Assessment of Secondary Sulfate Aqueous-Phase Formation Pathways in the Tropical Island City of Haikou: A Chemical Kinetic Perspective

Sulfate (SO42−) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical–chemical characteristics. The mechanisms of secondary SO42− aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM2.5 samples were collected in the tropical island city of Haikou, and water-soluble inorganic ions, as well as water-soluble Fe and Mn, were analyzed. The results showed that non-sea-salt SO42− (nss-SO42−) was the dominant species of water-soluble inorganic ions, accounting for 40–57% of the total water-soluble inorganic ions in PM2.5 in Haikou. The S(IV)+H2O2 pathway was the main formation pathway for secondary SO42− in wintertime in Haikou, contributing to 57% of secondary SO42− formation. By contrast, 54% of secondary SO42− was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+H2O2, S(IV)+Fe×Mn, and S(IV)+NO2 pathways contributed equally to secondary SO42− formation. The ionic strength was the controlling parameter for the S(IV)+NO2 pathway, while pH was identified as a key factor that mediates the S(IV)+H2O2 and S(IV)+Fe×Mn pathways to produce secondary SO42−. This study contributes to our understanding of secondary SO42− production under low PM2.5 concentrations but high SO42− percentages.


Introduction
Sulfate (SO 4  2− ) is an important component of water-soluble inorganic ions in fine particulate matter (PM 2.5 , aerodynamic diameter ≤ 2.5 µm), accounting for 44-60% of the mass fraction in PM 2.5 [1][2][3][4][5].It is well known that sources of SO 4 2− in the atmosphere include primary emission sources (such as sea salt and dust) and secondary SO 4 2− formation (the oxidation of SO 2 to SO 4 2− in the atmosphere), with the latter being the dominant contributor of fine particulate SO 4  2− [6,7].Atmospheric secondary SO 4 2− formation pathways include gas-phase reactions, aqueous-phase reactions in aerosols or clouds, and heterogeneous reactions on aerosol surfaces (Figure S1).The gas-phase reaction is the oxidation of SO 2 by OH• to produce gaseous sulfuric acid (H 2 SO 4(g) ) [8]; subsequently, gas-phase H 2 SO 4(g) reacts with alkaline substances (such as NH 3(g) and CaCO 3 ) to produce particulate SO 2− .The heterogeneous reactions are generally referred to as the direct oxidation of SO 2 to SO 4 2− on aerosol surfaces [9,10].Among these formation pathways, aqueous-phase reactions are thought to be the most important reactions for secondary SO 4 2− formation and have attracted the most attention.Due to strong public concerns about air pollution in China and the important contribution of secondary SO 4 2− to PM 2.5 , the aqueous-phase formation of secondary SO 4 2− in aerosols or cloud/fog droplets has received extensive interest in the past decades in air-polluted areas in China.Based on WRF-CMAQ analysis, Cheng et al. reported that S(IV)+NO 2 was the dominant pathway for secondary SO 4 2− formation under a pH ranging from 5.4 to 6.2 during haze pollution periods in Beijing [11].By combining observational datasets with an observation-based model for simulating secondary inorganic aerosol, Xue et al. found that the S(IV)+NO 2 pathway was prevalent during haze-fog events in Shanghai, Nanjing, and Guangzhou [12].By combining atmospheric measurements and laboratory simulations, Wang et al. reported that the aqueous oxidation of SO 2 by NO 2 was a key pathway for secondary SO 4 2− formation during air-polluted periods [13].By using online observations and developing an improved solute intensity-dependent chemical thermodynamics and kinetics model, Gao et al. reported that the S(IV)+H 2 O 2 pathway dominates sulfate formation in Tianjin during haze pollution periods [14].By coupling a laboratory simulation and a state-of-the-art multiphase model, Song et al. suggested that the TMI-catalyzed pathway was the most important one for secondary SO 4 2− formation in North China [15].Although much research has been conducted on the secondary SO 4 2− aqueous-phase formation rate, previous studies have mainly focused on haze pollution periods [11][12][13][14][15][16][17].Few studies have explored the secondary SO 4 2− aqueous-phase formation rate under the condition of low PM 2.5 concentration.In particular, in China, after the implementation of the Clean Air Act and the strengthening of the government resolve on air pollution, the PM 2.5 concentration decreased to low levels.However, the changes in secondary SO 4 2− aqueous-phase formation rates remain unclear.According to the 14th Five-Year Plan of Hainan Province, the annual average PM 2.5 concentration in Hainan Province should be lower than 11 µg m −3 .However, the average PM 2.5 concentration in recent years in Haikou has been 16.7 µg m −3 , especially in winter, during which the PM 2.5 concentration can be as high as 30 µg m −3 .As a pilot zone for ecological conservation and a free trade port with Chinese characteristics, Hainan Province still has the daunting task of managing its atmospheric PM 2.5 to establish its ecological environment as a world leader.Previous observations in Haikou in 2011-2012 found that SO 4 2− was the most abundant inorganic ion in PM 2.5 [18], but the formation mechanisms and sources of secondary SO 4 2− formation remain undocumented.The multi-resolution emission inventory for China (MEIC) reported that the total emissions of SO 2 in Hainan Province in 2020 was 36,114 t, and industry emissions were thought to be the dominant source [19,20].However, gas-phase SO 2 is not equal to SO 4 2− in PM 2.5 .If one wants to deeply understand the accumulation of secondary SO 4 2− in PM 2.5 , the first step is to clarify secondary SO 4 2− chemical formation mechanisms.In this study, we collected PM 2.5 samples from September 2021 to August 2022 and analyzed the concentrations of watersoluble inorganic ions, and water-soluble Fe and Mn.This study aims to (1) calculate the formation rate of secondary SO 4 2− using chemical kinetic models and (2) explore the influences of ionic strength and pH on secondary SO 4 2− formation rates.

