Complexation strength between organic carbon and transition metal ions dominates the photochemical conversion of SO₂ to sulfates

The photooxidation of SO₂ on organic carbon (OC) is a critical pathway for sulfate formation, yet the role of interactions between OC and transition metal ions (TMIs) in this process remains scarcely understood. We systematically investigated potential influences of TMIs (Fe³⁺, Cu²⁺ and Mn²⁺) on the conversion of SO₂ to sulfates on OC from coal combustion under irradiation. For OC and its coexistence with TMIs under experimental conditions adopted here, the steady-state uptake coefficients of SO₂ and sulfate masses were (0.39–22.24) × 10⁻⁶ and (2.36–26.05) × 10⁻³ mg, respectively. Fe³⁺ exhibited a significantly antagonistic effect, whereas Cu²⁺ and Mn²⁺ enhanced SO₂ uptake and sulfate generation on OC. Spectroscopic evidences, including absorbance decreasing and fluorescence quenching, confirmed the complexes formation of TMI with chromophores in OC. Fe³⁺ owned the strongest binding affinity with chromophores, followed by Cu²⁺ and Mn²⁺. This variation in the coordination strength dominated the generation of reactive species, such as free electrons (e⁻), superoxide radicals (•O${}_{\mathrm{2}}^{-}$), H₂O₂ and hydroxyl radicals (•OH), and •OH acted as a pivotal trigger to drive the sulfate production. Extended investigations confirmed a good linear relationship between •OH intensity (I) and sulfate mass (M_sulfates): M_sulfates = (1.68 ± 0.11) × 10⁻² × I − (0.43 ± 0.08) × 10⁻², demonstrating that the regulatory effects of metal ions on the sulfate production were generally governed through their ability to suppress or facilitate the •OH generation.

1 Introduction

Fine particulate matter (PM_2.5) is a complex mixture of inorganic and organic species, and plays a central role in the persistent haze events (Fan et al., 2016; Huang et al., 2014; Peng et al., 2021). The rapid accumulation of sulfates in PM_2.5 significantly influences atmospheric radiative forcing, air quality and human health (Zhang et al., 2020a; Salana et al., 2024). Sulfates primarily stem from SO₂ oxidation, including reactions with hydroxyl radicals (•OH) in the gas phase and aqueous oxidation in cloud and fog droplets involving various oxidizing species (Tsona and Du, 2019; Liu et al., 2020a; Jiang et al., 2009; Hoyle et al., 2016; Liu and Abbatt, 2021). However, only considering these pathways usually leads to a pronounced gap between the modeled and observed sulfate concentrations during the haze events (Li et al., 2017; Eckhardt et al., 2015; Zheng et al., 2015).

To bridge this gap, several heterogeneous pathways have been proposed, involving reactions of SO₂ on PM_2.5, mineral dust (e.g., Arizona test dust and metal oxides), inorganic salt (e.g., nitrates and NaCl), soot and organic aerosols (Zhang et al., 2020c, 2019, 2022; Dupart et al., 2012; Yang et al., 2024a; Cao et al., 2024b). Although these heterogeneous reactions have narrowed the discrepancies between model prediction and field observation concentrations of sulfates, substantial uncertainties and controversies still remain (Wang et al., 2014). This may be attributed to the fact that most studies mainly focus on simplified single-component systems and overlook the extreme complexity of multicomponent aerosols under realistic atmospheric conditions. Recent evidence highlighted that the potential interactions among distinct components in aerosols can significantly influence the transformation pathways of atmospheric pollutants (Zhang and Chan, 2023; Zhang et al., 2023). For instance, organic carbon (OC) in soot can donate electrons to elemental carbon (EC) and enhance the electron-hole separation under irradiation, thereby generating more •OH and promoting the conversion of SO₂ to sulfates (Zhu et al., 2022). OC-derived photosensitizers activated both O₂ and Cl⁻ in NaCl-OC under ultraviolet light, producing •OH and Cl• that synergistically oxidized SO₂ to sulfates (Tang et al., 2023).

OC from coal combustion can serve as a photosensitive electron donor under irradiation, initiating the formation of reactive oxygen species (ROS), which subsequently oxidized SO₂ to sulfates (Yang et al., 2025). In atmospheric aerosols, OC often coexisted with various TMIs, especially in some regions influenced by the combustion, vehicular emissions, and industrial processes (Yang et al., 2024b; Deng et al., 2022; Zhang et al., 2020b; Shiraiwa et al., 2017; Hua et al., 2025). This coexistence was not a simple superposition of their specific roles. OC usually contained diverse functional groups such as carboxyl, hydroxyl and carbonyl moieties, which can interact with TMIs via the complexation, metal-proton ion exchange and electrostatic adsorption, forming organometallic complexes in aerosols (Li et al., 2022; Wang et al., 2021c, b). Furthermore, the binding strength between OC and TMIs varied with the specific type of TMIs, which was related to the differences in ionic radius, oxidation state and electronic configuration that influenced coordination chemistry and redox reactivity (Wang et al., 2021a; Pan et al., 2020). Fe, Cu and Mn are the most abundant transition metals in atmospheric aerosols and cloud water (Singh and Gupta, 2017). Their soluble ionic forms, including Fe³⁺, Cu²⁺ and Mn²⁺, can catalyze or regulate SO₂ oxidation and sulfate formation (Liu et al., 2025). The potential roles of OC and these TMIs in the heterogeneous photochemical oxidation of SO₂ haven't been systematically investigated, which may restrict the exact incorporation of OC photochemistry into the sulfate formation prediction models.

