Experiments of the liquid-phase ⚫OH oxidation of aromatic OSs were carried out in a 150 mL custom-built quartz reactor thermostated by a water jacket. OH radicals were generated through the aqueous photolysis of 10 mM H2O2 (30 %, Hu Shi) under irradiation from a 300 W Xenon arc lamp to simulate sunlight. Three commercial aromatic OSs (i.e., phenyl sulfate (≥98 %, Macklin), p-tolyl sulfate (≥98 %, Macklin), and 4-ethylphenyl sulfate (≥98 %, Sigma-Aldrich)) were used as representative of aromatic OSs. The solution containing each aromatic OS, H2O2, and dissolved O2 was introduced into the quartz reactor with a total volume of 100 mL, and was agitated by an electromagnetic stirrer. Subsequently, the reactor was sealed, and the lamp was ignited to start the reaction. Given the varied pH values in aqueous environments in the atmosphere (from 1 to 9) (Herrmann et al., 2015; Pye et al., 2020), the solution was adjusted to pH 3 (using 36 %–38 % HCl, Hu Shi) and pH 8 (using phosphate buffer (Na2HPO4 and NaH2PO4)) to represent acidic and alkaline conditions, respectively. All experiments were performed at 298 K at least in duplicate. Details of the experiments carried out in this study are summarized in Table S1 in the Supplement.

Kinetic experiments were performed with 0.05 mM of each aromatic OS in the presence of a reference compound (i.e., sodium benzoate (BA, 98 %, Macklin)) over the reaction time of 2 h. Two sets of control experiments were carried out. One involved irradiating a solution of each aromatic OS alone to investigate the effects of light only. Another set combined each aromatic OS with H2O2 in the dark to preclude the interference from H2O2. Reaction progress was tracked by withdrawing 1 mL aliquots at 30 min intervals for direct analysis via either ultrahigh-performance liquid chromatography (UPLC, Agilent 1260) or ion chromatography (IC, Dionex ICS-600), without any intermediate processes or dilution. Mechanism experiments were conducted the same as the kinetic experiments, except that BA was not added. After reactions, 0.5 mL of the sample was taken and was immediately stored at -20 °C prior to product analysis using ultra-performance liquid chromatography equipped with a quadrupole time-of-flight mass spectrometer (UPLC-Q-TOF-MS, Bruker Impact HD). Control experiments of each aromatic OS and H2O2 in the dark were also performed for comparison in order to eliminate the possibility of detected products resulting from analytical artifacts. To enhance the detection of optical changes, we also conducted experiments at higher aromatic OS concentrations (0.5 or 1 mM) over an extended period of 8 h. For these experiments, the sample was taken every 4 h for immediate measurement of its absorbance spectra using a UV–vis spectrophotometer (Duetta™, Horiba Scientific) and its excitation-emission matrix (EEM) fluorescence spectra by a fluorescence spectrometer (Duetta™, Horiba Scientific). After reactions, the sample was also analyzed by ultrahigh-performance liquid chromatograph coupled with a photodiode array detector and an Orbitrap mass spectrometer (UPLC-PAD-MS, Thermo Fisher Scientific) to investigate how optical changes were affected by the formation of chromophores. 0.5 mL sample was diluted with 0.5 mL H2O and 0.25 mL acetonitrile, and then the resulting solution was stored at -20 °C before analysis.