Sampling
The sampling location was set on a rooftop (24 m above the ground) of the State Key Laboratory of Marine Resource Utilization in the South China Sea, Hainan University (20 • 06 N,110 • 32 E), Haikou, China (Figure S2).PM 2.5 samples were collected using a high-volume sampler equipped with a PM 2.5 cascade impactor and quartz filters (TISSUQUARTZ-2500QAT-UP, PALL Corporation, New York, NY, USA).The sampling duration was 24 or 48 h.Before sampling, all filters were combusted at 450 • C for 6 h in a muffle furnace.After sampling, the filters were stored in clear Ziplock bags and immediately refrigerated at −20 • C. A total of 200 PM 2.5 samples were obtained from 1 September 2021 to 30 August 2022.We divided the sampling periods into four seasons (autumn (September to November 2021), winter (December 2021 to February 2022), spring (March to May 2022), and summer (June to August 2022)).The meteorological parameters (temperature and RH) were downloaded from http://www.weather.com.cn(accessed from 1 September 2021 to 30 August 2022), and there were no significant seasonal variations for RH in contrast with obvious seasonal differences for temperature (Figure S3a).Hourly air pollutants (such as PM 2.5 , PM 10 , SO 2 , NO 2 , O 3 , CO, Figure S3b-d) were obtained from an air quality monitoring station (https://map.zq12369.com;accessed from 1 September 2021 to 30 August 2022) located 300 m from our sampling site.

Chemical Analysis
Analysis of Water-Soluble Ions, and Water-Soluble Fe and Mn One-quarter of the filters were placed in clear 50 mL centrifuge tubes, and 30 mL of Milli-Q water (18.2Ω) was added to immerse the filters, followed by ultrasound for 30 min and then rest for 30 min at room temperature.The extracts were filtered through a membrane filter (0.22 µm).Then, 5 mL extracts were used for water-soluble ion measurements and 9.5 mL extracts were used for water-soluble Fe and Mn analysis.The water-soluble ions (Na + , NH 4 + , K + , Mg 2+ , Ca 2+ , Cl − , NO 3 − , SO 4 2− ) were analyzed by Ion Chromatography (DIONEX AQ-1100, Dionex Aquion RFIC ThermoFisher, CS12A for cations, and AS22 for anions).The anionic eluents comprised 4.5 mM Na 2 CO 3 mixed with 1.4 mM NaHCO 3 , and the cationic eluents comprised 20 mM methanesulfonic acid (MSA).The concentration of each ion in the blank was subtracted from the measured ion concentrations of each sample to remove possible contamination during the test.
The water-soluble Fe and Mn were analyzed by inductively coupled plasma optical emission spectrometry (Agilent 5100, Agilent Technologies Inc., Santa Clara, CA, USA).Before analysis, 0.5 mL of pure nitric acid (65%) was added to the 9.5 mL extracts.All concentrations were corrected for background concentrations with duplicate filter blanks.The detection limits for water-soluble Fe and Mn were 0.1 µmol L −1 and 0.11 µmol L −1 , respectively (3 × blank standard deviation).The accuracy of instrument testing was ensured by inserting a quality control solution every 10 samples, and the 44 repeat analyses of quality control solutions for water-soluble Fe and Mn were 100 ± 0.3 µg L −1 .The concentration of nss-SO 4 2− in PM 2.5 is as follows:

