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. 2025 Nov 10;6(1):74–81. doi: 10.1021/acsenvironau.5c00168

Oxidation of Organic Sulfur at the Air–Water Interface of Microdroplets: A New Way of Atmospheric Particle Precursor Formation

He Tang †, Xihe Yang †, Fengjian Chu †, Shiwen Zhou †, Wangyu Li †, Yongqi Li ‡, Haiyan Zhang ‡, Zechen Yu ‡,*, Hongru Feng †,*, Yuanjiang Pan †,*
PMCID: PMC12828607  PMID: 41583862

Abstract

Dimethyl sulfide (CH3SCH3), methanethiol (CH3SH), and dimethyl disulfide (CH3SSCH3) are the most predominant marine organic sulfur compounds that drive cloud formation by acting as a critical precursor of cloud condensation nuclei. However, discrepancies of natural aerosol radiative effects and climatic impacts between climate model simulations and satellite observations highlight an incomplete understanding of organic sulfur oxidation pathways. Here, we demonstrate a previously unrecognized rapid oxidation mechanism at the air–water interface of microdroplets, where volatile sulfides convert to particle precursors within milliseconds. Combining DFT calculation, QM/MM simulation, and sequential oxidation of possible intermediates, we proposed a new pathway of atmospheric particle precursor formation at the air–water interface of microdroplets, which is different from the traditional gas-phase oxidation pathway, offering a mechanistic solution to re-evaluate the model simulation gaps in natural aerosol radiative effects and climatic impacts.

Keywords: organic sulfur, microdroplet reaction, atmospheric particle, mass spectrometry, air−water interface

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1. Introduction

Sulfuric acid (H2SO4) and methanesulfonic acid (CH3SO3H) are key oxidation products of organic sulfur species. H2SO4 and CH3SO3H promote the formation of polymers in secondary organic aerosol (SOA), leading to increased PM2.5 concentrations, and the salt form of H2SO4 and CH3SO3H also facilitates the formation of cloud condensation nuclei. − Globally, the most predominant organic sulfur species are dimethyl sulfide (DMS, CH3SCH3) and methanethiol (CH3SH), followed by dimethyl disulfide (DMDS, CH3SSCH3). , Extensive research has focused on the oxidative formation of H2SO4 and CH3SO3H from organic sulfur species in the gas phase and its implications for aerosol generation, especially for DMS. −

Despite these well-characterized mechanisms, systematic discrepancies persist between climate model simulations and satellite observations suggest an underestimation of natural nucleation processes. − Atmospheric modeling revealed a large missing sulfate budget. − Such deficits strongly suggest missing chemical routes mediating the conversion of organic sulfur species to nucleation precursors. Investigating the process could help reconcile the discrepancies between observations and model simulations, thus addressing current knowledge gaps and advancing the comprehensiveness of atmospheric research.

Recent advances highlight microdroplet chemistry as a potential contributor to the unresolved reactivity. Water microdroplets exhibit air–water interfacial electric fields exceeding 109 V/m, − capable of generating radicals such as •OH. Sea spray aerosols represent a dominant source of atmospheric microdroplets, mediating chemical exchanges between marine and tropospheric systems. Prior investigations demonstrate spontaneous oxidation of thiols and thioethers on microdroplet surfaces via •OH or H2O•+. , Nevertheless, the specific reaction pathways of ambient H2SO4/CH3SO3H formation remain uncharacterized.

2. Materials and Methods

2.1. Materials

Dimethyl sulfide (C2H6S, 99%), sulfurous acid (H2SO3, 99% SO2 ≥ 6% in water), sulfuric acid (H2SO4, 98%), potassium peroxymonosulfate (≥42% KHSO5 basis), and 2,2,6,6-tetramethylpiperidinyl-1-oxide (TEMPO, 99%) were purchased from Shanghai Aladdin Biochemical Technology­(Shanghai, China). Methanesulfonic acid (CH4SO3, 98%), methanethiol (CH4SH, 10% in propanediol), and disulfide (C2H6S, 98%) was purchased from Macklin­(Shanghai, China). DMSO-d6 was purchased from Cambridge Isotope Laboratories (Tewksbury, MA). HPLC-grade methanol was purchased from Sigma-Aldrich (St. Louis, MO). 98% Water-18O (98%) was purchased from Adamas (Shanghai, China). Water used for the HPLC mobile phase was purchased from A.S. Watson Group Ltd. (Hong Kong, China). Nitrogen was generated by NiGen LC-MS 40-1 from Claind (Villa Carlotta, Italy). The concentrations of DMSO-d6, CH3SO3H, H2SO3, H2SO4, and KHSO5 used in the experiments were 100 ppm in pure water. The concentrations of DMS in the gas-configuration nanoESI experiment were 1000 ppm in pure water.

