The following reagents were purchased from the Sigma-Aldrich (dithiothreitol, > 99 %; 5,5'-dithiobis (2-nitrobenzoic acid), > 99 %; benzoic acid, > 99 %; syringol, > 99 %; furfuryl alcohol, > 99 %). Methanol (≥ 99 %), acetonitrile (≥ 99 %), Na₂CO₃ (≥ 99 %) and NaHCO₃ (≥ 99 %) were purchased from Acros Chemicals. Sulfuric acid (≥ 99 %) was obtained from Sinopharm. (NH4)2SO₄ (GR), H₂O₂ (29 %–32 %) and KOH (≥ 99 %) were supplied by Aladdin and Alfa Aesar, respectively. 2,2,6,6-Tetramethylpiperidine (≥ 98 %) and 5,5-dimethyl-1-pyrroline N-oxide (≥ 97 %) were purchased from Anpel Laboratory Technologies (Shanghai) Co., Ltd.

All chemicals were used as received without further purification. All solutions were prepared with ultrapure water (resistivity ≥ 18.2 MΩ cm) produced by a Milli-Q purification system.

Coal and maize straw were collected from Lingwu (Ningxia) and Shangqiu (Henan Province), respectively, and combusted in a self-built stove designed to simulate domestic fuel burning conditions. Smoke particle collection followed the procedures described in our previous study (Ye et al., 2025). Briefly, the stove was connected to a stainless steel dilution tunnel and residence chamber. Smoke particles emitted from maize straw and coal combustion were collected on pre-baked quartz fiber filters (20.3 × 25.4 cm, Whatman) using two samplers equipped with cyclone with a 2.5 µm aerodynamic cutoff.

One quarter of each filter was cut into strips and placed in extraction bottles. The samples were ultrasonically extracted three times with 30 mL Milli-Q water. The combined extracts were filtered through a 0.45 µm PTFE membrane and subsequently diluted to approximately 15 mg C L⁻¹ for photoaging experiment based on suggested TOC level (0.5–1.4 mmol C L⁻¹) by Cook et al. (2017) for cloud water. Photochemical reactions were performed in a Rayonet RPR-200 photoreactor equipped with 14 lamps, following the procedure described in our previous study (Ye et al., 2025). The irradiance intensity on the solution surface was 2.4 mW cm⁻² in the wavelength region of 290–400 nm (centered at 313 nm), as measured by a radiometer (Photoelectric Instrument Factory of Everfine Corporation, Hangzhou, China). The intensity is slight lower than natural sunlight levels (6.16 mW cm⁻²) measured at noon during winter at Jiangsu University of Technology (Wang et al., 2025). Before photooxidation, 10 mM H₂O₂ was added to the reaction solution to generate ⚫OH with certain concentration, consistent with previous study (Arciva et al., 2022; Cao et al., 2025).

Water-soluble organic carbon (WSOC) concentrations were determined using a total organic carbon (TOC) analyzer (TOC-L CPH, Shimadzu, Japan). Metal element concentrations (Fe and Cu) were quantified by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7800). Eight water soluble inorganic ions (Na⁺, Cl⁻, SO42-, NO3-, K⁺, NH4+, Ca²⁺ and Mg²⁺) were also detected for both smoke extracts. Details can be found in Sect. S1 in the Supplement. These detection limits were determined based on three times the standard deviation of blank sample. The method detection limits ranged from 5–20 µg L⁻¹ for anions and 0.5–2 µg L⁻¹ for cations. The detection limits of Fe and Cu are 2.0 and 0.8 µg L⁻¹, respectively.

The UV-vis absorption spectra were monitored using UV-vis spectrophotometer (Shimadzu, Japan) over wavelength range of 200–700 nm. The mass absorption coefficients (MACs, m² gC⁻¹), defined as absorbance normalized by WSOC concentration, were calculated as follows: 1MACλ=AλC×L×ln⁡10 Where Aλ represent the absorbance at wavelength λ. C refers to the WSOC concentration of reaction solution. L is the optical path length (1 cm in this study).