Aerosol Water Content (AWC), Aerosol pH, and Ionic Strength
The AWC, aerosol pH, and ionic strength were calculated using ISORROPIA II.ISOR-ROPIA II calculates the compositions and phase state of Na + -K + -Ca 2+ -Mg 2+ -NH 4 + -SO 4 2− -NO 3 − -Cl − -H 2 O.This model (ISORROPIA v2.1) was developed by Athanasios Nenes and Christors Fountoukis at the University of Miami, Carnegie Mellon University, and the Georgia Institute of Technology.It has two input units (µmol m −3 air and µg m −3 air), two modes (forward and reverse modes), and two aerosol states (stable and metastable states).In this study, we chose µg m −3 air, forward mode, and metastable state and then input the concentration of water-soluble inorganic ions, RH, and thermodynamic temperature to calculate the AWC, aerosol pH, and ionic strength.Detailed information about ISORROPIA II can be in the work of Fountoukis and Nenes [23].ISORROPIA II has been widely used to calculate AWC, aerosol pH, and ionic strength [14,15,17,24,25].The aqueous-phase formation steps of secondary SO 4 2− include the transformation of SO 2 into S(IV) and the oxidation of S(IV) into secondary SO 4 2− by various oxidants.The detailed calculations are as follows.

The S(IV) Concentration
Assuming that gas-phase X is in equilibrium with aqueous X in aerosol water, the concentration of dissolved X ([X(aq)], (M)) can be expressed using Equation (3) [X(aq)] = H(X)p(X) (3) where X represents the concentration of SO 2 or other oxidants (NO 2 , H 2 O 2 , and O 3 ) and p(X) is the partial pressure of X in the atmosphere (atm).H(X) represents Henry's law constant for X, and the unit of Henry's law constant is M −1 atm −1 .The concentrations of SO 2 •H 2 O, HSO 3 − , and SO 3 2− in aerosol water are given by Equations ( 4)-(6) [26], [ where H SO2 is Henry's law constant for SO 2 (M −1 atm −1 ), [H + ] = 10 −pH M, and pH is calculated via ISORROPIA II.K s1 (M) and K s2 (M) are the first and second dissociation equilibrium constants for HSO 3 − and SO 3 2− , respectively.The detailed calculations for H SO2 , K s1 , and K s2 are described in Text S1.

The Oxidation Rate of S(IV) by H 2 O 2
The reaction rate of the S(IV)+H 2 O 2 pathway was given by Hoffmann and Calvert [29]: where k (S(IV)+H2O2)1 can be estimated using Equation ( 14) [11,26], When the temperature is 298 K, k (S(IV)+H2O2)1 (298 K) is equal to 7.45 × 10 7 M −1 s −1 .E/R (K) is listed in Table S2.α is equal to 13 M −1 [14].[H + ] (M) is the H + concentration, calculated using ISORROPIA II.[HSO 3 − ] (M) is the only species of S(IV) that reacts with H 2 O 2 to form secondary SO 4 2− , calculated using Equation (5).[H 2 O 2 ] is the mole fraction of H 2 O 2 (nmol mol −1 ), estimated using an empirical equation [14,30], where T is the ambient temperature ( • C).The influences of ionic strength on the reaction rate of the S(IV)+H 2 O 2 pathway are given in detail in Text S3.

Mass Transport Limitations Rate
The total reaction rate of oxidation of S(IV) into secondary SO 4 2− (R H,aq ) is affected by both the rate of chemical reactions (R aq ) and the rate of limiting mass transfer (J aq,lim ) in different media and across interfaces.By following Cheng et al. [11], where R H,aq (M s −1 ) is the total reaction rate of oxidation of S(IV) into secondary SO where 3600 s h −1 is the time conversion factor; 96 g mol −1 is the molar mass of SO 4 2− ; AWC is the aerosol water content (mg m −3 ), calculated using ISORROPIA II; ρ w is the density of water, which is 1 kg L −1 ; and R H,aq (M s −1 ) can be calculated using Equation (21).