Our previous work mainly focused on the intrinsic photo-reactivity of OC and its dependence on the physicochemical properties (Yang et al., 2025). In this study, laboratory experiments and theoretical calculations were conducted to elucidate the influences of TMIs (Fe³⁺, Mn²⁺ and Cu²⁺) on the photooxidation of SO₂ to sulfates on OC from coal combustion. Differences in the photochemical activity were quantified by measuring SO₂ uptake coefficients and sulfate yields. Changes in the optical properties were examined to evaluate the potential interactions between OC and TMIs. Mechanism insights into the reactive species generation, including e⁻, •O${}_{\mathrm{2}}^{-}$, H₂O₂ and •OH, clarified how these interactions regulated the SO₂ photooxidation. Finally, further investigation involving multiple metal ions commonly within atmospheric aerosols highlighted the broader significance of OC and TMIs interactions in the sulfate formation.

2 Materials and methods

2.1 Preparation of OC and OC@TMIs samples

As shown in Fig. S1 in the Supplement, OC was obtained with a custom combustion and sampling apparatus. The coal in the combustion experiment originated from Shanxi Province, China. The details of OC sampling and preparation are provided in Sect. S1.1 in the Supplement. The background concentrations of Fe, Mn and Cu in OC were measured through ICP-MS, and they varied in the range of 1.23–7.70 µg L⁻¹. The details of ICP-MS measurements are provided in Sect. S1.2. 1 mL of the extracted OC solution was dropped onto 6.0 × 1.5 cm prebaked quartz-fiber filters, and the solvents were dried under a N₂ stream (100 mL min⁻¹) in the dark at 298 K.

The sources and purities of salt chlorides used here are provided in Sect. S1.3. Stock solutions of metal salts (1.5 mM, in ultrapure water) were mixed with the OC solution at a 1:2 volume ratio, yielding mixtures containing 0.5 mM metal ions and 543.9 ± 35.4 µg C mL⁻¹ (Fig. S1). This metal-ion concentration was close to the levels reported in heavily polluted atmospheric conditions (Li et al., 2022). Each mixture was ultrasonicated for 30 min at 25 °C in the dark. The resulting products were designated as OC@TMIs, including OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺. Here, OC@TMIs referred to the actual sample prepared by introducing a specific TMI into OC. OC+TMIs was defined as a reference sample, where the interaction between OC and TMIs was assumed to be nonoccurrence. The property of OC+TMIs represented the arithmetic sum of that measured separately for OC and metal ion. Thus, OC+TMIs was utilized as a reference for evaluating synergistic or antagonistic effects of OC and TMIs. In the OC@NaCl control experiment, the NaCl concentration was 1.5 mM. 1.5 mL of the mixed solution was deposited on the inner surface of a quartz flow tube (20 cm length, 1.0 cm inner diameter). The flow tube was rotated slowly to ensure the uniform wetting of the surface. The coating was obtained by drying the samples under a N₂ stream (100 mL min⁻¹) in the dark environment at 298 K to prevent photochemical interference. Because a single sample could not be used sequentially for all experimental measurements, separate subsamples from the same initial batch of OC@TMIs were used to minimize potential variability.

2.2 Flow tube experiments and SO₂ uptake coefficients

The SO₂ uptake experiments were performed at ambient pressure in a horizontal cylindrical quartz flow tube reactor (34 cm length, 1.6 cm i.d.), as shown in Fig. S1. The details of the flow reactor and reaction procedure are given in Sect. S1.4. The changes in the SO₂ concentration were measured with a SO₂ analyzer (Thermo 43i). The calculation of the SO₂ uptake coefficient is described in Sect. S1.5.

2.3 Analysis methods

2.3.1 Measurements of sulfate products

The functional group changes during the photochemical reaction of SO₂ on the samples were characterized using in situ attenuated total reflection infrared (ATR-IR) spectroscopy (Nicolet iS50, Thermo Scientific), which was equipped with a mercury-cadmium-telluride (MCT) detector. Descriptions of experimental process are given in Sect. S1.6.

The sulfate ions were quantified using an ion chromatography (IC) system (CIC-D120+, Shenghan) with an analytical column (AS11-HC, Thermo) and a conductivity detector. The testing procedures are described in Sect. S1.7.

2.3.2 Optical property measurements

The optical properties of OC, TMIs, and OC@TMIs were analyzed using the UV-vis absorption spectroscopy and the excitation-emission matrix (EEM) fluorescence spectroscopy. The UV-vis spectra were recorded by the UV-vis spectrophotometer (UV-2550, Shimadzu) with a 1.0 cm pathlength cell in the wavelength range of 200–600 nm. A mixed solution of methanol and water (2:1, $v/v$) was used as the blank background. Samples were diluted to one-fifteenth of the original concentrations. UV-vis analysis was carefully performed under identical testing conditions to minimize handling-related variations.

The EEM fluorescence spectra were measured via a fluorescence spectrophotometer (FP8550, Jasco) at 700 V voltage and 3-D mode in the wavelength ranges of 200–400 nm for excitation (Ex.) and 300–520 nm for emission (Em.). The fluorescence spectra were recorded at 5 nm intervals and a scan speed of 2400 nm min⁻¹ in a 1 cm path-length quartz cuvette. Samples were diluted to one-fifteenth of the original concentrations to minimize the inner-filter effect (IFE) and reabsorption artifacts, which can distort the excitation–emission spectra at high sample concentrations. Instrument calibration, correction for inner-filter effects and normalization of fluorescence intensity to the Raman units (RU) of solvent were conducted before EEM spectral analysis. The EEM data were modeled with the parallel factor analysis (PARAFAC) through the DOMFluo toolbox (version 0.2.0) for MATLAB. Based on the evaluation of 2–4 components using EEM profiles and residual error patterns, a 3-component model was ultimately selected (Fig. S4).