The EEM spectra were recorded using a three-dimensional fluorescence spectrophotometer (FluoroMax Plus, HORIBA Scientific). Parallel factor analysis (PARAFAC) model was applied to EEM spectra to resolve the fluorescent compounds using the DOMFluor toolbox in MATLAB 2021b. Details of the determination and modeling procedures are provided in our previous study (Ye et al., 2025). Three fluorescence components (C1, C2 and C3) were identified from PARAFAC model. The fluorescence index (FI), humification index (HIX), and biological index (BIX) were further calculated to characterize the fluorescent properties of the samples. The calculation methods for these indicators were shown in the supplement and in our previous study (Ye et al., 2025).

The molecular compositions, degree of unsaturation, and aromaticity of WSOM were characterized using FT-ICR MS coupled with negative electrospray ionization (ESI-). Solid-phase extraction (SPE) was employed for sample pretreatment prior to FT-ICR MS determination, following procedures similar to those described in previous studies (Yang et al., 2025). Briefly, the reaction solution was adjusted to pH 2 and pass through SPE cartridges (Oasis HLB, Waters, USA) preconditioned with 15 mL methanol and 10 mL Milli-Q water. The retained organic matter was subsequently eluted with 10 mL methanol. The eluate was then concentrated to approximately 0.5 mL using a rotary evaporator and stored at -20 °C until analysis. Prior to analysis, the sample was re-dissolved in 4 mL of methanol and filtered through a 0.22 µm PTFE membrane. FR-ICR MS analysis was performed with a capillary voltage of 4.0 kV, and samples were introduced into the ESI source at a flow rate of 120 µL h⁻¹. Mass spectra were acquired over the m/z range of 150–800 Da. To improve the signal-to-noise ratio and dynamic range, each spectrum was averaged from 200 scans. Blank samples were analyzed under the same procedure. Notably, no water-insoluble precipitates larger than 0.22 µm were observed during photooxidation. However, filtration of the reaction solution through a 0.22 µm membrane prior to analysis may resulted in the loss of some newly formed oligomers.

The Composer software (Sierra Analytics, USA) was utilized to process the FT-ICR MS spectra and assign elemental compositions to recalibrated peaks, with a mass tolerance of ± 1.0 ppm and a signal-to-noise ratio (S/N) threshold of ≥ 4. Based on the assigned molecular formulas, WSOM compounds were categorized into four groups: CHO, CHON, CHOS, and CHONS. To evaluate the degree of unsaturation and aromaticity, double bond equivalent (DBE) and aromaticity index (AI) were calculated as follows:

2DBE=12×(2c+2-h+n)3AI=1+c-0.5o-s-0.5hc-0.5o-s-n The intensity-weighted averaged characteristic parameters can be expressed as: 4Pw=∑Pi×Ii/∑Ii where P represents DBE, AI, molecular weight (MW), oxygen-to-carbon (O / C) or hydrogen-to-carbon (H / C) ratio. Pi represents the corresponding parameter value for each individual compound I, and Ii represents the relative abundance of its molecular formula.

Molecular formulas were further classified into seven compound classes based on their H / C and O / C ratios (Ning et al., 2025): lipid-like (1.5 < H / C ≤ 2.0, 0 ≤ O / C≤ 0.3); aliphatic -like (1.5 < H / C ≤ 2.2, 0.3 < O / C ≤ 0.67); lignin-like (0.67 < H / C ≤ 1.5, 0.1 ≤ O / C < 0.67); carbohydrate-like (1.5 < H / C ≤ 2.5, 0.67 < O / C < 1.2); unsaturated hydrocarbon-like (0.67 < H / C ≤ 1.5, O / C < 0.1); unsaturated aromatic-like (0.2 ≤ H / C ≤ 0.67, O / C < 0.67), and tannin-(0.6 < H / C ≤ 1.5, 0.67 ≤ O / C ≤ 1.2). The saturated compounds were defined as the sum of lipid-like and aliphatic components.

