The team, led by Yuan Dai and corresponding authors Junfeng Wang of Nanjing University of Information Science and Technology and Xinlei Ge of Southeast University, exploited a natural experiment created by the COVID-19 pandemic. They compared emission control periods, when lockdowns sharply curtailed traffic and industry, with adjacent normal periods in both winter and summer. During these control periods, nitrogen oxide concentrations fell by 39 percent in winter and 34 percent in summer, while sulfur dioxide barely moved, staying between 3.5 and 3.9 parts per billion. That asymmetry pushed the gas-phase sulfur-to-nitrogen ratio up by 71 percent in winter and 60 percent in summer, while ozone rose about 15 percent because less of it was being destroyed by fresh nitrogen oxide emissions.

What makes the study unusual is its single-particle approach. Instead of averaging the composition of all particles together, as bulk measurements do, the researchers used a single-particle aerosol mass spectrometer to classify each particle by its dominant chemical signature. They sorted the population into black carbon particles, black carbon mixed with organic carbon, organic-rich particles without black carbon, and black-carbon-free inorganic particles, then subdivided these into 19 subclasses based on sulfate, nitrate, and cyanide-related fragments. Fresh combustion particles, marked by cyanide ions and small sizes of roughly 100 to 600 nanometers, could be distinguished from aged particles coated in secondary sulfate and nitrate.

By defining three complementary sulfur-to-nitrogen ratios, one for the gas phase, one for the particle phase, and one based on the relative numbers of sulfur-containing versus nitrogen-containing particles, the team could watch the same chemical signal move through different domains of the atmosphere. During emission control periods, the particle-phase ratio rose by roughly 57 percent on average across particle classes, and the mean vacuum aerodynamic diameter of particles grew by 27 percent. In other words, when the air became relatively sulfur-rich, particles not only changed their internal chemistry but also grew larger, likely through the accumulation of sulfate coatings.

The most striking finding concerns which particles respond most strongly. Particles containing both black carbon and organic carbon showed the greatest sulfate enrichment relative to black-carbon-free inorganic particles, in both seasons. In summer, the enhancement factor for black carbon particles climbed from 1 to 3 in the morning to 6 to 9 near solar noon, suggesting that photochemically active hours favor sulfate formation on soot surfaces, possibly through black-carbon-mediated release of nitrous acid and nitrogen dioxide and light-induced reactive oxygen species that accelerate the conversion of sulfur dioxide to sulfate. At night, when humidity exceeded 80 percent and sunlight vanished, organic-rich particles took over as the dominant site of sulfate enrichment, consistent with aqueous-phase oxidation by hydrogen peroxide and ozone and with the formation of organosulfates.

Mixed black carbon and organic carbon particles showed the largest enhancement of all, in both day and night and in both seasons. The authors propose a synergistic mechanism: the black carbon fraction provides reactive or catalytic surfaces, while the organic fraction helps sulfate accommodate and remain trapped within the particle matrix. Light-absorbing organic compounds such as brown carbon may even act as photosensitizers, speeding up sulfur dioxide conversion under ultraviolet light. In winter, when photochemistry is weak, organic-driven aqueous pathways dominate, yet the mixed particles still hold their advantage, possibly aided by the larger accessible surface area of soot’s tangled morphology and by coating softening that promotes sulfate retention.

Humidity emerged as the master regulator of this entire cross-phase coupling. The researchers divided their observations into four relative humidity regimes anchored to known phase transitions of ammonium nitrate and ammonium sulfate, the two dominant secondary inorganic salts in fine particulate matter. Below 55 percent relative humidity, particles are largely solid, gas-particle exchange is kinetically limited, and the three sulfur-to-nitrogen ratios decouple from one another. The particle population remains externally mixed, with substantial chemical differences between particle types. Crossing the deliquescence point of ammonium nitrate near 61 percent humidity triggers a semi-solid transition that activates particle surfaces and pushes the particle-phase ratio upward, especially for black-carbon-containing classes.

Above 85 percent relative humidity, the picture changes dramatically. Particles take up enough water to become predominantly aqueous, kinetic barriers to gas-to-particle partitioning collapse, and all three sulfur-to-nitrogen ratios converge to similarly low, stable values, with correlations between them reaching coefficients of 0.68 to 0.81. The differences between particle types nearly vanish as the aerosol evolves toward an internally mixed state, in which water-mediated multiphase processing homogenizes composition across the population. This humidity-dependent transition from chemical heterogeneity to uniformity is precisely the kind of behavior that models treating particles as a well-mixed average cannot capture.

To move beyond correlation, the team combined causal inference with interpretable machine learning. A causal network analysis, using the Grow-Shrink and Greedy Equivalence Search algorithms, identified directed links running from the gas-phase ratio to the particle-phase ratio and onward to the number-based ratio, with ozone and boundary layer height acting as intermediaries between the chemistry and the meteorological environment. A machine-learning model, an XGBoost framework optimized by a Sparrow Search Algorithm, achieved coefficients of determination between 0.71 and 0.91 across particle classes. Shapley value analysis confirmed that the gas-phase ratio was the dominant driver of particle-phase partitioning, and that the particle-phase ratio in turn led the number-based ratio, supporting the sequential propagation picture. Relative humidity exerted a consistent negative effect, while solar radiation and ozone contributed positively.

The implications reach beyond Chinese cities. As clean air policies worldwide tighten nitrogen oxide controls faster than sulfur dioxide controls, the gas-phase sulfur-to-nitrogen ratio may rise again, potentially alongside higher ozone levels and a more oxidizing atmosphere. The study shows that such shifts will not affect all particles equally: soot particles will accumulate sulfate coatings differently than organic or purely inorganic ones, with knock-on consequences for how particles take up water, scatter light, and act as cloud condensation nuclei. The authors argue that future air-quality models should incorporate particle-type heterogeneity and humidity-dependent multiphase processes, rather than relying on bulk averages, if they are to predict how secondary aerosols will evolve under the emission regimes of the coming decades.

Subject of Research: Cross-phase sulfur-nitrogen partitioning and aerosol mixing-state evolution observed at the single-particle level

Article Title: Cross-phase partitioning of sulfur-nitrogen ratios and aerosol mixing-state evolution based on single-particle observations

Article References: Dai, Y., Wang, J., Chen, M., Zhang, S., Li, H., Zhang, Y., Wu, Y., Wang, M., & Ge, X. (2026). Cross-phase partitioning of sulfur-nitrogen ratios and aerosol mixing-state evolution based on single-particle observations. Atmospheric Chemistry and Physics, 26(19), 14015-14029. https://doi.org/10.5194/acp-26-14015-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14015-2026

Keywords: aerosols, single-particle mass spectrometry, sulfur-to-nitrogen ratio, secondary aerosol formation, black carbon, relative humidity, mixing state, nitrogen oxides, sulfur dioxide, COVID-19 lockdown, air quality modeling, multiphase chemistry