The second-order rate constant of each aromatic OS with ⚫OH was measured by the competition kinetic method using 0.05 mM sodium benzoate as the reference compound (Smith et al., 2015). It is noted that one of the important principles of this method is that aromatic OS and the reference compound are consumed only by ⚫OH oxidation in the liquid phase. Control experiments of the direct photolysis of each aromatic OS without the addition of H2O2 as well as the reactions of each aromatic OS, BA, and 10 mM H2O2 without illumination were carried out to preclude interferences from other reactions. Figures S1 and S2 in the Supplement show that the influences of either hydrolysis or photodegradation on the kinetic measurements of these three aromatic OSs are negligible, verifying that the decay of reactants results from ⚫OH oxidation. Therefore, in this study, the second-order rate constant for aromatic OS (kOS) can be calculated using Eq. (E1) based on following Reactions (R1) and (R2). E1ln⁡[OS]0[OS]t=kOSkrefln⁡[ref]0[ref]tR1Aromatic OS+⚫OH→Products,kOSR2Reference+⚫OH→Products,kref where [OS] and [ref] are concentrations of aromatic OS and the reference compound (BA) (time=0 and t). The time dependence of aromatic OS consumption is shown in Fig. S3. kref is the rate constant of BA with ⚫OH. kref value at pH 3 and 8 was reported as 4.3 ± 0.8×109 and 6.3 ± 0.2×109 M-1s-1, respectively (Buxton et al., 1988; Richards-Henderson et al., 2014). Figure 1 displays the relative kinetic plots for aromatic OSs oxidized by ⚫OH under acidic (pH 3) and basic (pH 8) conditions. These plots exhibit strong linearity (R2≥0.99), with the slope of each linear fit corresponding to the kOS/kref. According to the slope and value of kref, the kOS can be calculated.

The concentrations of aromatic OSs and BA were detected using a UPLC coupled with a UV detector operating at 254 nm. Chromatographic separation was performed on a ZORBAX Eclipse Plus C18 column (4.6mm×250mm, 5 µm) maintained at 40 °C. The mobile phase consisted of acetonitrile and 0.1 % formic acid aqueous solution (20:80, v/v) delivered at a flow rate of 0.8 mLmin-1, with an injection volume of 10 µL. Quantification of aromatic OSs and BA was achieved by calibration curves (Fig. S4) based on their corresponding peak areas in the chromatogram.

Inorganic sulfate was analyzed by IC with an analytical column (AS 11-HC, 4mm×250mm, IonPac) and a guard column (AG11-HC, 4mm×250mm, IonPac). The eluent was 20 mM potassium hydroxide at a flow rate of 1 mLmin-1.

Reaction products were detected using a UPLC-Q-TOF-MS. Separation was achieved on a C18 column (4.6mm×250mm, particle size=5µm; ZORBAX Eclipse Plus) at 40 °C, with a mobile phase of pure water and acetonitrile (40:60, v/v) at a flow rate of 1 mLmin-1. The mass spectrometer was equipped with an electrospray ionization (ESI) source operated in the negative (-) ionization mode. The instrumental conditions for the (-) ESI-MS analysis were as follows: capillary voltage, 4000 V; gas temperature, 200 °C; dry gas flow rate, 5 Lmin-1; and nebulizer pressure, 0.4 bar. Data were collected over the mass range of 50–500 Da. We also conducted complementary chromophore product analyses using a UPLC-PAD-MS. Separation was carried out on a C18 column (4.6mm×250mm, particle size=5 µm; ZORBAX Eclipse Plus) at 40 °C, with a binary mobile phase consisting of acetonitrile and 0.1 % formic acid (20:80, v/v) delivered at a flow rate of 0.8 mLmin-1. Mass spectrometric detection was conducted in negative ionization mode over a mass range of 50–500 Da, with the spray voltage set at -3.0 kV, the capillary temperature at 320 °C, the S-lens RF level at -50 V, the sheath gas (nitrogen) pressure at 2.76×105 Pa, and the auxiliary gas (nitrogen) flow rate at 3.33 Lmin-1.

The light absorption spectra of samples during the processes of reactions were collected using a UV–vis spectrophotometer with a scanning interval of 1 nm in the range of 250–700 nm. A reference absorption spectrum of hydrochloric acid solution (pH 3) or phosphate buffer solution (pH 8) was recorded in the same cuvette before sample analysis for baseline correction.