2.2. Microdroplets Generation

Air–water interface oxidation device: Microdroplets were produced by nanoESI, which consists of a fine glass capillary tip and a metal wire with high voltage. Experimental conditions were taken as follows: DC voltage of 3 kV; fine glass capillary tip with an inner diameter of 2 μm; the tip of the glass capillary is 10 mm away from the mass spectrometer; pure water as the nanoESI solvent. The fine glass capillary tip was pulled using borosilicate glass capillaries (ID 0.78 mm, item no. BF100-78-10, Sutter Instruments, Novato, CA) by a Sutter Instrument pipet puller P-1000 (Novato, CA).

ESI analysis was conducted by ultraperformance liquid chromatography Vanquish system (Thermo Fisher Scientific, CA) in tandem with Orbitrap Exploris 120 (Thermo Fisher Scientific, San Jose, CA) under the following conditions: flow rate 0.2 mL/min with mobile phase MeOH: H2O = 1:1, v/v; ion spray voltage, 3500 V; ion transfer tube temperature, 320 °C; Vaporizer temperature, 275 °C; RF lens, 70%; sheath gas, 35 Arb; aux gas, 7 Arb; injection time, 100 ms. Data acquisition was carried out using Xcalibur 2.2 SP1 software (Thermo Fisher Scientific, San Jose, CA).

Pneumatic spray: Pure pneumatic spray for mass spectrometry detection is very unstable and has a very low response. So we conducted the experiment using a pneumatic spray system with an inner diameter of 50 μm. The sheath gas is air. The DMS was mixed with water and then injected by using a syringe at a rate of 10 μL/min. The spray was collected in a round-bottom flask with an outlet. The collected product was redissolved in water and analyzed by LC-MS.

2.3. Gas-Configurable NanoESI Device

The support structure was fabricated by using 3D printing with photosensitive resin. The box features a 1 mm diameter hole for the insertion of the nanoESI spray needle, as well as three holes with a diameter of 4 mm. One of these holes is oriented toward the mass spectrometer inlet, while the other two are positioned on either side of the spray needle hole to facilitate the introduction of carrier gas. During operation, the lid is closed, creating a sealed system after ventilation. The operating conditions include a total carrier gas flow rate of 2 L/min with a split of 1 L/min directed to each of the two holes, while the nanoESI conditions remain consistent with those previously described.

2.4. DFT Computational Methodology

Theoretical calculations were based on density functional theory (DFT) combined with transition state theory (TST). Geometry optimizations and electronic structure analyses were performed at the B3LYP/6–31+G­(d,p) computational level. Transition states were validated through intrinsic reaction coordinate (IRC) trajectories and vibrational frequency examinations to ensure thermodynamic feasibility (Details in Text S2–S3).

2.5. Free Energy Profile for the Oxidation at the Gas Interface and at the Air–Water Interface

Quantum mechanics/molecular mechanics (QM/MM) approach combined with multiscale metadynamics (SMS-MetaD) were employed to investigate the oxidation mechanism of dimethyl sulfide (DMS). All simulations were performed using the CP2K software package, where the quantum mechanical (QM) region was treated with the BLYP-D3 density functional theory and the DZVP-MOLOPT basis set, while the molecular mechanics (MM) region utilized the TIP3P water force field. The simulation system was constructed in a periodic simulation box of 17.5 × 17.5 × 37.0 Å3 under the NVT ensemble, with temperature maintained at 300 K via a Nose-Hoover chain thermostat. To enhance sampling efficiency, we defined the collective variable dcv­(oxidation) = dS1O1 + dS2O2 – dO1O2 to describe the oxidation process. Enhanced sampling was conducted using the MetaD method with a Gaussian height of 0.1 kcal/mol and a width of 0.05 Å, deposited every 0.05 ps. The total simulation time amounted to 200 ps with an integration time step of 0.1 fs. Reaction energy barriers and pathways were determined by analyzing the evolution of the collective variable trajectory and constructing the corresponding free energy landscape (Detailed process shown in Text S4).