Seasonal Differences in H2O2, AWC, Aerosol pH, Ionic Strength, Fe(III)×Mn(II) and S(IV)
The concentrations of H2O2 ranged from 0.3 to 1.5 ppb, displaying significant seasonal variations, with the highest values in summer (1.3 ± 0.1 ppb) and the lowest values In general, Fe in aerosol is entirely sourced from mineral dust, while Mn originates from both mineral dust and anthropogenic activities [37,38].The concentrations of water-soluble Fe and Mn were 7.4 ± 6. and temporal differences in water-soluble Fe and Mn are influenced by many factors, such as sources, aerosol aging, and aerosol acidity [43].

The Aqueous-Phase Formation Rates of Secondary SO 4 2−
Figure 3 shows the daily secondary SO 4 2− aqueous-phase formation rates by various formation pathways.The results showed that secondary SO 4 2− aqueous-phase formation rates by the S(IV)+NO 2 , S(IV)+H 2 O 2 , and S(IV)+Fe×Mn pathways were 4-7 orders of magnitude faster than the S(IV)+O 3 , S(IV)+Fe, and S(IV)+Mn pathways.Previous studies during Beijing haze periods found that S(IV)+O 3 was an important pathway for secondary SO 4 2− production when pH > 5.8 [11,51].As shown in Figure S5, when the pH is between 4.5 and 8, S(IV) partitioning shifts in favor of SO 3  2− , and the rate constants for SO 3 2− +O 3 (k (S(IV)+O 3 )3 = 1.5 × 10 9 M −1 s −1 ) are almost 4-5 orders of magnitude faster than for SO 2 •H 2 O+O 3 (k (S(IV)+O 3 )1 ) and HSO 3 − +O 3 (k (S(IV)+O 3 )2 (Table S2), highlighting that the S(IV)+O 3 pathway is important for secondary SO 4 2− when aerosol pH > 4.5.However, in our observations, the pH range was 0.05-4.3(Figure 2c), SO 2 •H 2 O and HSO 3 − were the dominant species of S(IV) (Figure S5), and the rate constants of SO 2 •H 2 O+O 3 and HSO 3 − +O 3 were significantly lower than S(IV)+NO 2 , S(IV)+H 2 O 2 , and S(IV)+Fe×Mn (Table S2); thus, we ignored the S(IV)+O 3 pathway for secondary SO 4 2− formation.Using model simulations and in-field observations, previous studies also found that secondary SO 4 2− production by the S(IV)+O 3 pathway was unimportant under conditions with pH < 4.5 [10,11,51,52].In addition, we also neglected the S(IV)+Fe and S(IV)+Mn pathways for secondary SO 4 2− formation due to their extremely low rate constants (Table S2).Although rate constants k (Fe +O2)2 under pH range of 3 and 4.5 were high in the S(IV)+Fe×Mn pathway (Table S2), the extremely low concentrations of Fe(III) (Figure S6) limited secondary SO 4 2− formation by k (Fe+O2)2 [Fe(III)] 2 [S(IV)] under pH range from 3 to 4.5.The aqueous-phase formation rates of secondary SO 4 2− by S(IV)+Mn reactions also displayed low rates, which was attributed to the lowest rate constant of k (Mn+O2) (Table S2).Therefore, we did not consider the S(IV)+O 3 , S(IV)+Fe, and S(IV)+Mn pathways due to their low contributions to secondary SO 4 2− production.
+O2)2 under pH range of 3 and 4.5 were high in the S(IV)+Fe×Mn pathway (Table S2), the extremely low concentrations of Fe(III) (Figure S6) limited secondary SO4 2− formation by k(Fe+O2)2 [Fe(III)] 2 [S(IV)] under pH range from 3 to 4.5.The aqueous-phase formation rates of secondary SO4 2− by S(IV)+Mn reactions also displayed low rates, which was attributed to the lowest rate constant of k(Mn+O2) (Table S2).Therefore, we did not consider the S(IV)+O3, S(IV)+Fe, and S(IV)+Mn pathways due to their low contributions to secondary SO4 2− production.