The excitation-emission matrix (EEM) fluorescence spectra were recorded by a fluorescence spectrometer. The excitation wavelength (Ex) and emission wavelength (Em) of EEM were both set to the range of 250–600 nm. The scanning intervals were set to 5 and 2 nm. Hydrochloric acid solution (pH 3) or phosphate buffer solution (pH 8) was used as a blank to correct the data as well.

The kOS values for the reactions of three aromatic OSs (i.e., phenyl sulfate, p-tolyl sulfate, and 4-ethylphenyl sulfate) with ⚫OH are summarized in Table 1. At pH 3, the kOS value of phenyl sulfate was measured as 4.3 ± 0.1×109 M-1s-1. This value is comparable to the literature result of phenyl sulfate at pH 2 using pimelic acid as the reference compound (5.34 ± 0.06×109 M-1s-1) (Gweme and Styler, 2024). The slight difference may be attributed to the reference compound selection and the experimental conditions. Values of kOS for the other two aromatic OSs were similar to that of phenyl sulfate (Table 1). The similar kOS value among these three aromatic OSs suggests that the substituent carbon chain length on the aromatic ring has a negligible effect on the reaction kinetics. This observation is quite different from that for alkyl OSs, which shows that kOS is strongly dependent on the carbon number of OS molecule contained (Lai et al., 2024; Gweme and Styler, 2024). Lai et al. (2024) reported that kOS of ethyl sulfate (3.8 ± 0.1×108 M-1s-1) was approximately five times higher than that of methyl sulfate (7.5 ± 0.1×107 M-1s-1). Gweme and Styler (2024) also found that kOS value increased with increasing carbon chain length for methyl sulfate (1.03 ± 0.21×108 M-1s-1), ethyl sulfate (4.01 ± 0.17×108 M-1s-1), and propyl sulfate (1.22 ± 0.03×109 M-1s-1). This distinct behaviour may be ascribed to the different mechanisms for aromatic OSs and alkyl OSs oxidized by ⚫OH. For aromatic OSs, the OH radical predominantly attacks the aromatic ring with multiple addition sites (Bloss et al., 2005; Garmash et al., 2020). While alkyl OSs react primarily through hydrogen abstraction, the increasing carbon chain length can enhance reactivity through the inductive effect of -CHx groups, the increasing electron density at the hydrogen abstraction site, and the stabilization of resulting alkyl radicals (Dorfman and Adams, 1973; Monod and Doussin, 2008). As such, aromatic OSs have higher reactivity compared to alkyl OSs. The negligible effect of carbon number on the reactivity of aromatic compounds was also observed in other homologues of aromatic compounds. For example, Schuler and Albarran (2002) reported a similar rate constant for the reactions of toluene (8.1×109 M-1s-1) and benzene (7.8×109 M-1s-1) with ⚫OH. It is noted that the kOS values are lower than those of their parent aromatic hydrocarbons. This reduction in reactivity may be attributed to the electron-withdrawing effect of the -OSO3- groups, which can reduce the reactivity of the aromatic ring toward ⚫OH (Lai et al., 2024).

