Zhang Si¹， Wang Gehui²

1.School of Environmental and Municipal Engineering， Xi’an University of Architecture and Technology， Xi’an 710055， China； 2.School of Geographic Sciences， East China Normal University， Shanghai 200241， China

Abstract

Significance China’s sustained clean-air actions have sharply reduced sulfur dioxide （SO₂） emissions， yet sulfate aerosol concentrations have declined much more slowly. From 2007 to 2017， SO₂ fell by 89.1%， whereas sulfate decreased by only 58.9%.This decoupling indicates that sulfate abundance is governed not only by precursor emissions but also by evolving multiphase chemistry. As fine-particle composition shifts toward nitrate-， ammonium-， and organic-rich mixtures， changes in aerosol pH， liquid water content， ionic strength， and interfacial structure can enhance SO₂ uptake and oxidation. This review examines how these changes sustain sulfate production under low-SO₂ conditions， with a focus on NO₂ interfacial oxidation pathway and O₃ aqueous oxidation pathway， as well as their implications for coordinated PM_2.5-O₃ control.

Progress Sulfate forms through gas-phase OH oxidation， cloud and fog chemistry， and heterogeneous or aqueous reactions involving H₂O₂， O₃， NO₂， organic peroxides， and transition metals. Recent experiments show that aerosol particles cannot be treated simply as dilute bulk solutions. Concentrated electrolytes， interfacial ion gradients， liquid water content， and organic coatings can alter gas uptake and reaction rates by orders of magnitude. For the NO₂ oxidation pathway， dissolved SO₂ is converted to HSO₃⁻ and SO₃²⁻， which are subsequently oxidized at the air-water interface， directly producing sulfate and HONO. The reaction is favored by weakly acidic to near-neutral particles， abundant NH₃， high relative humidity， sufficient liquid water， and interfacial enrichment of anions. In contrast， strong acidity， low humidity， cation-rich interfaces， and some organic coatings suppress it. Measured interfacial rate coefficients exceed traditional bulk solution values by three to four orders of magnitude. HONO photolysis also produces OH radicals， coupling sulfur， nitrogen， and radical chemistry while indirectly accelerating gas-phase SO₂ oxidation. Modeling and paired gas-particle sulfur isotope observations support the atmospheric importance of this pathway during haze. O₃ oxidation proceeds by oxygen transfer to dissolved sulfite and is strongly controlled by acidity， water content， ionic strength， and particle composition. It becomes especially efficient in fresh sea-salt droplets， where chloride and high ionic strength promote alkalinity and reaction kinetics， and on hygroscopic alkaline mineral dust， where surface basicity facilitates SO₂ hydrolysis. Therefore， no single pathway dominates universally. NO₂ oxidation is predominant in weakly acidicurban aerosols， and O₃ oxidation becomes dominant in marine or dust-influenced environments. Divergent apportionments reflect both regional variability and incomplete model treatments of interfaces and concentrated aerosol media.

Conclusions and Prospects The nonlinear response of sulfate to SO₂ abatement is a dynamic outcome of particle composition， conversion efficiency， atmospheric oxidation capacity， and pollution stage. Future studies should combine single-particle and microdroplet experiments， in situ interfacial spectroscopy， molecular simulation， field observations， and multiscale modeling. Simultaneous measurements of gaseous and particulate sulfur isotopes， HONO， radicals， oxidants， aerosol pH， liquid water， ionic strength， metals， and organics are needed to separate direct sulfate production from oxidation feedback effects and to quantify precursor decline versus enhanced conversion. Kinetic parameterizations should vary with composition， pH， humidity， phase state， and organic coatings， rather than relying on fixed dilute-solution constants. Coordinated observations across coastal sea-salt regions， cold regions of northeastern China， dust-affected areas of northwestern China， and urban clusters should identify the conditions under which dominant pathways shift. These advances will enable more reliable evaluation of coupled SO₂， NOx， NH₃， and O₃-precursor controls and support region-specific strategies for joint PM_2.5 and O₃ mitigation.

Keywords： sulfate aerosol； NO₂ interfacial oxidation pathway； O₃ aqueous oxidation pathway； atmospheric oxidation capacity

Get Citation:  Zhang Si， Wang Gehui. Multiphase mechanisms of formation of rapid sulfate aerosol under low-SO₂ emission conditions［J］. China Powder Science and Technology， 2027， 33（2）： 223-234.

Received:  2026-07-15， Revised： 2026-08-17， Online： 2026-09-11。

Funding： The research was supported by the National Natural Science Foundation of China Young Scientists Fund (Grant No. 42407137)

DOI：10.13732/j.issn.1008-5548.2027.02.016

CLC No.:  X131； TB44

Type Code:  A

Serial No.:  1008-5548（2027）02-0223-12