2.3.3 Electrochemical measurements

The electron donating capacities (EDC) of the samples were determined using an electrochemical workstation (CHI760F, Shanghai Chenhua Co., Ltd.) equipped with a three-electrode system. The testing procedures are described in Sect. S1.8. The EDC was calculated as the following Eq. (1) (Ma et al., 2024),

$$\begin{array}{}\text{(1)}& \mathrm{EDC}={\displaystyle \frac{\int \frac{I_{\mathrm{ox}}}{\mathrm{F}}{d}_t}{m_{\text{organic carbon}}}}\end{array}$$

where I_ox means the OC oxidation curves; F is the constant of Faraday, which is equal to 96 485 s A mol⁻¹ e⁻; m_{organic carbon} represents the amount of organic carbon of samples.

2.3.4 Electron paramagnetic resonance spectrometer

ROS under irradiation were analyzed using an electron paramagnetic resonance spectrometer (EPR, A300, Bruker). For the radical detection, 20 µL of 0.05 M 5,5-dimethylpyrroline-Noxide (DMPO, in methanol) and 5-tert-butoxycarbonyl-5methyl-1-pyrroline-N-oxide (BMPO, in water) were used as spin traps for superoxide radicals (•O${}_{\mathrm{2}}^{-}$) and •OH, respectively. The parameters were set as follows: magnetic field range of 3450–3550 G, central field range of 3500 G, scan time of 30 s and microwave power of 20 mW.

2.3.5 H₂O₂ detection

The H₂O₂ generation was analyzed by the TiOSO₄ colorimetric method (Liu et al., 2023; Li et al., 2023). TiOSO₄ reacted with H₂O₂ to form a yellow titanium-peroxide complex (Ti(IV)O${}_{\mathrm{2}}^{\mathrm{2}+}$), which exhibited a characteristic absorbance peak at 405 nm. The H₂O₂ concentration was determined using the H₂O₂ standard curve (Fig. S5), and the testing procedure is given in Sect. S1.9.

2.3.6 X-ray photoelectron spectroscopy

The X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo) was employed to investigate the valence state variations of metal ions under irradiation. Fresh samples (3 mL, prepared by 2 mL OC extraction with 1 mL ionic solution) or the ones photoaged by SO₂ were combined with SiO₂ (100 mg) and dried by rotary evaporation. Before the XPS analysis, the samples were stored in a vacuum environment to minimize the changes in the valence state of TMIs. Aluminum Kα radiation (hv =  1486.68 eV, spot size 500 µm) was used as the excitation source for XPS analysis. The measurements were conducted under ultrahigh vacuum conditions of approximately 10⁻⁹ mbar. The instrument was operated at 15 kV and 10 mA. Signals were accumulated over 5–10 scans, and the spectra were recorded with a pass energy of 30 eV and a step size of 0.05 eV. Beam-induced effects were assessed by collecting consecutive C 1 s spectra at the same sample position. The peak positions and spectral shapes remained consistent, indicating that beam-induced effects were negligible under the measurement conditions. Binding energies were calibrated using the C 1 s peak at 284.8 eV. Peak deconvolution was conducted to resolve the contributions of different valence states.

2.3.7 Density functional theory calculations

Molecular structures were constructed in GaussView 6. Density functional theory (DFT) calculations were carried out through the quantum chemistry software Gaussian 16. Geometry optimizations and frequency calculations were performed at the B3LYP/LANL2DZ level. The SMD water solvation model was used to represent aqueous solvation effects. Grimme's D3 dispersion correction was employed to account for van der Waals interactions. All optimized structures were confirmed as local minima without imaginary frequencies. Single-point energy calculations were conducted at the B3LYP/def2-TZVP level using the optimized geometries. O₂ adsorption on OC and the corresponding IGMH diagrams were analyzed using Multiwfn (Lu and Chen, 2011; Lu, 2024). The calculations of complexation energy between OC and TMIs and O₂ adsorption energy are described in Sect. S1.10.

3 Results and discussion

3.1 Dependence of SO₂ uptake on metal ions

Figure 1a–c illustrates the temporal evolution of SO₂ concentration on OC, TMIs and OC@TMIs under irradiation. All errors denoted the standard deviations of three independent replicates. Because chloride salts were used here, OC@NaCl control experiments were conducted. The decreasing trend of SO₂ concentration on OC@NaCl was the same as that on OC, suggesting no measurable effect of Cl⁻ on SO₂ uptake (Fig. S6). The SO₂ loss on FeCl₃ or MnCl₂ was less than that on OC, while it was similar for CuCl₂ and OC. When SO₂ was exposed to OC@Fe³⁺, the SO₂ concentration decreased by 1–2 ppb, which was obviously lower than that for OC+Fe³⁺. This indicates that the coexistence of Fe³⁺ with OC markedly suppressed the photochemical uptake of SO₂. The SO₂ loss on OC@Cu²⁺ and OC@Mn²⁺ was markedly greater than that on OC+Cu²⁺ and OC+Mn²⁺, respectively. This suggests that both Cu²⁺ and Mn²⁺ enhanced the photochemical reaction of SO₂ with OC. Dark control experiments showed that SO₂ uptake on OC@TMIs was slightly different from that on OC in the absence of light, indicating that the non-photochemical effects of TMIs to SO₂ uptake was limited (Fig. S7).

