitted during combustion, contributing to the initial hygroscopicity of freshly emitted particles. During photochemical aging, rapid oxalate formation is observed, accompanied by a moderate increase in hygroscopicity, while PAH signals decrease on a slower timescale. The decay rates of individual PAHs are similar but show a clear dependence on relative humidity, indicating that PAH degradation is controlled by viscosity-dependent radical diffusion into the particles. In contrast, highly oxidized products form on much shorter timescales, suggesting that these reactions are largely confined to the particle surface. At elevated relative humidity, surface oxidation continues, whereas it rapidly ceases under dry conditions. These observations highlight the central role of relative humidity in controlling the microphysical properties, optical effects, and cloud activation potential of aged wildfire smoke.

How to cite: Zimmermann, R., Rosewig, I. E., Kalamašņikovs, A., Hakkim, H., Ihalainen, M., Hartikainen, A., Somero, M., Yli-Pirilä, P., Sippulä, O., Peltokorpi, S., Buchholz, A., Liqing, H., Virtanen, A., Vakkari, V., Walte, A., and Passig, J.: The role of relative humidity for the formation of oxidized shells on aged wildfire particles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18015, https://doi.org/10.5194/egusphere-egu26-18015, 2026.

Carbonaceous aerosols are a major component of atmospheric fine particulate matter and play a crucial role in climate forcing due to their light-absorbing properties. Their influence on wintertime radiative forcing is particularly pronounced, while the source-specific absorption and molecular properties remain poorly understood in semi-arid regions¹. To address this knowledge gap, we conducted a 24-hr three-week PM_2.5 winter campaign in a semi-arid urban region of northwestern India. We performed measurement of BC concentration using multi-wavelength AE-33, filter-based elemental carbon-organic carbon (EC–OC) analysis, water-soluble organic carbon (WSOC) quantification and brown carbon (BrC) optical characterization, and molecular characterization of isolated HULIS fractions (HULIS-A at pH=2 and HULIS-N at pH=7) using UHPLC-ESI-ToF-MS.

eBC remained consistently elevated, with most daily mean values above 15 µg/m³ and episodic peaks exceeding 26 µg/m³, indicating sustained wintertime loading. Fossil-fuel derived BC (BC_ff) dominated throughout the day, with early-morning (12.61 µg/m³) and evening (15.52 µg/m³) peaks, 2.84–15.52 µg/m³, while biomass-burning BC (BC_bb) showed characteristic morning and late-evening enhancements of 8–11 µg/m³, consistent with residential biomass or wood burning in colder periods. Further, we observed WSOC/OC ratio of 0.75–0.80 with average WSOC levels of 35 µg/m³. A strong near-UV absorption, with SUV₂₅₄ (MAC₂₅₄ ≈ 5.0–5.2 m²/g) and MAC₂₅₄ > 5 m²/g, along with a high AAE (4.04) confirmed the presence of strongly light-absorbing BrC from biomass-burning precursors and secondary processing^2,3.

The molecular characterization of HULIS showed that CHO- and CHON-rich ions were present not only in the semi-volatile oxygenated organic aerosol (SVOOA) domain but also in regions associated with biomass-burning organic aerosol (BBOA). This indicates the simultaneous presence of both primary BB products and secondarily aged organics. HULIS-A exhibited stronger near-UV absorption, confirming its dominant contribution to wintertime chromophores. In DBE–N_C space, HULIS-A displayed a much broader distribution compared to HULIS-N, extending into regions characteristic of unsaturated aromatics, phenolic and nitro-aromatic BrC precursors, and cata-PAH-like structures, all of which are known carriers of strong near-UV absorption.

Together, the optical measurements (MAC spectra, UV–Vis absorption) and high-resolution molecular analysis indicate that wintertime WSOC at this semi-arid site is strongly enriched in both primary BBOA-linked chromophores and secondary OOA-derived oxygenated species. Among these, HULIS-A emerges as the principal carrier of light-absorbing organic matter, driving enhanced shortwave absorption during winter.

References:

(1)        Laskin, A.; Laskin, J.; Nizkorodov, S. A. Chemistry of Atmospheric Brown Carbon. Chem. Rev. 2015, 115 (10), 4335–4382. https://doi.org/10.1021/cr5006167.