Table 1 shows that the carbon chain length has an insignificant effect on the kOS values of aromatic OSs at pH 8 as well. kOS values of phenyl sulfate, p-tolyl sulfate, and 4-ethylphenyl sulfate were calculated as 5.9 ± 0.1×109, 6.4 ± 0.2×109, and 6.3 ± 0.1×109 M-1s-1 at pH 8, respectively. These values were higher than those measured at pH 3. It should be noted that different matrices (HCl vs. phosphate buffer) were used to adjust the solution pH, and the ionic strength of the solution is different at different pHs. The ionic strength of solution at pH 3 and pH 8 was estimated as 1×10-3 and 6.9×10-3 M, respectively. Previous study reported that a substantial increase in ionic strength from ca. zero to 6.5 M only resulted in a tenfold decrease in kOS value of phenyl sulfate (Gweme and Styler, 2024). Therefore, the relatively low ionic strength variation between pH conditions in this study may not account for the observed differences in kOS values of aromatic OSs. Gweme and Styler (2024) measured the kOS of phenyl sulfate at pH 2 and pH 9, observing that it is pH independent. They attributed this pH independence to phenyl sulfate remaining fully deprotonated (pKa=-2.2) across the entire experimental pH range. However, previous studies demonstrated that even though methoxyphenol, benzene-diols, and highly substituted phenols mainly exist in their protonated form within the pH range of 2–6, their k values at pH 2 were generally lower than those at pH 5 or 6 (Arciva et al., 2022). One possible explanation is that the acidic condition could hinder ⚫OH attack on aromatic systems or reduce the lifetime of hydroxycyclohexadienyl radical intermediates, slowing irreversible diol formation (Smith et al., 2015). Another possible explanation is the uncertainty of the k value for the reaction of the reference compound with ⚫OH (Arciva et al., 2022). Therefore, the difference and uncertainty of the rate constant of the reference compounds selected may also explain the discrepancy between the findings of Gweme and Styler (2024) and our study.

In this study, products generated from the liquid-phase reactions of three aromatic OSs with ⚫OH were characterized by mass spectrometry. The identified species were consistent across both pH 3 and pH 8 conditions. The observation that product intensities were substantially higher under illumination conditions than in dark controls implies that these products arise from ⚫OH oxidation, not from analytical artifacts or hydrolysis (Fig. S5). Table S2 lists the identified products from the reaction of phenyl sulfate with ⚫OH. The predominant signals corresponded to hydroxyphenyl sulfate (C6H5O5S-, m/z 189) and dihydroxyphenyl sulfate (C6H5O6S-, m/z 205), aligning with previous work (Gweme and Styler, 2024). Additionally, the multiple -OH group addition products (e.g., C6H5O7S-) were also detected. As illustrated in Fig. 2, ⚫OH-initiated oxidation of phenyl sulfate follows a mechanism analogous to conventional aromatic compounds (e.g., benzene). The reaction initiates via the addition of ⚫OH to the aromatic ring, generating hydroxycyclohexadienyl radicals (OH-PS radicals) (Lay et al., 1996; Minakata et al., 2015). OH-PS radicals rapidly react with O2 to yield phenolic compounds that can undergo further multi-step ⚫OH additions to form these polyhydroxy products (C6H5OnS-, n=5–7). Alternatively, OH-PS radicals can also react with O2 to form peroxyl radicals (RO2⚫). The reversible cyclization of RO2⚫ and the subsequent O2 addition generate bicyclic RO2⚫. Bicyclic RO2⚫ can react with RO2⚫ to produce ring-opening products as shown in Table S2 (Wang et al., 2013; Dong et al., 2021). Fragmented OS formation resulting from ring-opening pathways during ⚫OH oxidation of aromatic OSs has not been reported previously. Notably, some of these ring-opening fragments (e.g., C2H3O5S-, C5H7O8S-) have the same formula as OSs detected in the atmosphere, and their precursors were regarded as biogenic VOCs (Cai et al., 2020; Wang et al., 2022; Huang et al., 2023). For example, previous studies inferred that m/z 139 (C2H3O5S-) is produced from reactions related to isoprene and its derivatives (Cai et al., 2020; Wang et al., 2022). In this study, we found that this compound can also be formed through the oxidation of phenyl sulfate by ⚫OH, providing the additional pathway for its formation in the atmosphere.