3. Results and Discussion

In this work, we experimentally simulated air–water interface reactions of microdroplets under atmospheric conditions to probe the rapid oxidation of volatile sulfides and the formation of an atmospheric particle precursor. Since DMS/CH3SH/DMDS are nearly insoluble in water, an air–water interface oxidation device (Figure , photographs of experimental setup shown in Figure S1A) was designed to enable mass transfer of volatile organic sulfur compounds to microdroplets in the atmosphere. Gaseous sulfur compounds were exposed to water microdroplets from submicrometer to several micrometers in diameter. This setup overcomes the solvent limitations of poorly soluble substances and provides a more accurate representation of the conditions at the ocean surface. In addition, we also confirmed the generation of the products (Figure S16) through pneumatic spray collection (experimental details shown in Supporting Information, photographs of experimental setup shown in Figure S1C, MS spectrum). Except for DMS/DMDS/CH3SH, all other compounds in this study were dissolved in pure water for the air–water interface reactions (materials and methods are shown in Supporting Information).

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Schematic diagram of the device and spectra of DMS oxidation products. (A) Schematic diagram of the air–water interface oxidation device. (B, C, E, and F) Spectra of DMS oxidation at the air–water interface in MS negative ion mode. (D) Spectrum of DMS oxidation at the air–water interface in MS positive ion mode.

DMS can be oxidized immediately at the air–water interface of microdroplets within a microsecond time scale, while no oxidation products were detected in ESI (Figure S2B,E). At the air–water interface, DMS was oxidized to produce HSO4 – (m/z 96.96) and CH3SO3 – (m/z 94.98), key species that serve as critical precursors for atmospheric particle nucleation. Oxidation intermediates of DMSO (m/z 79.02, [DMSO+H]+), CH3SO2 – (m/z 78.99), HSO3 – (m/z 80.96), SO2 •– (m/z 63.96), SO3 •– (m/z 79.96), SO4 •– (m/z 95.95), SO5 •– (m/z 111.95), and HSO5 – (m/z 112.95) were also observed (Figure ). In pneumatic spray, it was observed that a series of ions were detected, including CH3SO3 –, CH3SO2 –, and HSO4 –.

We found that the air–water interface exhibits distinct characteristics compared with gas-phase oxidation. Our mass spectrometry results indicate that oxidation at the air–water interface preferentially promotes the significant formation of CH3SO3H over H2SO4 (Figure ). In contrast, gas-phase oxidation mainly produces H2SO4, with CH3SO3H generated in much smaller amounts. This difference implies a potential compensatory pathway for CH3SO3H production at the air–water interface.

CH3SH and DMDS are also important natural organic sulfur compounds that can undergo rapid oxidation at the air–water interface. Mass spectrometric analysis in negative ion mode demonstrates that their oxidation products were CH3SO2 –, HSO4 –, SO2 •–, SO3 •–, SO4 •–, SO5 •–, and HSO5 –, exhibiting identical mass spectral signals to those derived from DMS oxidation (Figures S3, S4). CH3S•+ (m/z 46.99), CH3SH2 + (m/z 49.01), CH3SOH2 + (m/z 65.01, MS2 in Figure S5), C2H6S2H+ (m/z 95.00), and C2H6S2OH+ (m/z 110.99) ions (Figure S6) were detected in positive ion mode for CH3SH, while C2H6S2H+ (m/z 95.00) and trace amount of C2H6S2OH+ (m/z 110.99) ions (Figure S7) were detected for DMDS. The relatively significant formation of C2H6S2OH+ from CH3SH suggests that it may arise from the combination of CH3SO and CH3S radicals.

We hypothesize that under the influence of the electric field at the air–water interface, CH3SH generates free radicals, which then either combine to form DMDS or are oxidized to CH3SOH. CH3SOH can subsequently be oxidized to CH3SO2H. Alternatively, CH3SH may be directly oxidized to CH3S­(H)­O, which, under the electric field, loses a H atom and generates CH3SO• that can be further oxidized to CH3SO2H or combine with CH3S• to form C2H6S2O.

