The research team, led by Kaitao Chen and Gehui Wang of East China Normal University together with colleagues at Southwest Jiaotong University, carried out an intensive observation campaign in central Chengdu from mid-December 2024 to early January 2025. Their instrument package, an In-situ Gas and Aerosol Compositions monitor coupled to ion chromatography and a total organic carbon and nitrogen analyzer, allowed them to measure WSON in both the gas and particle phases simultaneously, at three-hour resolution, something few studies anywhere have attempted. Most previous work on organic nitrogen has measured only the particle phase, which makes it impossible to see the dynamic exchange between gas and aerosol that ultimately determines how much secondary organic aerosol forms.

What the measurements revealed was unambiguous. Particulate WSON averaged 2.3 plus or minus 1.4 micrograms of nitrogen per cubic meter of air, roughly twice the gaseous concentration of 1.2 plus or minus 0.9 micrograms per cubic meter. On average, 70 percent of the total WSON resided in the particle phase, a partitioning coefficient of 0.70 plus or minus 0.14. That figure is markedly higher than those recorded in Shanghai, around 0.49, or Xianghe in the North China Plain, around 0.43, and the regional comparison proved diagnostic: the city ranking in partitioning efficiency matched the ranking in aerosol liquid water content, while temperature, relative humidity, and particle acidity showed no consistent pattern across the three sites.

Chengdu’s geography makes it an ideal natural laboratory for this chemistry. The Sichuan Basin is a deep, enclosed depression more than two thousand meters deep, ringed by mountains that strangle the dispersion of pollutants. In winter, the Qinling range blocks cold air intrusions, letting warm, moist air stagnate over the city; temperature inversions occur on 95.4 percent of winter days, with an average inversion layer depth of nearly 290 meters. During pollution episodes the boundary layer compresses by roughly half, trapping emissions from vehicles, cooking, and industry near the surface. During the campaign, average PM2.5 concentrations reached 76 plus or minus 38 micrograms per cubic meter, with 14 of the 27 sampling days classified as polluted and peak levels hitting 199 micrograms per cubic meter.

The partitioning behavior of WSON shifted with the weather in a chemically revealing way. During warmer, drier periods, when the organic fraction of the aerosol exceeded its water fraction, WSON condensed into the organic phase, and the partitioning coefficient dropped to 0.61 plus or minus 0.11. During colder, more humid periods, when relative humidity averaged 92 percent and the aerosol water fraction was roughly 4.5 times the organic fraction, the compounds dissolved into the aqueous phase instead, and the coefficient rose to 0.71 plus or minus 0.13. In other words, the same molecules follow two different uptake pathways depending on which condensed medium dominates, but the humid, water-rich route is the more efficient one, and it is the route that prevails during Chengdu’s worst haze.

Thermodynamic modeling with the ISORROPIA-II model identified the source of all that particle water. Ammonium nitrate alone contributed approximately 61.3 percent of the aerosol liquid water content, followed by ammonium sulfate at 14.4 percent and organic matter at just 7.7 percent. Across a nationwide statistical analysis, the fraction of aerosol mass made up of liquid water correlated strongly with the WSON partitioning coefficient, with a correlation coefficient of 0.78. The implication is a chain reaction: nitrogen oxides and ammonia react to form ammonium nitrate, ammonium nitrate pulls in water, and that water then dissolves gaseous organic nitrogen compounds, converting them into additional particulate mass and thickening the haze.

The team paid particular attention to amines, a class of reduced nitrogen compounds with outsized influence on atmospheric chemistry. Methylamine and dimethylamine were overwhelmingly gaseous, with gas-phase concentrations three to five times their particle-phase values, yet their partitioning coefficients nearly doubled during polluted periods. Amines are potent players in new particle formation: the sulfuric acid-amine-water nucleation mechanism can boost formation rates by more than a thousandfold compared with ammonia alone, and amines also participate in Maillard-type reactions in the aqueous phase that generate light-absorbing brown carbon. In Chengdu, amine nitrogen accounted for nearly half of gas-phase WSON, and the compounds correlated with vehicular tracers such as nitrogen dioxide and carbon monoxide, implicating traffic as a significant source.

Perhaps the most technically interesting result concerns what does not control amine partitioning. Classical theory predicts that amines, as weak bases, should partition strongly into acidic particles, protonating to form nonvolatile aminium salts, and that their uptake should therefore be highly pH-dependent. The researchers tested this using S-curve analysis, computing theoretical partitioning fractions from Henry’s law constants, acid dissociation constants, and activity coefficients from the E-AIM model. The observed data fell within the theoretical envelope, but showed no significant dependence on pH or temperature. The reason is that atmospheric aqueous phases are acidic enough, in the pH 0 to 5 range, that amines exist almost entirely in their protonated form regardless of small pH variations. What mattered instead was aerosol liquid water, with which the amine partitioning coefficients correlated significantly, both day and night.

The amines that did enter the particle phase appear to be locked there as salts. Their partitioning coefficients correlated strongly with nitrate and sulfate concentrations, and the coefficients rose with the nitrogen and sulfur oxidation ratios, indicators of secondary inorganic aerosol formation. The chemistry is straightforward acid-base neutralization: gaseous amines dissolve, deprotonate, and combine with nitrate or sulfate to form stable amine nitrate or amine sulfate salts within the PM2.5. Notably, the degree of methylation worked against uptake, since the partitioning coefficients followed the order ammonium, then methylamine, then dimethylamine; each hydrophobic methyl group substituted on the amino group weakens the molecule’s affinity for water and pushes it back toward the gas phase. Rapid reaction with hydroxyl radicals, giving atmospheric lifetimes of only 12.6 hours for methylamine and 4.3 hours for dimethylamine, further limits their accumulation in particles.

The policy implications are as concrete as the chemistry. Because ammonium nitrate supplies the majority of the particle water that drives WSON and amine uptake, curbing nitrogen oxide and ammonia emissions would deliver a double benefit: it would directly reduce nitrate mass in PM2.5 and indirectly starve the aqueous uptake pathway that converts gaseous organic nitrogen into additional aerosol. In a basin where secondary sources already account for roughly 40 percent of PM2.5 and nitrate has overtaken sulfate as the dominant inorganic salt, that dual mechanism offers a way to interrupt the self-amplifying loop of humid winter haze. For the many cities worldwide that share Chengdu’s combination of enclosed topography, high humidity, and ammonia-rich emissions, the study provides both a mechanistic explanation and a practical target.

Subject of Research: Gas-particle partitioning of water-soluble organic nitrogen and amines in winter urban haze

Article Title: Simultaneous measurements of gas- and aerosol-phase water-soluble organic nitrogen in the winter urban atmosphere of Chengdu, China

Article References: Chen, K., Li, Z., Li, R., Chen, L., Li, R., Xiao, B., Lv, S., Wu, C., Li, R., Zhang, J., & Wang, G. (2026). Simultaneous measurements of gas- and aerosol-phase water-soluble organic nitrogen in the winter urban atmosphere of Chengdu, China. Atmospheric Chemistry and Physics, 26(19), 14329-14343. https://doi.org/10.5194/acp-26-14329-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14329-2026

Keywords: water-soluble organic nitrogen, WSON, amines, aerosol liquid water, secondary organic aerosol, PM2.5, winter haze, Chengdu, Sichuan Basin, ammonium nitrate, gas-particle partitioning, ISORROPIA-II