Previous studies revealed that some aliphatic OSs (e.g., methyl sulfate, ethyl sulfate, 2-methyltetrol sulfate, and α-pinene-derived OS) can be converted into inorganic sulfate during ⚫OH oxidation (Kwong et al., 2018; Xu et al., 2022, 2024; Lai et al., 2025). The reaction is initiated through hydrogen abstraction from the alkyl group, forming an alkyl radical (R⚫) followed by rapidly reacting with O2 to form RO2⚫. The self- or cross-reactions of RO2⚫ can further produce an alkoxy radical (RO⚫). Typically, the formation of inorganic sulfate results from the production of ⚫SO4-, which is generated from the decomposition of α-OSO3- alkoxy radical (defined as the containing of -OSO3- group at the α-position of RO⚫). Additionally, a recent study proposed an alternative mechanism for the formation of inorganic sulfate, proceeding via sulfite radicals (⚫SO3-) (Xu et al., 2024). In this pathway, a β-OSO3- alkoxy radical (-C(O⚫)-C(OSO3-)-) undergoes C-C bond cleavage, yielding an α-OSO3- alkyl radical, subsequently generating non-sulfate products and ⚫SO3-. Upon the formation of ⚫SO4- and ⚫SO3-, inorganic sulfate can be formed through the further reactions of these radicals. In this study, the formation of inorganic sulfate during the reaction of phenyl sulfate with ⚫OH was also examined. Figure S6a shows that the SO42- peak in IC increased progressively with reaction time. The formation of inorganic sulfate was further supported by the evidence of the observed prominent HSO4- peak at m/z 97, which is assigned to HSO4- in the mass spectra. The intensity in the extracted ion chromatograms (EIC) was substantially higher than in dark controls (Fig. S6b), ruling out in-source fragmentation or hydrolysis as the source of HSO4-. The results of IC and mass spectrometry suggest that in addition to new OSs, inorganic sulfate can be formed during the reaction. The mechanism of inorganic sulfate formation is elucidated to be produced from the elimination of the sulfate group from phenyl sulfate, as well as the ipso-addition followed by disproportionation reaction as shown in Fig. 2. Phenyl sulfate can undergo ipso-addition to form OH-PS radical, the ipso-OH-adduct can either rapidly eliminate HSO4-, or undergo bimolecular reactions with other isomers of the OH-PS radical to yield phenol upon elimination of HSO4- as well. However, compared to other OH addition pathways (o-add, m-add, and p-add), there is only very little room for the ipso-addition. It is noted that a previous study has shown that benzoic acid can undergo decarboxylation reactions (Singla et al., 2004). Another possible pathway for HSO4- production is proposed to occur via the elimination of the sulfate group from phenyl sulfate, similar to the decarboxylation mechanism of benzoic acid.

Tables S3 and S4 summarize the identified products from the liquid-phase reactions of p-tolyl sulfate and 4-ethylphenyl sulfate with ⚫OH, respectively. The mechanisms of p-tolyl sulfate and 4-ethylphenyl sulfate oxidized by ⚫OH are similar to that of phenyl sulfate as mentioned above. Similar to phenyl sulfate, the addition of ⚫OH to the aromatic ring predominantly yields phenolic compounds, such as C7H7O5S-, C7H7O6S-, C8H9O5S-, and C8H9O6S-. Further oxidation initiated by addition of ⚫OH can also generate fragmented products, such as C5H5O6S- and C4H5O7S- (Tables S3 and S4). Moreover, the presence of alkyl substituents for p-tolyl sulfate and 4-ethylphenyl sulfate can enable additional hydrogen abstraction pathways (Forstner et al., 1997; Baltaretu et al., 2009; Liu et al., 2017), leading to the formation of aromatic aldehydes (e.g., C7H5O5S-, C8H7O5S-). In addition to new OSs formed, the formation of inorganic sulfate was also observed during the process of either p-tolyl sulfate or 4-ethylphenyl sulfate oxidized by ⚫OH. For p-tolyl sulfate, the gradual increase of the SO42- peak with reaction time in IC as well as the pronounced signal of m/z 97 (HSO4-) observed in mass spectra provide robust evidences for the formation of inorganic sulfate during the reaction (Fig. S7). For 4-ethylphenyl sulfate, SO42- peak in IC was found to overlap with that of the compound itself (Fig. S8a). The inference that 4-ethylphenyl sulfate converts to inorganic sulfate is supported by comparing the intensity of HSO4- (m/z 97) peak of samples collected from illumination and dark conditions (Fig. S8b).