Oxidation pathways were investigated through sequential oxidation experiments (species detected in ESI and at the air–water interface are shown in Table S1. Mass errors are shown in Tables S2 to S16). DMSO is the probable primary product of DMS oxidation. Deuterated DMSO (DMSO-d6) was first used to track the oxidation pathway of the air–water interface. We found that CD3SO3 – (m/z 98.00) and CD3SO2 – (m/z 82.00) were generated, confirming that DMSO undergoes oxidation processes analogous to those of DMS, yielding identical intermediates and products (Figure S8). In contrast, the control ESI spectrum (Figure S9) showed no detectable oxidized product. This confirms that DMSO is a key intermediate product of DMS oxidation.

CH3SO3H contains one methyl group, which originates from a monomethylated precursor, probably CH3SO2H. Oxidation of CH3SO3H generates SO3 •– and SO5 •– (Figure S10A). The absence of HSO4 – means that it follows a pathway different from that of H2SO4 generation. H2SO3 lacks a methyl group and is derived from DMSO via demethylation (Figure S8). As Figure S10C shows, oxidation of H2SO3 generates HSO4 –, SO4 •–, SO5 •–, SO2 •– and SO3 •–. The formation of HSO4 – suggests that H2SO3 is a precursor of H2SO4. Further oxidation of H2SO4 generates SO3 •–, SO4 •–, SO5 •–, and HSO5 – (Figure S11A), identifying it as the precursor to both SO5 •– and HSO5 –. These species can interconvert at the air–water interface. Subsequent reaction of HSO5 – at the air–water interface generates SO2 •–, SO3 •–, SO4 •–, SO5 •–, HSO4 – (Figure S12A), indicating that HSO5 – can be converted to form SO5 •–, which can be quenched by H2O to regenerate HSO5 –.

As a result, the proposed oxidation process of DMS unfolds as follows (Figure B): Initially, •OH adds to DMS to form DMSO; DMSO is then demethylated to generate CH3SO2H; CH3SO2H is further oxidized to CH3SO3H. Meanwhile, CH3SO2H can be oxidized and demethylated to generate H2SO3; H2SO3 is further oxidized to H2SO4. H2SO4 transferred to HSO4 – and reacts with •OH to produce HSO5 –, which can subsequently be oxidized to SO5 •– and decomposed to produce SO4 •–. These compounds are both involved in atmospheric active species and serve as crucial precursors for the formation of atmospheric particulate matter.

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Schematic diagram of the air–water interface oxidation process of DMS/CH3SH/DMDS. (A) Schematic diagram of the air–water interface oxidation process, showing fast particle formation. (B) Proposed oxidation process of DMS. (C) Proposed oxidation process of CH3SH and DMDS.

Figure C demonstrates that CH3SH and DMDS undergo different initial oxidation pathways compared to DMS, but they converge onto the same route. At the air–water interface, CH3SH undergoes initial conversion to the CH3S• and then either reacts with •OH to directly produce CH3SOH, or two CH3S• dimerize to form DMDS. CH3SOH can be oxidized by •OH to generate CH3SO2H, subsequently following the same pathway as for DMS. CH3SH can also react directly with •OH to produce CH3SOH, which can lose a H atom to form a CH3S­(O) radical that combines with CH3S• to generate C2H6S2O. DMDS reacts with •OH to produce a small amount of C2H6S2O, but it primarily undergoes direct oxidative cleavage to generate CH3SO2H and a series of subsequent compounds.

The air–water interface generates •OH, which participates in the oxidation of compounds. ,, To validate the oxidation mechanism, we conducted oxidation experiments using H2 18O. As a result, we observed the incorporation of the 18O atom into DMS oxidation products and various intermediates (Figure ), providing direct evidence that reactive species generated from water mediate these oxidations. Although 98% H2 18O was used, the proportion of 18O in the oxidation products was low.

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MS spectra of DMS oxidation at the air–water interface oxidation by applying heavy water (H2 18O) as a microdroplet generation source. (A) DMS oxidation products detected in MS positive ion mode, MS range 75–85. (B) DMS oxidation products detected in MS negative ion mode, MS range 50–130. (C) MS range 110–120, magnified by 7 times from panel (B). (D) MS range 60–70, magnified by 4 times from panel (B). (E) MS range 78–88, magnified by 8 times from panel (B). (F) MS range 92–102.