High-resolution aerosol mass spectrometer (HR-AMS, Aerodyne Res. Inc.) was used to characterize the bulk chemical composition of aqSOA, including average elemental ratios (i.e., O / C and H / C) and some specific fragment ions. The average oxidation state of carbon (OSc = 2 × O / C - H / C) was used to index the oxidation degree of aqSOA. 10 mg L⁻¹ ammonium sulfate was added into the solution as an internal standard for quantifying SOA mass concentration. The aqSOA mass yield was calculated as follows: 5aqSOAyield=[Org]t-[Org]0[WSOC]0-[WSOC]t=[Org]t×[SO42-]t[SO42-AMS]t-[Org]0×[SO42-]0[SO42-AMS]0(WSOC0-WSOCt)×M/12 Where [SO42-]_t and [SO42-]₀ denote sulfate concentrations (mg L⁻¹) in the solution at irradiation time t and zero, respectively. Here, [SO42-]_t was equal to [SO42-]₀ assuming sulfate was not loss during irradiation. The [Org] and [SO42-AMS] denote the apparent concentrations of aqSOA and sulfate measured by HR-AMS. [WSOC]_t and [WSOC]₀ were WSOC concentrations in the solution measured by TOC at irradiation time t and zero, respectively. M represents the averaged molecular weight of mixed solution which can be estimated by FT-ICR MS.

Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMXnano, Germany) was used to detect ROS. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP) were used as spin-trapping agents for ⚫OH and ¹O₂ to identify the DMPO-OH adducts (1:2:2:1) and the TEMPO adducts (1:1:1), respectively (Hu et al., 2025b; Wang and Wang, 2020). The 10 mL of aqueous extracts were diluted into 100 mL using deionized water and pH was adjusted at 5.0 ± 0.1 with 0.1 M H₂SO₄ solutions. Then 100 mM of two spin-trapping agents were added into the 50 mL of acidified aerosol extract solutions. After photodegradation of smoke extracts, 200 µL of solutions were transferred from the reaction solutions and immediately analyzed by EPR. The EPR parameters were set as following: modulation frequency of 100 kHz; center field of 3460 G, modulation amplitude of 1 G, microwave power of 25 mW, sweep width of 200 G, sweep time of 150 s, number of scans of 20.

Figure 1 showed the reconstructed FT-ICR mass spectra of WSOM for two fresh extracts samples. Based on the intensity of each negative ion, the average molecular formulas for coal and maize smoke extracts calculated as C_18.0H_24.0O_6.9N_0.90S_0.41 and C_21.0H_21.7O_7.4N_0.86S_0.04, respectively, showing higher C in maize smoke extracts, consistent with previous finding (Fan et al., 2016). In this study, these identified molecular formulas were classified into four main compound groups based on their compositions: CHO, CHON, CHOS, and CHONS. The relative abundances of the four groups were determined by normalizing the magnitude of each peak to the total magnitude of all identified peaks. Most peaks were located within the m/z range of 200–400. The greatest peak magnitudes were mainly distributed within the m/z range of 250–350. Distinct peak distribution patterns were observed for both smoke extracts. For example, several CHOS compounds with high relative abundance, such as C₁₂H₂₆O₄S, C₁₇H₂₈O₃S, C₁₈H₃₀O₃S, were identified in the coal smoke extract, whereas the high-abundance CHO and CHON compounds, including C₈H₁₀O₄, C₇H₇NO₄, and C₁₈H₁₈O₇ were predominant in the maize smoke extract. It should be noted that peak magnitude is not indicative of a compound's concentration in a sample due to inherent biases of C18 extractions and electrospray ionization efficiencies.