Kinetic and mechanism results show that aromatic OSs can undergo rapid ⚫OH oxidation to form a series of functionalized and fragmented compounds. The changes of optical properties resulting from the formation of these compounds were also investigated. Figure S9 shows the time-dependent absorption spectra of aromatic OSs during ⚫OH oxidation at pH 3. As the reaction progressed, the consumption of reactants accompanied by an increase in absorbance across 250–400 nm. To establish the relationship between light absorption and organic compounds, chromophores formed in the reaction were identified by correlating UV absorption bands with the retention time based on UPLC-PAD-MS analysis. For phenyl sulfate, Fig. 3a and b display that phenyl sulfate (m/z 173) was the prominent chromophore with the retention time of 5.67–6.16 min at the beginning of the reaction, exhibiting a characteristic absorption peak at 262 nm (Fig. S10). After the liquid-phase ⚫OH oxidation, five major chromophores were observed as shown in Fig. 3c. Chromophore #4 was assigned to the unreacted phenyl sulfate. Figure 3c shows that Chromophore #1, #2, and #3 eluted at 3.47–3.72, 4.00–4.18, and 5.46–5.60 min, respectively. These newly formed chromophores exhibit red-shifted absorption peaks (Fig. S10), likely due to the electron-donating effect of hydroxyl groups increasing aromatic ring electron density (Hems and Abbatt, 2018). The results of EIC suggest that these chromophores correspond to co-eluting mixtures containing C6H5O5S- isomers (m/z 189), along with C6H5O6S- (m/z 205), C6H5O7S- (m/z 221) and C6H7O8S- (m/z 239) (Fig. 3d). Among these compounds, C6H5O6S- exhibited the highest intensity. Chromophore #5, eluting at 14.98–15.67 min, remained unidentified. Its later elution time suggests a larger molecular structure and lower polarity (Fleming et al., 2020). Additionally, there may exist other chromophores unidentified since these five chromophores cannot fully explain the total light absorption as shown in Fig. S10.

For p-tolyl sulfate, the increase in absorbance, contributing by the formation of chromophores, was also observed after ⚫OH oxidation. The primarily newly formed chromophore (Chromophore #1), eluting at 5.23–5.78 min, was identified as C7H5O5S- (m/z 201) based on the corresponding EIC (Fig. S11). A blue-shift peak at 258 nm was observed upon the formation of C7H5O5S-, which is associated with the generation of a carbonyl (C=O) functional group (Fig. S12). Other newly formed chromophores were characterized as Chromophore #2 and Chromophore #3. Chromophore #2 corresponded to a mixture of C7H7O5S- (m/z 203), C7H5O6S- (m/z 217), and C7H7O6S- (m/z 219) with absorption band at 274 nm, and Chromophore #3 was assigned to an isomer of C7H7O5S- with the absorption band at 266 nm (Figs. S11 and S12). Figures S13 and S14 show the characterization of chromophores formed from liquid-phase reaction of 4-ethylphenyl sulfate with OH radicals. After reactions, Chromophore #2 (C8H7O5S-, m/z 215) with a characteristic absorption peak at 254 nm was the dominant contributor to total light absorption. Four additional chromophores were also identified: Chromophore #1, a mixture of C7H5O5S- (m/z 201), C8H9O5S- (m/z 217), C8H7O6S- (m/z 231), and C8H9O6S- (m/z 233), with absorption peak at 258 nm; Chromophore #3, an isomer of C8H7O6S-, with absorption peak at 262 nm; Chromophore #4, an isomer of C8H9O5S-, with absorption peak at 274 nm; and Chromophore #5, another isomer of C8H9O5S-, also with a characteristic absorption peak at 274 nm.