Previous literature indicates that heavier isotopes preferentially accumulate in the interiors of microdroplets, while lighter isotopes are more likely to be found at the air–water interface. Consequently, oxidation processes occurring at the air–water interface primarily involve •OH rather than heavy isotopes such as •18OH. In this study, •OH was captured, and a heavy 18O was reinserted into sulfur oxide, thereby confirming the role of •OH in the oxidation process.

Radical capture experiments of •18OH at the air–water interface of H2 18O using TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxide) revealed the presence of [TEMPO+OH+H]+ and [TEMPO+18OH+H]+ ions (Figure S13), confirming the formation of •18OH and •OH, and their participation in the oxidation process. In addition, the involvement of O2 in the air is also essential. We constructed a gas-configurable nanoESI device to conduct experiments under the conditions of a single gas environment (Figure S1B). We found that when argon was used as the carrier gas, the response of the oxidation products H2SO4 and CH3SO3H in the mass spectrum was extremely low (Figure S14B). In contrast, when compressed air was used as the carrier gas, the oxidation products were clearly observed in the mass spectrum (Figure S14A). This indicates that the participation of oxygen is of significant importance.

In addition to the experiments described above on the reaction between gaseous DMS and microdroplets, we also mixed DMS with H2 18O and conducted experiments at different distances for comparison. This further confirmed the reaction pathway and mechanism. It is noted that the air–water interface can spontaneously generate singlet oxygen (1O2), superoxide anions (•O2 –), and •OH. Among these reactive species, 1O2 and •O2 – are entirely derived from O2, while •OH originates solely from H2O. At a distance of 3 mm, fully 18O-substituted products of SO2 •–, SO3 •–, HSO3 –, HSO4 –, SO4 •–, CH3SO3 – and HSO5 – ions could be detected (Figure S15). The MS response of CH3SO2 – was too low to detect its 18O-substituted product. The intensities of SO5 •–and its 18O-substituted products were relatively high, but only up to three 18O-substituted ions could be detected (S18O3O2 •–). This indicates that SO5 •– is not entirely formed from •OH generated from water in the microdroplets, but rather through the reaction between SO3 •– and O2. The substitution proportion of 18O is high at 3 mm, rapidly decreases at 6 mm, and remains constant at 9–15 mm. The change in the proportion of the 18O verifies that heavier isotopes preferentially accumulate in the interiors of microdroplets, while lighter isotopes are more likely to be found at the air–water interface.

The results of sequential oxidation experiments and DFT calculations showed that the oxidation process proceeds in the presence of •OH and O2 (Figure S17). It is important to clarify that the preliminary DFT calculations in here (Figure S17) primarily serve to theoretically validate the plausibility of the proposed radical oxidation pathway. The role of the air–water interface in this pathway is supported by two lines of evidence: Model calculations demonstrate that the air–water interface environment significantly reduces reaction energy barriers. For a rate-determining step in the reaction pathway, the calculated energy barrier from DMS to DMSO in the gas phase is 14.74 kcal/mol, which decreases to 12.72 kcal/mol in the air–water interface model (Figure S18). Radical scavenging experiments directly confirm the in situ generation of abundant •OH radicals at the air–water interface, which act as highly reactive initiators and mediators to trigger and drive the rapid progression of the oxidative chain reaction.

The PAM-OFR (Potential Aerosol Mass–Oxidation Flow Reactor) reaction (detailed methods shown in Text S1) demonstrated the gas-phase oxidation of DMS by •OH. As Figure S19 shows, the experiments identified significant oxidation products, including DMSO, CH3SOH, CH3SO2H, and CH3SO3H. Notably, although CH3SOH is a key intermediate in the gas-phase oxidation of DMS, its absence during oxidation at the air–water interface reveals distinct reaction pathways between the gas-phase and the newly observed air–water interface, representing a new oxidation mechanism.

In addition to the •OH oxidation pathway mentioned above, we also observed ions resulting from the combination of the NO X and SO X . NO X can participate in the oxidation of atmospheric compounds. , This phenomenon may be attributed to the strong electric field at the air–water interface, , generating a small amount of reactive species in situ, such as nitro or nitroso compounds. , We captured the corresponding ions NO2SO3 – and NO3SO3 – (Figure S20), indicating that the in situ generated NO X may be involved in oxidation. Simultaneously, in a gas-configurable nanoESI device, a small amount of NO2SO3 – and NO3SO3 – ions could be observed when compressed air was used as the carrier gas, whereas no relevant ions were detected when argon was employed as the carrier gas. However, the low abundance of these ions indicates that this type of oxidation is not a dominant factor in the oxidation of organic sulfides.