Figure 1 (a–c) Temporal variations of the SO₂ concentration during the heterogeneous reaction of SO₂ with OC, TMIs, OC+TMIs and OC@TMIs under irradiation (For OC+TMIs, SO₂ uptake was the arithmetic sum of that measured for OC and TMIs in the separate experiments). (d) Initial uptake coefficients (γ_i) and steady-state uptake coefficients (γ_ss) of SO₂ under irradiation. Reaction conditions: irradiance of 1.05 × 10¹⁶ photons cm⁻² s⁻¹, 40 ppb SO₂, 298 K and 60 % RH.

The initial (γ_i) and steady-state (γ_ss) uptake coefficients of SO₂ are summarized in Fig. 1d. The γ_ss was consistently lower than the γ_i due to the gradual aging and the depletion of photoactive components during the reaction. The γ_ss on OC@Fe³⁺ only accounted for 5 % of that on OC+Fe³⁺, confirming that Fe³⁺ significantly restrained the photochemical SO₂ uptake on OC. In contrast, γ_ss for OC@Cu²⁺ and OC@Mn²⁺ was 1.83 and 2.93 times of the corresponding one for OC+Cu²⁺ and OC+Mn²⁺, respectively. This definitely suggests that the coexistence of Cu²⁺ and Mn²⁺ with OC has synergistic effects on the photochemical uptake of SO₂, with Mn²⁺ exhibiting a stronger enhancement than Cu²⁺. Overall, these contrasting behaviors reveal that the addition of TMIs to OC can either restrict (Fe³⁺) or facilitate (Cu²⁺ and Mn²⁺) the photochemical uptake of SO₂, highlighting the distinct roles of TMIs in modulating the OC reactivity.

3.2 Dependence of sulfate formation on metal ions

The contour maps of in-situ ATR-IR spectra of OC, TMIs and OC@TMIs exposed to SO₂ under irradiation are displayed in Fig. 2a. Several characteristic sulfate (SO${}_{\mathrm{4}}^{\mathrm{2}-}$ or HSO${}_{\mathrm{4}}^{-}$) bands were identified for all samples in the 1300–1000 cm⁻¹ region (Zhang et al., 2023, 2019). It was noted that these bands intensified progressively with the time, reflecting the continuous accumulation of sulfates.

Figure 2 (a) Contour map of in-situ ATR-IR spectra of OC, FeCl₃, CuCl₂, MnCl₂, OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺ exposed to SO₂ under irradiation. Reaction conditions: irradiance of 7.40 × 10¹⁵ photons cm⁻² s⁻¹, 2 ppm SO₂, 298 K and 60 % RH. (b) Mass of sulfates produced on OC, TMIs, OC+TMIs and OC@TMIs after the 10 h reaction with SO₂ under irradiation (For OC+TMIs, its sulfate mass was the arithmetic sum of that measured for OC and TMIs in the separate experiments). Reaction conditions: irradiance of 1.05 × 10¹⁶ photons cm⁻² s⁻¹, 200 ppb SO₂, 298 K and 60 % RH.

The IC results provided quantitative evidence of sulfate formation under irradiation (Fig. 2b). For OC@Fe³⁺, sulfate production was much lower than that on OC or FeCl₃. Specifically, the sulfate mass on OC@Fe³⁺ was merely 12 % of that for OC+Fe³⁺, confirming that Fe³⁺ restrained the formation of sulfates on OC. In contrast, the sulfate mass on OC@Cu²⁺ and OC@Mn²⁺ was 1.13 and 1.52 times larger than that for OC+Cu²⁺ and OC+Mn²⁺, respectively. These confirm that both Cu²⁺ and Mn²⁺ promote the photochemical oxidation of SO₂ to sulfates on OC. Only limited sulfate formation was observed on OC@TMIs under dark conditions, indicating a minor contribution from the non-photochemical pathways (Fig. S8). Taken together with the SO₂ uptake data in Fig. 1, TMIs exerted markedly different influences on SO₂ conversion to sulfates on OC.

3.3 Spectroscopic evidence for complexation between OC and TMIs

The complexation between OC and TMIs may occur and alter the optical properties of OC (Wang et al., 2024). Changes in optical absorption and steady-state fluorescence signals were used to probe these interactions. Figure S9a–c displays the UV-vis spectra of OC, TMIs, OC+TMIs and OC@TMIs. OC and OC@TMIs exhibited a sharp increase in the absorbance at the shorter wavelengths. FeCl₃, CuCl₂ and MnCl₂ owned very weak absorbance. OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺ presented lower absorbance than OC+Fe³⁺, OC+Cu²⁺ and OC+Mn²⁺, respectively. This decrease in the absorbance suggests the occurrence of interactions between OC and TMIs. The background concentrations of Fe, Mn and Cu in OC were measured by ICP-MS, and they varied in the range of 1.23–7.70 µg L⁻¹. This accounted for only 0.0045 %–0.0276 % of TMI concentrations used for OC@TMIs. Therefore, the contribution of intrinsic Fe, Mn and Cu to the TMI dependent spectral changes was considered to be very limited. The FT-IR spectra of OC are displayed in Fig. S10, and the peak assignments are summarized in Table S1 in the Supplement. OC contained carboxyl, hydroxyl and carbonyl groups. These oxygen-containing groups with π bond electrons and lone pairs can serve as the complexation sites by donating electrons to the vacant orbitals of TMIs (Wang et al., 2021c). Coordination with TMIs can perturb the electronic structure of OC chromophores and affect $\mathit{\pi }-{\mathit{\pi }}^{\ast }$ or $n-{\mathit{\pi }}^{\ast }$ electronic transitions, which may induce a hypochromic effect in the UV-vis spectra (Qin et al., 2024). To further compare the spectral changes induced by TMIs, differential absorbance was calculated as ΔAbs = Abs(OC+TMIs) − Abs(OC@TMIs). Notably, as shown in Fig. S9d, there was the largest ΔAbs between OC@Fe³⁺ and OC+Fe³⁺ in the entire spectral range, indicating the strongest complexation of OC with Fe³⁺ among three TMIs.