Furthermore, fluorescence evolutions during liquid-phase ⚫OH oxidation of aromatic OSs were investigated as shown in Fig. 4. The initial maximum excitation/emission (Ex/Em) wavelengths of phenyl sulfate, p-tolyl sulfate, and 4-ethylphenyl sulfate at pH 3 were Ex/Em=255/275, 260/284, and 260/284 nm, respectively. The different initial fluorescence intensities among these three aromatic OSs may be attributed to the substituent effect of the compound. Compared to phenyl sulfate, p-tolyl sulfate and 4-ethylphenyl sulfate contain additional methyl and ethyl groups, respectively. These electron-donating substituents extend the conjugation system, lowering the π→π∗ transition energy and resulting in both emission redshift and fluorescence enhancement (Cao et al., 2023). During the reaction, the fluorescence intensity initially decreased due to phenyl sulfate consumption, followed by an increase from fluorescent product formation. After 8 h of illumination, a redshifted fluorescence peak emerged at Ex/Em=260/284 nm, implying the formation of products with expanded conjugated systems (e.g., C6H5O5S-, C6H5O6S-, and C6H5O7S-) (Tang et al., 2020). The fluorescence intensity of p-tolyl sulfate and 4-ethylphenyl sulfate monotonically decreased with the reaction time and showed a redshift in the fluorescence band at Ex/Em=(250-300)/(400-500). Previous studies uncovered that the emission wavelengths of 400–500 nm are indicative of humic-like substances (HULIS), which can significantly contribute to the light-absorbing properties of organic aerosols (Bianco et al., 2014). Previous studies revealed that the oxidation of non-photolyzable phenolics by ⚫OH can yield HULIS-like fluorescent products (Chang and Thompson, 2010; Tang et al., 2020). Here, multi-hydroxy products from p-tolyl sulfate (e.g., C7H7O5S-, C7H7O6S- and C7H7O7S-) and 4-ethylphenyl sulfate (e.g., C8H9O5S-, C8H9O6S-, and C8H9O7S-) may exhibit spectral features resembling aerosol HULIS.

Employing phenyl sulfate as the representative, spectral changes at pH 8 were also examined. Previous studies have demonstrated that the light absorption properties of carbonyl compounds (e.g., aldehydes) and nitrophenols exhibit pronounced pH-dependence owing to protonation-deprotonation equilibria (Calvert and Schnitzler, 2023; Chen et al., 2020a). In this study, phenyl sulfate remains deprotonated across the pH range of 3–8, resulting in negligible spectral variations in the initial solution (Figs. S9a and S15). However, the temporal evolution of the reaction revealed substantially enhanced absorbance at pH 3 compared to pH 8, particularly within the 300–400 nm range. Figure S16 shows the molecular composition of chromophores from the reaction of phenyl sulfate with ⚫OH at pH 8. Chromophores #1, 2, and 4 were identical to Chromophores #1–3 at pH 3 but exhibited stronger absorption due to their higher concentrations. An additional chromophore #3, eluting at 4.99–5.23 min at pH 8, contributed significantly to absorption but the detailed composition of this chromophore is unknown. Compared to pH 3, the solution at pH 8 exhibited an enhanced peak intensity at 4.99–5.23 min, while the peak at 14.98–15.67 min was reduced, which corresponded to distinct changes in the relative contributions to total absorption. For fluorescence spectra, phenyl sulfate exhibited an initial maximum fluorescence peak at Ex/Em=255/279 nm at pH 8 (Fig. S17), displaying minimal variation compared to the pH 3 conditions (Fig. 4a). However, the temporal evolution of its fluorescence spectrum differed obviously at different pH values. Under basic conditions (pH 8), fluorescence decreased monotonically without recovery and no red shift of fluorescence peak occurred even after 8 h.