Volatile sulfur compounds at the air–water interface are highly reactive, generating a large number of free radicals that lead to various reactions with other species in the atmosphere, thereby increasing the complexity of atmospheric chemistry. The exclusive presence of SO5 •– ions at the air–water interface (complete absence in ESI experiments) underscores the unique oxidative properties of the air–water interface. SO5 •– and other sulfide radical species may serve as reactive atmospheric species, participating in reactions with atmospheric organic carbon.

The aerosol formation process can be divided into three stages: stage I, involves the formation of small molecular precursors; stage II, reaches a critical value for cluster formation; and stage III, results in the formation of growable nanoparticles. Sulfates are significant components of small molecular precursors. We observed various ion-driven small cluster formation from the oxidation of DMS in the mass spectra, including [(SO3)­CH3SO3]− (m/z 174.94), [(SO3)­HSO4]− (m/z 176.92), [(CH3SO3H)­CH3SO3]− (m/z 190.97), [(CH3SO3H)­HSO4]− and [(H2SO4)­CH3SO3]− (m/z 192.95), [(H2SO4)­HSO4]− (m/z 194.93), [(CH3SO3H)2CH3SO3]− (m/z 286.96), and [(H2SO4)2HSO4]− (m/z 292.90) (Figure , same products of CH3SH and DMDS shown in Figures S3C and Figure S4B), as proved by MS2 spectra in Figure S21. These species can combine with amines or other nucleating particles in the atmosphere, leading to stabilized clusters and ultimately forming cloud condensation nuclei or smog. These findings confirm the environmental significance of organic sulfur oxidation in real environments and highlight the new pathway of atmospheric nucleating particles formation at the air–water interface.

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Mass spectra of DMS oxidation products and small clusters. (A) Full spectrum of DMS oxidation products. (B) Spectrum of [(CH3SO3H)2CH3SO3]− and [(H2SO4)2HSO4]− formation. (C) Spectrum of [(SO3)­CH3SO3]−, [(SO3)­HSO4]−, [(CH3SO3H)­HSO4]− and [(H2SO4)­CH3SO3]−, and [(H2SO4)­HSO4]− formation. (D) Spectrum of [(CH3SO3H)­CH3SO3]− formation.

4. Conclusion

This study shows a previously overlooked oxidation mechanism for volatile organic sulfides alongside novel critical atmospheric formation pathways for H2SO4, CH3SO3H, and the nucleating cluster. These findings provide direct evidence addressing the discrepancies between atmospheric models and observations in both anthropogenically influenced regions and remote oceanic areas. The proposed mechanism further elucidates the formation of SOA in coastal areas through acid catalysis and liquid-phase reaction principles. As SO2 emissions in China decline annually, the mechanism of SOA formation is shifting from being dominated by SO2 to one dominated by NO X . − This study shows that even with substantial reductions or complete elimination of anthropogenic SO2 emissions, natural sources like DMS can still generate H2SO4 and other acidic organosulfur compounds. Therefore, sulfate-dominated SOA formation may continue, with natural pathways likely playing a significant role. The identified pathways provide a molecular-scale explanation suitable for accelerated aerosol formation under diverse environmental conditions, from marine atmospheres to urban pollution hotspots, offering new insights for more accurate climate prediction and emission control strategies. text.

Supplementary Material

vg5c00168_si_001.pdf (2.3MB, pdf)

Acknowledgments

The authors are grateful for the financial support provided by the National Natural Science Foundation of China (22336004, 22476172, 42305093).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenvironau.5c00168.

  • Additional texts including details on the methodology of PAM-OFR reaction, DFT calculations, and QM/MM simulation; additional figures including photographs of experimental setups; MS and MS2 spectra of microdroplets reaction products; schematic diagram of PAM-OFR reaction process; gas-phase oxidation pathways of DMS; additional tables including species detected in ESI and at the air–water interface; mass error of all important species appeared in the spectrum (PDF)

§.

H.T. and X.Y. contributed equally to this work. This manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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