EEM fluorescence spectroscopy combining with the PARAFAC analysis were employed to further investigate the complexation interactions between TMIs on OC. Three chromophores are identified in Figs. 3a–c and S11, and their peak positions and assignments are given in Table S2. C1 (Ex $/$ Em = 265(320) $/$ 380 nm) and C2 (Ex $/$ Em = 285(370) $/$ 420 nm) were assigned to chromophores with low and high oxidation states, respectively. C3 (Ex $/$ Em = 200(260) $/$ 340 nm) belonged to protein-like chromophores (Li et al., 2022; Wang et al., 2021b). Fluorophores at longer emission wavelengths, such as C2, typically owned abundant polar groups, including hydroxyl and carboxyl, which acted as primary binding sites for TMIs (Pan et al., 2020; Lu et al., 2019). The relative contributions of different components to OC shifted upon the addition of TMIs (Fig. 3d), reflecting the varied complexation strengths of TMIs with OC (Ma et al., 2022; Yan and Korshin, 2014). For OC@Fe³⁺ in comparison with OC, the ratio of C1 obviously increased by 14 %, while it decreased by 9 % and 5 % for C2 and C3, respectively. By contrast, OC@Cu²⁺ and OC@Mn²⁺ showed smaller changes in the C1–C3 proportions. These mean that Fe³⁺ forms the most stable complexes with OC due to its high charge density and trivalent oxidation state, which would enhance the ligand attraction and enable higher coordination numbers (Liu et al., 2022). Cu²⁺ and Mn²⁺, with lower charge densities and less favorable coordination geometries, exerted weaker complexation effects with OC.

Figure 3 (a–c) Three components extracted by the PARAFAC analysis. (d) Relative fluorescence contributions of C1–C3 in OC, OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺. (e) Total fluorescence volume of OC, OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺.

As a comprehensive measure of fluorescence intensity, the total fluorescence volume (TFV) was used to evaluate the overall impact of metal complexations (Fig. 3e). The addition of TMIs led to a reduction in TFV, confirming the fluorescence quenching occurrence. Fe³⁺ caused the most pronounced reduction of TFV (85.2 %), followed by Cu²⁺ (13.4 %) and Mn²⁺ (1.4 %). This strong quenching effect was attributed to the high binding affinity of Fe³⁺ with highly oxidative and protein-like fluorophores. The complexation between OC and TMIs can disrupt the $\mathit{\pi }-{\mathit{\pi }}^{\ast }$ conjugation and deactivate fluorescence centers, which contributed to a significant reduction in TFV (Kuramochi et al., 2018). In addition, the complexation of Fe³⁺ with OC could promote the intersystem crossing (ISC) from the singlet state (¹OC^∗) to the triplet state (³OC^∗) (Ruzi et al., 2017; Treacy and Rovis, 2024), enhancing nonradiative decay and further suppressing the fluorescence emission.

To directly demonstrate the complexation strength between TMIs with species in OC, DFT calculations were conducted using Phenol (PH), benzoic acid (BA) and a model substance (MS) (Fig. S12). PH and BA were chosen according to FT-IR analysis (Fig. S10), as they represented species with hydroxyl and carboxyl groups commonly present in OC. MS simultaneously contained these functional groups with a π-conjugated aromatic backbone to better reflect the structural features of ambient OC. Section S3.1 and Fig. S13 illustrate the complexation structure of three molecular models with Fe³⁺, Cu²⁺ and Mn²⁺. As summarized in Table S3, all molecular models exhibited the strongest complexation energy with Fe³⁺, followed by Cu²⁺ and Mn²⁺. This was well consistent with the results observed in the UV-vis and fluorescence spectra. Although these models cannot capture the full molecular heterogeneity of OC, the consistency between the calculated results and experimental trends helped to understand the complexation effects of OC with TMIs. Complexation strength is commonly described using stability constants (log K). Because OC was a heterogeneous mixture with multiple potential binding sites, log K may not be exactly determined. Weak and strong complexation described here was relative sense, according to UV-vis spectral change, fluorescence quenching and complexation energy.

3.4 Mechanistic insights into TMIs-regulated oxidation of SO₂ to sulfates

As described through Reactions (R1)–(R6), previous studies have proposed the photochemical conversion pathways of SO₂ to sulfates on OC (Yang et al., 2025, 2024a). Upon accepting photons, OC is excited to ³OC^∗, which subsequently generates free electrons (Reactions R1–R2) (Yang et al., 2024a; Wang et al., 2020). These electrons would reduce adsorbed O₂ to •O${}_{\mathrm{2}}^{-}$ (Reaction R3) (Zhang et al., 2022). •O${}_{\mathrm{2}}^{-}$ can combine with protons to form hydroperoxyl radicals (HO₂•), which are converted into O₂ and hydrogen peroxide (H₂O₂) (Reaction R4) (Zhang et al., 2020c). H₂O₂ undergoes the photolysis to produce •OH (Reaction R5), and •OH drives the oxidation of adsorbed SO₂ or H₂SO₃ or HSO${}_{\mathrm{3}}^{-}$ to sulfates (Reaction R6) (Liu et al., 2020b).