To better illustrate the differences in the ⚫OH oxidation behavior between the two extracts, the concentrations of inorganic ions and transition metals (Fe and Cu) in the fresh WSOM were also measured, as shown in Table S1. It can be seen that the concentrations of Cl⁻ and NH4+ ions in maize were much higher than those in coal, whereas the concentration of SO42- ions in coal was higher than that in maize. During the photochemical reaction, the concentrations of these ions showed little change. In addition, the concentrations of Fe and Cu ions were very low, almost below the detection limits of the instrument. These results indicate that the influence of ions on the photochemical reaction can be neglected, especially that of Fe and Cu.

The intensity weighted average values of various molecular parameters – including molecular weight (MW), elemental ratios (H / C and O / C), DBE, AI – for maize and coal smoke extracts before and after photoaging were summarized in Table 1. As listed in Table 1, a total number of 5596 and 5107 molecular formulas were identified for fresh coal and maize extracts, respectively, within the m/z range of 100–600, indicating the complicated molecular compositions of WSOM. For coal WSOM, the MW decreased slightly 313 g mol⁻¹ in the fresh sample to 296 and 288 after 11 and 23 h photooxidation, respectively. The MW for maize remained nearly unchanged during OH-photooxidation. The dominant species in both smoke extracts were CHO and CHON compounds, with higher abundance observed in maize than in coal smoke extract (Fig. 5). Molecular composition analysis further revealed that maize smoke WSOM was largely composed of CHO and CHON, together accounting for 98.6 % of the total peak area. CHO compounds constituted more than half of all identified molecular formulas in both WSOM samples (74.5 % for maize and 58.9 % for coal). In contrast, S-containing compounds (CHOS and CHONS) were much more abundant in coal smoke extracts (29.4 % in total) than in maize (1.4 % in total). Similarly, previous studies reported that the fractions of S-containing CHOS and CHONS species in crop-derived WSOM were relatively low (3 %–9 % in peak area) (Li et al., 2024b). Interestingly, S-containing compounds in coal smoke decreased by nearly 50 % after photodegradation, whereas their abundance increased markedly in maize smoke extracts. Meanwhile, the proportion of CHON compounds in coal smoke increased under photoaging, which may be attributed to the photochemical transformation of CHONS species and/or the oxidation of reduced CHN compounds. Conversely, the CHON proportion in maize smoke decreases with reaction time, likely due to the progressive degradation of nitroaromatic compounds commonly present in biomass burning emissions (Lin et al., 2016).

The molecular-level parameters are summarized in Table 1. DBE values ranged from 2 to 9 for coal-smoke WSOM and from 2 to 11 for maize-smoke WSOM. The degree of unsaturation and aromaticity of molecular formulas can be valuated using the H / C ratio and DBE values, with lower H / C ratios and higher DBE indicating greater unsaturation and, to some extent, stronger aromatic character. As shown in Table 1, after 23 h of photooxidation, aged WSOM exhibited higher H / C ratios (1.75 vs. 1.32 for coal; 1.68 vs. 1.02 for maize), lower DBE values (3.85 vs. 7.12 for coal; 4.08 vs. 8.99 for maize), and reduced AI values (0.16 vs. 0.31 for coal; 0.16 vs. 0.48 for maize) compared to the fresh samples. These concurrent changes consistently indicate the breakdown of aromatic structures and an overall shift toward more saturated compounds. This trend agrees with previous findings from dark aqueous ⚫OH oxidation of BB smoke WSOC reported by Fan et al. (2024).