S(IV)+NO 2 Pathway Formation Rates and their Influencing Factors
The secondary SO 4 2− formation rates by the S(IV)+NO 2 pathway exhibited ranges of 4.2 × 10 −8 -6.3 × 10 −2 µg m −3 h −1 in autumn, 1.9 × 10 −7 -1.6× 10 −1 µg m −3 h −1 in winter, 5.6 × 10 −8 -2.6 × 10 −1 µg m −3 h −1 in spring, and 4.3 × 10 −7 -2.7 × 10 −2 µg m −3 h −1 in summer (Figure 3).Our calculated secondary SO 4 2− formation rates by the S(IV)+NO 2 pathway in autumn and winter were lower than those from previous studies in Beijing (2.0 × 10 −4 -5.9 µg m −3 h −1 ) [17] and Tianjin (~6.0 µg m −3 h −1 ) [14].In addition to the effect of substrate (S(IV) and NO 2 ) concentrations, ionic strength was the main factor that modified secondary SO 4 2− formation by the S(IV)+NO 2 pathway.As shown in Figure 4a, when ionic strength increased, the secondary SO 4 2− formation rate by the S(IV)+NO 2 pathway increased, which is consistent with previous studies [10,53].Two possible mechanisms have been proposed to explain secondary SO 4 2− formation by the S(IV)+NO 2 pathway.The first is an oxygen atom transfer reaction [28]: Toxics 2024, 12, x FOR PEER REVIEW 10 of 17 concentrations, ionic strength was the main factor that modified secondary SO4 2− formation by the S(IV)+NO2 pathway.As shown in Figure 4a, when ionic strength increased, the secondary SO4 2− formation rate by the S(IV)+NO2 pathway increased, which is consistent with previous studies [10,53].Two possible mechanisms have been proposed to explain secondary SO4 2− formation by the S(IV)+NO2 pathway.The first is an oxygen atom transfer reaction [28]: The second is an electron transfer reaction, followed by the reaction of hydroxyl radical with a sulfite radical [54]: For both mechanisms, the controlling step is the reaction of an ion with a neutral molecule [11].The positive trends between the chemical rate constant kS(IV)+NO2 and ionic strength (Figure S7) as well as the secondary SO4 2− formation rates by the S(IV)+NO2 pathway and ionic strength (Figure 4a) support the fact that increasing ionic strength enhances secondary SO4 2− formation rates by the S(IV)+NO2 pathway [14,53,55].

