A new study published in the journal Atmospheric Chemistry and Physics has taken a major step toward solving this puzzle. An international team of researchers led by Fobang Liu of Xi’an Jiaotong University and the Max Planck Institute for Chemistry, together with corresponding authors Kai Wang of China Agricultural University and Haijie Tong of Helmholtz-Zentrum Hereon, developed and applied a novel molecular classification framework called MCR-VK to link the molecular composition of organic aerosols directly to their capacity for generating reactive oxygen species. The work analyzed fine particulate matter collected from four Chinese megacities, a German semi-urban site, and a remote boreal forest, alongside laboratory-generated secondary organic aerosol from multiple precursors, providing an unprecedented molecular-level view of what makes some particles more oxidatively dangerous than others.

The central challenge the researchers faced is that atmospheric organic aerosol is chemically bewildering. Thousands of distinct molecular species coexist in any given sample, yet only between 10 and 30 percent of the organic aerosol mass can typically be identified as specific compounds using existing analytical techniques. Advances in ultrahigh-resolution mass spectrometry, particularly Fourier transform ion cyclotron resonance and Orbitrap instruments combined with soft ionization methods, have dramatically expanded scientists’ ability to assign molecular formulas to these complex mixtures. However, traditional molecular metrics such as double bond equivalents and aromaticity indices, while useful for characterizing structural features, do not fully capture the functional group chemistry that actually governs redox reactivity and ROS formation.

To overcome this limitation, the team built upon their recently developed maximum carbonyl ratio, or MCR, a metric that estimates the maximum number of carbonyl groups a molecule could contain based solely on its molecular formula. By incorporating MCR into the traditional Van Krevelen diagram, which plots the hydrogen-to-carbon ratio against the oxygen-to-carbon ratio, the researchers established the MCR-VK framework. This approach classifies individual organic compounds into two chemically meaningful categories: oxidized organic compounds, or OOC, with MCR values below 0.9, and unsaturated organic compounds, or UOC, with MCR values at or above 0.9. Higher MCR values indicate greater carbonyl functionality and electrophilic character, while lower MCR values reflect the dominance of highly oxidized, non-carbonyl species such as organic peroxides, both of which may contribute to oxidative stress in the respiratory system.

The researchers applied this framework to fine particulate matter samples collected during six field campaigns spanning dramatically different environments. These included the SMEAR II boreal forest station in Hyytiälä, Finland; an urban site in Mainz, Germany; and four major Chinese cities: Beijing, Xi’an, Shanghai, and Guangzhou. In parallel, they generated secondary organic aerosol in the laboratory from five precursors: isoprene, alpha-pinene, beta-pinene, limonene, and naphthalene, using both a quartz flow tube with ozone-driven ozonolysis and a potential aerosol mass chamber with hydroxyl radical photooxidation. Molecular composition was analyzed using an ultra-high-performance liquid chromatography system coupled to an electrospray ionization Orbitrap mass spectrometer, enabling the assignment of thousands of molecular formulas across all samples.

The results reveal striking compositional contrasts across environments. Isoprene-derived secondary organic aerosol exhibited the highest relative fraction of oxidized organic compounds at 98 percent, consistent with its dominance of highly oxygenated, low-MCR products such as 2-methyltetrols. Alpha-pinene and beta-pinene secondary organic aerosol were similarly dominated by oxidized compounds at 95 and 94 percent respectively, reflecting oxygen-rich ozonolysis products including pinonic acid, pinic acid, nopinone, and MBTCA. The boreal forest samples from Hyytiälä also showed a high oxidized fraction of 94 percent, comparable to monoterpene-derived laboratory aerosol, reflecting the dominant contribution of biogenic precursors at that site. In sharp contrast, naphthalene-derived secondary organic aerosol was characterized by a substantially higher fraction of unsaturated organic compounds at 64 percent, indicating strong contributions from aromatic, ring-retaining oxidation products such as quinones.