At the molecular-class level, CHON compounds in maize smoke initially exhibit a relatively high average molecular weight (MW, 329.45 g mol⁻¹) and DBE (10.52). Upon photolysis, the average MW decreases to 301.07 (11 h) and 296.44 g mol⁻¹ (23 h), while the DBE declines to 6.59 and 4.14, respectively. These changes reflect the progressive breakdown of conjugated structures and a corresponding reduction in aromaticity, consistent with the observed decrease in light absorbance (Fig. 2b). For coal smoke, a considerable fraction of CHONS species undergoes transformation into CHON compounds with lower aromaticity and DBE during photolysis. This conversion increases the relative abundance of CHON species while contributing to a decrease in overall molecular weight. In addition, the MW of CHO compounds in coal smoke decreases progressively with photolysis. In contrast, maize-smoke WSOM is dominated by CHO compounds, whose MW remains relatively unchanged during photolysis, resulting in only minor variation in the bulk molecular weight. This contrast primarily reflects differences in the initial WSOC compositions between coal and maize smoke. Overall, the marked decreases in AI and DBE for CHON compounds in both smokes indicate substantial loss of aromaticity, which in turn contributes to the reduction in light absorption.

Additionally, the O / C increased from 0.38 to 0.45 for coal and from 0.40 to 0.55 after 11 h of oxidation, followed by a decrease to 0.27 and 0.25 at 23 h, respectively. This tread indicates a transformation from OH-functionalization to fragmentation as photooxidation progressed. A decrease in DBE per carbon (DBE per C) was observed after 23 h of photodegradation – from 0.45 to 0.25 for coal and 0.62 to 0.27 for maize – further confirming the transformation of refractory aromatic-condensed structures into more polar and readily degradable small molecules. Figure S5 shows the relationship between DBE values and C atom numbers for four compound groups identified by FT-ICR MS.

The van Krevelen diagram (Fig. 6), which plots the O / C ratio as the x-axis and the H / C ratio on the y-axis, was used to elucidate the molecular distribution. For clarity, the corresponding detailed values were listed in Table S2. Lignin-like compounds dominated both coal and maize smoke WSOM, accounting for 58.2 % and 93.1 % of total intensity, respectively, indicating a greater abundance of phenolic organic species in maize smoke. Previous study also showed that CHO formulas were mainly lignin-pyrolysis products (Song et al., 2018). After photoaging, the lignin-like fraction decreased significantly, reflecting the degradation of aromatic phenolic species. Given that most lignin-like compounds possess strong light-absorbing properties, their decomposition directly contributed to the observed decrease in absorbance. In contrast, the intensity of saturated compounds (sum of lipids and aliphatic components) increased substantially after OH-induced photooxidation, from 33.8 % to 51.2 % at 23 h for coal and from 2.4 % to 69.8 % for maize. These observations suggest a significant increase in saturated aliphatic and O-enriched compounds after ⚫OH photooxidation.

As listed in Table S2, the initial increase followed by a decrease (from 11.2 % to 1.1 % for coal and 31.8 % to 2.1 % for maize smoke) in tannin-like compounds suggests that radical coupling, condensation, or addition reactions likely occurred during the early stage of the reaction, leading to a higher O / C ratio at 11 h compared to the fresh sample.

Condensed aromatic molecules, characterized by low H / C and O / C ratios but high AI, showed a slight decrease with photoaging, indicating the partial degradation of highly aromatic structures. Overall, the reduction in aromatic and lignin-like compounds aligns with the observed decline in the light-absorbing properties (see Sect. 3.2.1). In all, aromatic and lignin-like compounds were continuously transformed into lipid- and aliphatic-like compounds. During the initial stage (first 5 h), carbohydrate-like substance such as oxalate were generated (Fig. S6), but their abundance subsequently decreased, consistent with the pH variation that first declined (initial 3 h) and then increased again (Fig. S7). The formation of carboxylic acids can be further confirmed later by identifying their characteristic fragment ions using HR-AMS.