(2)        Weishaar, J. L.; Aiken, G. R.; Bergamaschi, B. A.; Fram, M. S.; Fujii, R.; Mopper, K. Evaluation of Specific Ultraviolet Absorbance as an Indicator of the Chemical Composition and Reactivity of Dissolved Organic Carbon. Environ. Sci. Technol. 2003, 37 (20), 4702–4708. https://doi.org/10.1021/es030360x.

(3)        Hecobian, A.; Zhang, X.; Zheng, M.; Frank, N.; Edgerton, E. S.; Weber, R. J. Water-Soluble Organic Aerosol Material and the Light-Absorption Characteristics of Aqueous Extracts Measured over the Southeastern United States. Atmospheric Chem. Phys. 2010, 10 (13), 5965–5977. https://doi.org/10.5194/acp-10-5965-2010.

How to cite: Gupta, A., Shrivastava, A., and Bhattu, D.: Wintertime Molecular and Optical Properties of Carbonaceous Aerosols in a Semi-Arid Environment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2513, https://doi.org/10.5194/egusphere-egu26-2513, 2026.

It has been predicted that boreal forest and peatland fires will increase in the future as the climate warms. It is important for climate models that the optical properties of the aerosols emitted during these fires are well described.  Laboratory experiments were carried out to study biomass burning (BB) aerosol emissions and their photochemical and dark aging to investigate (1) the emission factors of BrC (and BC) from different combustion processes, (2) chemical and optical properties of the emissions and (3) how different atmospheric aging conditions affect these properties. The experiments were performed at the ILMARI-facility of the UEF (www.uef.fi/ilmari) in November–December 2023. Details of the experiments were presented by Mukherjee et al. (2025) who also presented absorption properties of water-soluble and methanol-soluble organic carbon (WSOC and MSOC, respectively) analyzed from filter samples. In the present work we will present the optical properties of BB emissions from burning peat and dry and moist boreal forest floor surface (BFS) samples at high time resolution. Light scattering coefficient was measured with a 3-wl nephelometer, absorption with a 7-wl Aethalometer and a 3-wl photoacoustic spectrometer, and particle number size distributions (PNSD) with an SMPS. A Mie code was used for calculating scattering and absorption coefficients from the PNSDs by varying real and imaginary refractive indices (n_r and n_i, respectively), until the measured and modeled scattering and absorption agree within 1%. The flaming BB emissions were dark with single-scattering albedo (SSA) varying between 0.25 and 0.6. The darkest aerosols with SSA ≈ 0.30 ± 0.05 were measured from flaming dry BFS and the highest SSA > 0.95 from aged peat fires. Fitting lines with the n_r vs SSA show that the real refractive indices can be estimated from a logarithmic function n_r(450) = 0.191ln(SSA) + 1.792, r² = 0.402; n_r(525) = 0.218ln(SSA) + 1.773, r² = 0.524; n_r(635) = 0.295ln(SSA) + 1.862, r² = 0.734 and the imaginary refractive indices from polynomials: n_i(450) = -4.50SSA³ + 10.35SSA² - 8.05SSA + 2.17, r² = 0.97; n_i(525) = -2.91SSA³ + 6.79SSA² -  5.40SSA + 1.51, r² = 0.98; n_i (635) = -1.65SSA³ + 3.85SSA² - 3.16SSA + 0.96, r² = 0.98. The next steps are to calculate the mass absorption cross sections (MAC) of BC and BrC by combining the optical data with the soot particle aerosol mass  spectrometer (SP-AMS) data and the absorption coeffcients and refractive indices of WSOC measured with an online UV-IR spectrometer connected to a liquid-waveguide capillary cell (LWCC) and a particle-into-liquid sampler (PILS).

Reference

Mukherjee, A. et al.: Brown carbon emissions from laboratory combustion of Eurasian arctic-boreal and South African savanna biomass, Atmos. Chem. Phys., 25, 16747–16774, 2025

Acknowledgement

 This work was supported by the Research Council of Finland via the project “Black and Brown Carbon in the Atmosphere and the Cryosphere” (BBrCAC) (decision number 341271)

How to cite: Virkkula, A., Li, D., Barreira, L., Hartikainen, A., Somero, M., Kokkola, T., Mukherjee, A., Karhu, J., and Sippula, O.: Single-scattering albedo and iterated refractive indices of fresh and aged black and brown carbon particles emitted from burning peat and boreal forest floor surface in chamber experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15338, https://doi.org/10.5194/egusphere-egu26-15338, 2026.