$$\begin{array}{}\text{(R1)}& {\displaystyle }\mathrm{OC}\stackrel{hv}{\to }{}^{\mathrm{3}}{\mathrm{OC}}^{\ast }\text{(R2)}& {}^{\mathrm{3}}{\mathrm{OC}}^{\ast }\to {\mathrm{OC}}^{+}+{\mathrm{e}}^{-}\text{(R3)}& {\displaystyle }{\mathrm{O}}_{\mathrm{2}}+{\mathrm{e}}^{-}\to \mathrm{•}{\mathrm{O}}_{\mathrm{2}}^{-}\text{(R4)}& {\displaystyle }\mathrm{•}{\mathrm{O}}_{\mathrm{2}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{HO}}_{\mathrm{2}}\mathrm{•}\to {\mathrm{O}}_{\mathrm{2}}+{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}\text{(R5)}& {\displaystyle }{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}\stackrel{hv}{\to }\mathrm{•}\mathrm{OH}\text{(R6)}& {\displaystyle }\mathrm{•}\mathrm{OH}+{\mathrm{SO}}_{\mathrm{2}}\mathrm{or}{\mathrm{H}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{3}}\left({\mathrm{HSO}}_{\mathrm{3}}^{-}\right)\to {\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\end{array}$$

To verify whether the above mechanism remained operative after the complexation between TMIs and OC, electrochemical and EPR analyses were conducted. Mediated electrochemical oxidation (MEO) data of OC without or with TMIs showed persistent occurrence of the photo-generated electron transfer (Figs. S14 and S15). BMPO was used as the spin-trapping agent. As displayed in Fig. 4a, the characteristic EPR signal of BMPO-•OH adducts was detected under irradiation, confirming the generation of •OH. Moreover, EPR signals of BMPO-•OH adducts followed the order: OC@Mn²⁺ > OC@Cu²⁺ > OC > OC@Fe³⁺, which well aligned with γ_ss and sulfate production. The critical role of •OH in the conversion of SO₂ to sulfates was further determined using NaHCO₃ to scavenge •OH (Yang et al., 2024a), where the sulfate formation was markedly restricted (Fig. S17). These demonstrate that •OH generation capacity is the primary determinant of sulfate formation on OC@TMIs.

Figure 4 (a) BMPO-•OH adduct EPR spectra of OC, OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺ under irradiation. (b) Electron donating capacity (EDC) of OC, TMIs, OC+TMIs and OC@TMIs (for OC+TMIs, EDC was the arithmetic sum of that measured for OC and TMIs in separate experiments). High-resolution XPS spectra of Cu 2p for OC@Cu²⁺ (c) before and (d) after the reaction, and Mn 2p for OC@Mn²⁺ (e) before and (f) after the reaction (R(Cu²⁺) and R(Mn²⁺) denoted the proportion of Cu²⁺ and Mn²⁺ on the surface, respectively).

According to Reactions (R1)–(R2), the photogeneration of electrons was an initial step that can affect the reaction activity of OC. Figure 4b summarizes the distinct regulatory roles of Fe³⁺, Cu²⁺ and Mn²⁺ in the EDC of OC under irradiation. The EDC of OC@Fe³⁺ (0.00496 µmol e⁻ (mg C)⁻¹) was substantially less than that of OC+Fe³⁺ (0.01904 µmol e⁻ (mg C)⁻¹), suggesting that Fe³⁺ significantly suppressed the photogeneration of electrons. This mainly stemmed from the strong complexation of Fe³⁺ and OC, which masked the redox-active moieties and dynamically quenched ³OC^∗, restraining free electron formation (Li et al., 2022; Liu et al., 2024). Figure 4b shows that EDC of OC@Cu²⁺ (0.02074 µmol e⁻ (mg C)⁻¹) and OC@Mn²⁺ (0.02147 µmol e⁻ (mg C)⁻¹) was higher than that of OC+Cu²⁺ (0.01959 µmol e⁻ (mg C)⁻¹) and OC+Mn²⁺ (0.01812 µmol e⁻ (mg C)⁻¹), respectively. This means that Cu²⁺ and Mn²⁺ enhance the electron-donating ability of OC. These larger EDC may be related to reversible redox cycling of Cu²⁺ $/$ Cu⁺ and Mn²⁺ $/$ Mn³⁺ in the photochemical process. In contrast, Fe³⁺ formed more stable complexes with OC that trapped the photogenerated electrons, thereby suppressing efficient redox cycling of Fe³⁺ $/$ Fe²⁺. Under irradiation, Cu²⁺ was reduced to Cu⁺ by accepting photogenerated electrons, which was subsequently re-oxidized to Cu²⁺ by O₂ or electron-deficient functional groups (e.g., quinones) (Pan et al., 2020). As displayed in XPS spectra of Cu 2p for OC@Cu²⁺ in Fig. 4c and d, the peaks at 932.5 and 934.3 eV corresponded to Cu⁺ and Cu²⁺, respectively (Biesinger et al., 2011). The proportion of Cu²⁺, denoted as R(Cu²⁺), decreased from 0.67 to 0.59 after photochemical SO₂ aging. This confirms the existence of reversible redox cycling of Cu²⁺ $/$ Cu⁺. Similarly, Mn²⁺ can be oxidized to Mn³⁺ by photogenerated holes (OC⁺) or reactive oxygen-containing intermediates on OC, and then Mn³⁺ was reduced by phenolic or aromatic groups on OC (Hansard et al., 2011). The Mn 2p${}_{\mathrm{3}/\mathrm{2}}$ spectra of OC@Mn²⁺ exhibited two peaks at 640.4 and 641.9 eV (Fig. 4e and f), which were ascribed to Mn²⁺ and Mn³⁺, respectively (Cerrato et al., 2010; Xing et al., 2025). The proportion of Mn²⁺ (R(Mn²⁺)) decreased from 0.41 to 0.16 after the reaction, indicating Mn²⁺ $/$ Mn³⁺ redox interconversion. These reversible redox cycles facilitated dynamic interfacial electron transfer by accepting and releasing photogenerated electrons. This bidirectional electron flow can amplify the outward electron flux from OC, thereby increasing the electron-donating capacity (Fulda et al., 2013; Li et al., 2021).