To further elucidate the photooxidation behaviors of both smoke extracts, the number proportions of resistant, degraded and newly produced molecules were summarized in Table S3. After 11 h of irradiation, 55.1 % and 58.2 % of the total formulas in fresh coal and maize, were degraded, resulting in 51.3 % and 64.6 % newly formed formulas. From 11 to 23 h, the numbers of newly produced and degraded molecules increased slightly for coal but decreased for maize. Figure 7 illustrates the O / C vs. H / C distributions of degraded and newly formed compounds after 11 and 23 h of photodegradation. For coal, most degraded compounds were located in high O / C regions, whereas some newly formed species with much lower O / C and higher H / C were likely associated with unsaturated hydrocarbons and lipid-like species. In contrast, maize exhibited a marked shift from low to high O / C and H / C compounds at 11 h, resulting in an increase in average O / C ratio. This trend suggests that maize compounds mainly underwent functionalization during the first stage – introducing oxygen-containing groups without breaking the carbon skeleton, thereby increasing O / C and slightly lowering or maintaining H / C. The shift toward higher O / C ratios in the van Krevelen diagram further supports the progression from aromatic to more oxygenated and saturated compounds for maize smoke WSOM.

Overall, these results reveal distinct degradation pathways and product characteristics for coal and maize smoke extracts.

The aqSOA spectra exhibited higher mass fractions of CxHy⁺ and CxHyO1+ ions but lower fractions of C_xH_yNp+ and C_xH_yO_xNp+ ions (Fig. 8). For corn-derived aqSOA, the fractions of C_xHy+ and CxHyO1+ both decreased by approximately 10 % with increasing photolysis time, while CxHyO2+ increased substantially from 15.95 % to 29.96 % after 23 h of photoreaction. In contrast, for coal-derived aqSOA, the fraction of CxHyO1+ increased with irradiation time, while no corresponding increase in CxHyO2+ was observed. This suggests that the overall oxidation degree of coal-derived aqSOA did not increase significantly relative to that prior to irradiation.

Table S4 summarizes the chemical properties, mass concentration and yield of the formed aqSOA and their evolution during the photoreaction. During the first 5–7 h, both f44 and OSc values increased, indicating a progressive enhancement in the oxidation state of aqSOA for both samples. In contrast, the H / C ratio exhibited only minor changes throughout the reaction, suggesting relatively stable bulk hydrogen content despite ongoing oxidation. A comparison between the two systems further reveals that maize-derived aqSOA undergoes a more pronounced increase in oxidation at the early stage, likely driven by functionalization reactions. As photochemical processing continues, however, a slight decline in oxidation is observed, which can be attributed to fragmentation processes. This trend is less evident in coal-derived aqSOA, highlighting differences in their underlying transformation mechanisms. Notably, the significantly higher O / C ratios and OSc of maize-derived aqSOA compared to those of the precursors suggest that aqueous-phase processing can serve as an effective source of oxygenated SOA in regions influenced by biomass burning emissions. The value of f43 remained relatively low value (less than 0.1) and is therefore not discussed further.

For coal samples, the aqSOA mass concentration ranged from 50.77 to 126.95 mg L⁻¹. It reached a minimum at 11 h and subsequently increased to 126.95 mg L⁻¹ at 23 h. Correspondingly, the aqSOA mass yield peaked at 148.44 % at 1 h, continuously decreased to 1.87 % within 9 h, and then increased again to 33.91 % at 23 h. The aqSOA mass yield of maize was significantly lower (less than 10 %) than that of coal samples, indicating that coal sample is more efficient at generating low-volatility species compared to maize. The possible reasons why coal-derived aqSOA is higher than that from maize are as follows. First, the fresh coal-derived CHOS compounds are dominated by species such as C₁₇H₂₈O₃S and C₁₈H₃₀O₃S, which are mainly organosulfates. These compounds have relatively high saturation and stability, and undergo little change upon photolysis, resulting in a high SOA mass yield measured by HR-AMS. In contrast, maize-derived WSOM is primarily composed of lignin-like substances with high DBE values, which are more susceptible to OH functionalization, forming saturated fatty acids or polyhydroxy acids (e.g., C₉H₁₈O₆ and C₉H₁₀O₇). These products can further undergo fragmentation into smaller, more volatile products (e.g., low-molecular-weight acids), leading to a lower aqSOA mass yield.