Urban fine particulate matter displayed a wide range of unsaturated compound fractions between 15 and 57 percent, reflecting the diversity of emission sources and atmospheric processing pathways in cities. The Chinese megacities of Beijing, Xi’an, Guangzhou, and Shanghai exhibited particularly high unsaturated fractions between 45 and 57 percent, indicating substantial contributions from aromatic and combustion-related organics. Mainz showed a considerably lower unsaturated fraction of just 15 percent, suggesting weaker aromatic emissions and a relatively greater biogenic influence compared with the Chinese megacities. Critically, the researchers validated their classification by demonstrating strong quantitative agreement with independent molecular indicators: the relative fraction of oxidized organic compounds correlated well with the fraction of highly oxygenated molecules, while the unsaturated fraction correlated strongly with the relative abundance of aromatic compounds.

When the team turned to reactive oxygen species measurements, a fascinating picture emerged. Hydrogen peroxide yields in aqueous particle extracts were quantified using a fluorometric probe, and radical yields were measured using electron paramagnetic resonance spectroscopy combined with a spin-trapping method. Across all sites, hydrogen peroxide dominated the ROS budget, accounting for approximately 78 to 97 percent of total ROS. Urban megacities such as Beijing, Guangzhou, Shanghai, and Xi’an exhibited substantially higher volume-normalized ROS yields, ranging from 57.8 to 260.5 picomoles per cubic meter of air, compared to just 13.0 picomoles per cubic meter at the boreal forest site. This spatial pattern tracked closely with fine particulate mass concentrations, indicating that environments with higher particle loadings contain greater amounts of ROS-generating substances per unit air volume.

However, when the researchers normalized ROS yields to particle mass rather than air volume, a completely different story unfolded. Mass-normalized ROS yields showed no correlation with particle loading whatsoever, ranging from 0.73 picomoles per microgram in Shanghai to 3.32 picomoles per microgram in Mainz, with the boreal forest site at Hyytiälä also exhibiting relatively high values of 2.49 picomoles per microgram despite its very low particle mass concentration. This divergence clearly demonstrated that intrinsic particle composition, rather than particle mass, governs ROS formation on a per-mass basis. Even more intriguingly, radical yields displayed a significant negative correlation with fine particulate mass concentration, meaning that although polluted environments generate higher total radical concentrations per unit air volume, individual particles in those environments are intrinsically less efficient at producing radicals than particles in clean, biogenic environments.

The most powerful result of the study is the establishment of the ratio between oxidized and unsaturated organic compound fractions as a robust predictor of ROS formation. Across urban sites, mass-normalized total ROS yields exhibited a strong positive exponential relationship with this ratio, with an R-squared value of 0.94, while hydrogen peroxide yields showed an even stronger correlation with an R-squared of 0.98. Laboratory-generated secondary organic aerosol further confirmed these composition-reactivity relationships, with total ROS yields following a strong positive linear relationship across all five precursors. Isoprene-derived aerosol, rich in oxidized compounds, exhibited exceptionally high ROS yields of 403 picomoles per microgram, while naphthalene-derived aerosol, dominated by unsaturated aromatic species, produced only 58 picomoles per microgram. These findings reveal that the molecular balance between oxygen-rich and unsaturated organic compounds provides a mechanistic basis for both the magnitude and spatial variability of ROS formation in atmospheric aerosols, offering a powerful new tool for assessing the health-relevant oxidative activity of fine particulate matter across diverse environments.

Subject of Research: Molecular composition of organic aerosols and reactive oxygen species formation in atmospheric fine particulate matter

Article Title: Linking molecular composition of organic aerosols to reactive oxygen species formation using an MCR-VK framework

Article References: Liu, F., Zhang, Y., Yue, S., Keskinen, H.-M. K., Petäjä, T., Kulmala, M., Fu, P., Hoffmann, T., Wang, K., & Tong, H. (2026). Linking molecular composition of organic aerosols to reactive oxygen species formation using an MCR-VK framework. Atmospheric Chemistry and Physics, 26(20), 14655-14669. https://doi.org/10.5194/acp-26-14655-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14655-2026

Keywords: organic aerosol, reactive oxygen species, fine particulate matter, MCR-VK framework, secondary organic aerosol, hydrogen peroxide, electron paramagnetic resonance, ultrahigh-resolution mass spectrometry, oxidative stress, air quality, biogenic emissions, urban pollution