For a long time, traditional models have significantly underestimated sulfate concentrations during periods of heavy pollution. A research team from Nanjing University and Fudan University published a study in *JACS*, using ozone isotope tracing technology to reveal a new mechanism: driven by a strong electric field of tens of millions of volts at the microdroplet interface, sulfate can be converted through a novel "direct oxidation channel" without the need for traditional hydrolysis. This discovery challenges existing atmospheric chemistry models and provides a new framework for understanding the causes of haze and atmospheric evolution.
1. Precise Oxygen Isotope Tracing: Resolving the Debate on Oxidation Pathways
The oxidation of SO₂ in the atmosphere can be driven by either oxygen O₂ or nitrogen dioxide NO₂, but when both are present simultaneously, traditional methods cannot distinguish their respective contributions.
The research team cleverly utilized the natural fingerprint differences of the trioxide isotopes Δ⁷O—the Δ⁷O of air O₂ is approximately 0.50 μm, the Δ⁷O of water is approximately 0.0 μm, and NO₂ also has its characteristic value. By measuring the Δ⁷O of the product sulfate and combining it with the isotopic mass balance equation, they quantitatively deduced the proportions of oxygen atoms contributed by O₂, SO₂, and water, respectively. The results clearly show that when O₂ and NO₂ coexist, the Δ⁷O value of sulfate is close to that under pure O₂ conditions. This proves that O₂ is the dominant oxidant, and this method provides a novel analytical tool for multi-oxidant competition systems.
2. A Novel Mechanism at the Microdroplet Interface: Direct Oxidation Channel Confirmed
Traditional aqueous solution theory suggests that SO₂, upon entering a droplet, rapidly hydrolyzes to bisulfite ions, undergoes complete oxygen exchange with water, and is subsequently oxidized. Of the four oxygen atoms in sulfate, three come from water and one from O₂; if oxidized by NO₂, all four oxygen atoms come from water. This known pathway is referred to as P1 in the paper.
However, isotopic data completely overturns this understanding.
In the water-labeled experiment, if only the P1 pathway is followed, the theoretically predicted sulfate Δ⁷O should be 6.6 μm, but the measured value is as high as 3.149 μm. This indicates that a large number of oxygen atoms do not originate from water, but directly from SO₂ itself. Based on this, the research team proposed a new pathway, named P2. In the P2 pathway, SO₂ is split by an electric field at the microdroplet interface, and the resulting oxygen radicals are directly oxidized to SO₃, which is then hydrolyzed to sulfuric acid. The entire process bypasses the hydrolysis and oxygen exchange steps, achieving an atom-economical conversion from SO₂ to sulfate.
3. Interfacial Electric Field Drives Reaction Rate Increase by Three Orders of Magnitude
The strong electric field on the surface of microdroplets can reach 10⁷ V/cm, sufficient to break down water and oxygen molecules to generate reactive oxygen species such as OH, O, and O₂⁻. The apparent rate constant of SO₂ oxidation measured in this study is 1.3 × 10⁻ s⁻, which is about two orders of magnitude higher than the conventional bulk non-catalytic oxidation rate. This explains why sulfate levels explode on high-humidity smoggy days.
The abundant presence of microdroplets provides a huge gas-liquid contact area for interfacial electric field reactions. Interfacial chemistry is no longer a supporting role to bulk chemistry, but the main one.
This groundbreaking discovery is underpinned by an experimental platform capable of stably and controllably generating microdroplet environments and accurately simulating atmospheric conditions. It is thanks to the PLR APD-01 atomizing photoreaction device that researchers were able to reproduce atmospheric microdroplet interfacial reactions in the laboratory and complete stable isotope labeling experiments lasting over ten hours.
1. Stable Atomization, Long-Term Operation
The device adopts vibrating diaphragm atomization technology, which can produce microdroplets with narrow size distribution and uniform surface charge. The atomizer can work continuously and stably for more than 10 hours, meeting the requirements of long-term isotope labeling experiments.
The droplet atomization intensity is adjustable, facilitating the study of the influence of different specific surface areas on the interfacial reaction path.
2. Precision Reaction Chamber for Efficient Gas-Liquid Contact
The effective volume of the reaction chamber is approximately 1.7 L. The gas phase enters from the bottom, forming a countercurrent contact with the atomized microdroplets at the top. This design maximizes the collision probability between SO₂ or NO₂ gas and the microdroplets, ensuring that the reaction yield is sufficient to meet the requirements of isotope mass spectrometry detection.
The inlet pressure is below 10 kPa, and the leakage rate is less than 0.1 Pa·L/s, meeting the requirements for high-precision gas material balance calculations. 3. Multi-wavelength LED light source to simulate solar photochemistry Provides multiple light source modules including 254 nm, 310 nm, 365 nm, and white light. It can simulate different wavelengths of illumination in atmospheric photochemical processes. The LED light source produces no ozone and does not interfere with the reaction system. The integrated design of the light source and reactor ensures uniform illumination and stable light intensity. It supports photocatalysis and dark reaction control experiments.
4. Precise Temperature Control, Eliminating Thermal Interference
The accompanying chiller stably controls the reaction temperature at 15℃, with an accuracy better than plusmn;1℃. The interfacial reaction rate is highly sensitive to temperature; precise temperature control ensures the repeatability of isotope data and avoids interference from thermochemical reactions on photochemical or interfacial electric field effects.
5. Low Dead Volume Sampling for Reaction Kinetic Tracking The reactor is equipped with PEEK sampling tubing and ball valves, resulting in a sampling dead volume of only 0.35 mL. Multiple samplings can be performed without disrupting the reaction system. Researchers can obtain sulfate samples at different time points to construct reaction kinetic curves and capture intermediate state information. This article reveals a novel oxidation pathway driven by the interfacial electric field of microdroplets through isotope tracing, solving a long-standing problem in atmospheric chemistry and opening a new chapter in gas-liquid interface chemistry research. The PLR APD-01 atomizing photoreaction device, as the core tool, provides a stable and precise experimental platform for the empirical verification of this mechanism, helping researchers capture and understand the reaction dynamics of microdroplets. It is hoped that this device will support more researchers in achieving original breakthroughs in aerosol chemistry, interfacial catalysis, and unconventional synthesis.
Literature information
Xiang Sun, Wenbo You, Yu Wei, Hao Yan, Liwu Zhang, Huiming Bao. Microdroplet Assisted Sulfate Formation Incorporating Versatile Oxygen Sources. Journal of the American Chemical Society. 2026, DOI: 10.1021/jacs.5c17911