The significant formation of carboxylic acids during the first 9 h of photoreaction is further evidenced by the Van Krevelen diagram (H / C versus O / C), in which aqSOA evolves along a slope of approximately -1 throughout the photooxidation process (Fig. S8). Consistently, the CHO2+ ion in the aqSOA AMS spectra – commonly used as a tracer for carboxylic functional groups – exhibits a continuous increase during the first 5 h of photoreaction (Fig. S9). After 3–9 h of reaction, the concentration of CHO2+ decreases, accompanied by a decline in f44. A plausible explanation is the occurrence of fragmentation reactions, during which the oxidation products initially formed through oligomerization or functionalization decompose into smaller, more oxidized species. This trend has also been reported in previous studies on the photooxidation of phenolic carbonyls (Jiang et al., 2021).

Aqueous photochemical aging of BB smoke can also alter its toxicity. The oxidative potential of the reaction solutions was evaluated using the dithiothreitol (DTT) assay, as described in our previous work (Ye et al., 2025). As shown in Fig. 9a, based on the DTT consumption rate (OP^DTT), OH-initiated photooxidation of smoke extracts led to an increase in OP^DTT during the first 1 h. Upon prolonged photoaging, the OP^DTT value decreased to 0.15 µM DTT min⁻¹ after 23 h, slightly lower than the corresponding initial values. Previous research results also suggested that aqueous OH oxidation of WS-BBOA components generally leads to a final reduction in OP^DTT (Wong et al., 2019; Jiang and Jang, 2018) during prolong irradiation, consistent to our findings. The temporal variation pattern of OP^DTT is comparable to that observed for aqueous oxidation of soybean straw extracts (Ye et al., 2025), but opposite to that of 4NC photodegradation (Lei et al., 2025). The DTT activity is likely associated with light-absorbing and fluorescent substances containing large conjugated electron systems, which can transfer electrons to participate in catalytic reaction, thereby contributing to DTT activity (Chen et al., 2019). The reduction in DTT activity after 23 h agrees with the decrease of lignin-like and aromatic compounds revealed by FT-ICR MS analysis.

Given that aqueous ⚫OH oxidation did not significantly reduce the total WSOC concentrations, the decrease in OP^DTT is likely attributed to the formation of non- or less DTT-active components. However, total WSOC decreased significantly upon aging; consequently, the OP^DTT normalized by WSOC increased over irradiation time, suggesting the possible formation of secondary toxic organic species during the aging processes. Previous published studies have also showed that photochemical aging of fresh particles can either enhance or diminish toxicity, depending on their sources and oxidation conditions (Fang et al., 2024). To further characterize the ROS-generation potential of WSOM from different combustion sources, we calculated the WSOC-normalized DTT consumption rate (OPWSOCDTT, OP^DTT divided by WSOC). The results showed that the mass-normalized DTT consumption rates gradually increased and reached a plateau at 132 and 82 pmol min⁻¹ µg⁻¹ for maize and coal smoke extracts, respectively, similar to finding from Wong et al. (2019). These values are higher than those reported for water extracts from PM_2.5 aerosol (22–68 pmol min⁻¹ µg⁻¹) (Verma et al., 2012). Based on the molecular-level differences after ⚫OH photooxidation, the reasons for the OPWSOCDTT changes induced by OH-photolysis in coal and maize are likely different. For coal, the increase in OPWSOCDTT may primarily result from the formation of CHON compounds after photolysis, whereas for maize, the increase is probably due to the production of more quinone species or OH addition products during the reaction (Tang et al., 2025; Wong et al., 2019).

Unfortunately, due to the limitations of current analytical techniques, it remains challenging or even impossible to attribute the observed DTT variation trends to specific molecular species.