As shown in Reaction (R3), •O${}_{\mathrm{2}}^{-}$ formation relied not only on the photogenerated electrons but also on the O₂ activation. To elucidate the effect of TMIs complexation on the O₂ activation, DFT calculations were carried out in the interaction of O₂ with MS (Fig. S18). Figure 5a displays that O₂ adsorption energy on MS was −82.03 kJ mol⁻¹, with O–O bond length of 1.20 Å. For MS@Fe³⁺, O₂ adsorption energy became more negative (−205.59 kJ mol⁻¹), reflecting stronger electrostatic binding due to high positive charge of metal center (Zhang et al., 2024). However, O–O bond length remained unchanged, suggesting negligible electron transfer into the π^∗ antibonding orbital of O₂. Thus, O₂ is strongly bound yet not appreciably activated. Complexation between OC with Cu²⁺ or Mn²⁺ obviously enhanced the adsorption of O₂ (−212.93 kJ mol⁻¹ for Cu²⁺ and −366.32 kJ mol⁻¹ for Mn²⁺). The O–O bond length slightly increased to 1.21 Å for MS@Cu²⁺, while it was markedly elongated to 1.32 Å for MS@Mn²⁺. These changes suggest electron transfer from the metal centers to the π^∗ antibonding orbital of O₂, boosting effective activation of O₂ and facilitating the •O${}_{\mathrm{2}}^{-}$ generation. Compared to Cu²⁺, Mn²⁺ exhibited stronger O₂ activation, which was attributed to its half-filled 3d⁵ configuration and moderate redox potential, both of which favored efficient π^∗ electron donation.

Figure 5 (a) O₂ adsorption energy and the bond length of O₂ for MS, MS@Fe³⁺, MS@Cu²⁺ and MS@Mn²⁺. (b) EPR spectra of DMPO-HO₂• $/$ •O${}_{\mathrm{2}}^{-}$ adduct for OC, OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺ under irradiation. (c) H₂O₂ formation from the photochemical reactions on OC, OC@Fe³⁺, OC@Cu²⁺ and OC@Mn²⁺.

Figure 6 (a) BMPO-•OH adduct EPR spectra under irradiation. (b) The correlation of M_sulfates with the generation of •OH.

Accordingly, Fe³⁺ inhibited electron generation and failed to activate O₂, whereas Cu²⁺ and Mn²⁺ promoted these two processes, with Mn²⁺ showing a stronger enhancement effect. This led to the •O${}_{\mathrm{2}}^{-}$ signal intensity order of OC@Mn²⁺ > OC@Cu²⁺ > OC > OC@Fe³⁺ (Fig. 5b). •O${}_{\mathrm{2}}^{-}$ directly contributed to H₂O₂ formation (Reaction R4), which was quantified with a TiOSO₄ chromogenic method (Fig. S20). Figure 5c shows that H₂O₂ production trend had the consistency with •OH generation ability among these samples (Fig. 4a), which ultimately governed the photoconversion of SO₂ to sulfates.

3.5 Discussion

The OC samples were collected from smoke aerosols generated during coal combustion. The TMIs concentrations selected here were atmospherically relevant soluble metal ion levels. These systems may roughly represent the coexistence of OC and TMIs for atmospheric aerosols in polluted regions affected by coal combustion emissions. High humidity and acidic conditions may promote metal dissolution and facilitate the complexation between soluble metal ions and OC. The photochemical lifetime of SO₂ (${\mathit{\tau }}_{{\mathrm{SO}}_{\mathrm{2}}}$) on OC@TMIs was calculated with Eq. (2) (Yang et al., 2021),

$$\begin{array}{}\text{(2)}& {\mathit{\tau }}_{{\mathrm{SO}}_{\mathrm{2}}}={\displaystyle \frac{\mathrm{4}}{\mathit{\gamma }\mathit{\nu }A}}\end{array}$$

where γ represents the uptake coefficient of SO₂; ν denotes the mean SO₂ molecular velocity; A corresponds to the aerosol surface area concentration. Based on field measurements during heavily polluted episodes, A ranged from 5.44 × 10⁻⁵ to 8.16 × 10⁻⁵ cm² cm⁻³ (Zhang et al., 2018; Yang et al., 2021). Under typical atmospheric conditions (40 ppb SO₂, 60 % RH), γ_ss for SO₂ on OC@TMIs were determined to be (0.39–22.24) × 10⁻⁶. Organic matters typically account for approximately 20 %–45 % of PM_2.5 mass (Wu et al., 2018). The ${\mathit{\tau }}_{{\mathrm{SO}}_{\mathrm{2}}}$ was estimated to be 1.8–379 d in the coexistence of OC and TMIs. By contrast, ${\mathit{\tau }}_{{\mathrm{SO}}_{\mathrm{2}}}$ on OC from coal combustion was much narrower, which ranged from 17 to 53 d under the same conditions (Yang et al., 2025).

To assess the atmospheric production of sulfates during the photooxidation of SO₂ on OC@TMIs, the sulfate formation rate (R) was estimated using Eq. (3),

$$\begin{array}{}\text{(3)}& R={\displaystyle \frac{d\left[{\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}\right]}{\mathrm{d}t}}={\left[{\displaystyle \frac{R_{\mathrm{P}}}{D_g}}+{\displaystyle \frac{\mathrm{4}}{\mathit{\gamma }\mathit{\nu }}}\right]}^{-\mathrm{1}}A\left[{\mathrm{SO}}_{\mathrm{2}}\right]\end{array}$$

where D_g refers to the diffusion coefficient of SO₂ (1.12 × 10⁻⁵ m² s⁻¹); R_P denotes the aerosol particle radius and is estimated via Eq. (4) (Li et al., 2020),

$$\begin{array}{}\text{(4)}& R_{\mathrm{P}}=\left(\mathrm{0.254}\times {\displaystyle \frac{\left[{\mathrm{PM}}_{\mathrm{2.5}}\right]}{\left({\mathrm{\mu }\mathrm{g}\mathrm{m}}^{-\mathrm{3}}\right)}}+\mathrm{10.259}\right)\times {\mathrm{10}}^{-\mathrm{9}}\end{array}$$

where [PM_2.5] denotes the mean mass concentration of PM_2.5, and a concentration range of 100–500 µg m⁻³ for the polluted episodes in typical Chinese cities was adopted here (Chu et al., 2020). Under these conditions, R was determined to be 0.07–5.95 µg m⁻³ h⁻¹, which was a substantially broader range than 0.43–1.33 µg m⁻³ h⁻¹ for OC (Yang et al., 2025). Broader ranges of ${\mathit{\tau }}_{{\mathrm{SO}}_{\mathrm{2}}}$ and R on OC@TMIs indicate that the coexistence of OC and TMIs significantly modulates the photochemical conversion of SO₂ to sulfates. These findings suggest that current model simulations may underestimate or overestimate sulfate formation on OC by overlooking such antagonistic or synergistic processes.

It has been well demonstrated that •OH was the dominant active species responsible for the oxidation of SO₂ to sulfates on OC@TMIs (Figs. 4a and S17). Other metal ions (Ca²⁺, Co²⁺, Zn²⁺, Mg²⁺, Ni²⁺, Cr³⁺ and Al³⁺) commonly presenting in atmospheric aerosols were expanded to examine whether this mechanism exhibited broader generality (Hua et al., 2024). As shown in Fig. 6a, the peaks of BMPO-•OH adducts were observed under irradiation, confirming the •OH generation on all OC@TMIs. Relative to OC, the coexistence of Ca²⁺, Co²⁺, Zn²⁺ and Mg²⁺ obviously promoted the •OH generation, while Ni²⁺, Cr³⁺ and Al³⁺ exerted an inhibitory effect. Figure S21 summarizes the mass of sulfates formed on OC@TMIs, which was consistent with the •OH generation trend. Importantly, a linearly positive relationship (R² = 0.97) between •OH intensity and sulfate mass (M_sulfates) was determined in Fig. 6b. M_sulfates can be parametrized as a function of •OH signal intensity (I) with an equation: M_sulfates = (1.68 ± 0.11) × 10⁻² × I − (0.43 ± 0.08) × 10⁻². The linear relationship between •OH intensity and sulfate formation was established using fresh OC from coal combustion and different TMIs at the same concentration. OC in aerosols generally stems from diverse sources and undergoes the atmospheric aging processes, which would significantly modify the compositions and structures of OC (Cao et al., 2021; Deng et al., 2022). TMIs types and concentrations also display temporal and spatial variations (Ito et al., 2016; Moreno et al., 2011). These may alter the complexation between OC and TMIs and consequently affect •OH generation and sulfate formation. Thus, there may be some uncertainties for the applicability of this linear relationship under ambient conditions. Nevertheless, this quantitative relationship highlighted the potential dual roles of metal ion types in the sulfate formation, challenging the traditional view that metal ions often promote the SO₂ oxidation (Cao et al., 2024a; Wang et al., 2022, 2021a). It also emphasized the critical role of •OH in the sulfate formation on OC@TMIs. Accordingly, incorporating the •OH generation ability of diverse OC@TMIs complexes into the atmospheric models may help to constrain sulfate source strength, predict haze evolution and assess associated health impacts in polluted environments.

4 Conclusions

It was well confirmed that the heterogeneous photooxidation of SO₂ to sulfates was strongly modulated by the interactions of OC with TMIs. As evidenced by SO₂ uptake coefficient and sulfate formation mass, Fe³⁺ exhibited a significantly inhibitory effect, whereas Cu²⁺ and Mn²⁺ promoted the conversion of SO₂ to sulfates, with Mn²⁺ playing a more obvious enhancement role. Spectroscopic analyses and DFT calculations consistently demonstrated that Fe³⁺ had the strongest binding affinity to OC chromophores, followed by Cu²⁺ and Mn²⁺. This difference in the complexation strength controlled the electron generation, O₂ activation, and the sequential formation of •O${}_{\mathrm{2}}^{-}$, H₂O₂, and •OH. In particular, a quantitative linear correlation was established between •OH intensity and sulfate mass across a wide range of metal ions. This clearly suggests that the regulation of sulfate formation on OC by metal ions is dominated by their capacity to suppress or enhance •OH generation.