S(IV)+H2O2 and S(IV)+Fe×Mn Pathway Formation Rates and Their Influencing Factors
The secondary SO4 2− formation rates by the S(IV)+H2O2 pathway in winter (3.0 × 10 −4 -9.5 × 10 −2 µg m −3 h −1 ) were higher than those in summer (2.6 × 10 −5 -9.1 × 10 −4 µg m −3 h −1 , Figure 3), which is consistent with a previous study in Tianjin, revealing higher values in winter than summer [14].By contrast, the S(IV)+ Fe×Mn pathway was more prevalent in summer ( 2  The second is an electron transfer reaction, followed by the reaction of hydroxyl radical with a sulfite radical [54]: For both mechanisms, the controlling step is the reaction of an ion with a neutral molecule [11].The positive trends between the chemical rate constant k S(IV)+NO2 and ionic strength (Figure S7) as well as the secondary SO 4 2− formation rates by the S(IV)+NO 2 pathway and ionic strength (Figure 4a) support the fact that increasing ionic strength enhances secondary SO 4 2− formation rates by the S(IV)+NO 2 pathway [14,53,55].
Different from the S(IV)+NO 2 pathway, there were no positive or negative relationships between ionic strength and secondary SO 4 2− formation rates by S(IV)+H 2 O 2 and S(IV)+Fe×Mn pathways (Figure 4b,c), indicating that ionic strength has less of an influence on the S(IV)+H 2 O 2 and S(IV)+Fe×Mn pathways to produce secondary SO 4 2− .The positive correlation between aerosol pH and secondary SO 4 2− formation rates by S(IV)+H 2 O 2 (Figure 5a) emphasizes that aerosol pH is the primary controlling factor for the S(IV)+H 2 O 2 pathway.The chemical rate constant k S(IV)+H2O2 is a function of pH; when the pH value increased from 0 to 2, the chemical rate constant k S(IV)+H2O2 showed an increasing trend (Figure S8a).In our in situ observations, when pH < 2, the secondary SO 4 2− formation rate by the S(IV)+H 2 O 2 pathway displayed a positive relationship with the pH value (Figure 5a).This phenomenon is supported by the simultaneous increase in the chemical rate constant k S(IV)+H2O2 (Figure S8a) and HSO 3 − concentrations (Figure S5) as pH increased from 0 to 2. In addition, our observed results showed that secondary SO 4 2− formation rates by the S(IV)+H 2 O 2 pathway under pH of 2~3 were higher than those under pH of 0~2 (Figure 5a).The influence of the chemical rate constant k S(IV)+H2O2 on the secondary SO 4 2− formation rate can be excluded due to k S(IV)+H2O2 at pH 0~2 being close to those at pH 2~3 (Figure S8a).However, the percentages of HSO 3 − to S(IV) at a pH of 2~3 (50-90%) were higher than those at pH of 0~2 (~50%, Figure S5).HSO 3 − is a unique species of S(IV) that can react with H 2 O 2 to form secondary SO 4 2− [29], which explains the high secondary SO 4 2− formation rates by the S(IV)+H 2 O 2 pathway [10,26].In addition, the higher pH in winter (Figure 2c) results in higher HSO 3 − concentrations than in other seasons, supporting the faster SO 4 2− formation rates by the S(IV)+H 2 O 2 pathway in winter than in other seasons (Figure 5a).
However, the percentages of HSO3 − to S(IV) at a pH of 2~3 (50-90%) were higher than those at pH of 0~2 (~50%, Figure S5).HSO3 − is a unique species of S(IV) that can react with H2O2 to form secondary SO4 2− [29], which explains the high secondary SO4 2− formation rates by the S(IV)+H2O2 pathway [10,26].In addition, the higher pH in winter (Figure 2c) results in higher HSO3 − concentrations than in other seasons, supporting the faster SO4 2− formation rates by the S(IV)+H2O2 pathway in winter than in other seasons (Figure 5a).The pH also has a direct influence on the secondary SO4 2− formation rate by the S(IV)+Fe×Mn pathway.pH determines not only the water-soluble concentrations of Fe(III) but also the hydrogen ion concentrations [56,57].When pH < 2.4, there was a positive relationship between the secondary SO4 2− formation rate by the S(IV)+Fe×Mn pathway and the pH value (Figure 5b).This phenomenon can be attributed to (1) the concentrations of water-soluble Fe(III) and Mn(II) maintaining the highest levels at low pH (Figure S6), (2) the S(IV) concentrations increasing as pH increased from 0 to 2.4 (Figure S8b), and (3) the concentrations of Fe(III) and Mn(II) being influenced not only by pH but also by the AWC.Lower AWC concentrations result in higher Fe(III) and Mn(II) concentrations, favoring secondary SO4 2− formation [10,15,58].This also explains the high rate of secondary SO4 2− formation by the S(IV)+Fe×Mn pathway in summer.The pH also has a direct influence on the secondary SO 4 2− formation rate by the S(IV)+Fe×Mn pathway.pH determines not only the water-soluble concentrations of Fe(III) but also the hydrogen ion concentrations [56,57].When pH < 2.4, there was a positive relationship between the secondary SO 4 2− formation rate by the S(IV)+Fe×Mn pathway and the pH value (Figure 5b).This phenomenon can be attributed to (1) the concentrations of water-soluble Fe(III) and Mn(II) maintaining the highest levels at low pH (Figure S6), (2) the S(IV) concentrations increasing as pH increased from 0 to 2.4 (Figure S8b), and (3) the concentrations of Fe(III) and Mn(II) being influenced not only by pH but also by the AWC.Lower AWC concentrations result in higher Fe(III) and Mn(II) concentrations, favoring secondary SO 4 2− formation [10,15,58].This also explains the high rate of secondary SO 4 2− formation by the S(IV)+Fe×Mn pathway in summer.
3.6.Comparison of Secondary SO 4 2− Formation Rates under Different PM 2.5 Levels The relative contributions of the S(IV)+NO 2 , S(IV)+H 2 O 2 , and S(IV)+Fe×Mn pathways to total secondary SO 4 2− production in different seasons are shown in Figure 6.In autumn and spring, the relative contributions of each pathway were comparable.In winter, the S(IV)+H 2 O 2 pathway was the main formation pathway, contributing to 57% of secondary SO 4 2− production, but in summer, the S(IV)+Fe×Mn pathway was the largest contributor (54%).A study in Tianjin also reported that the S(IV)+H 2 O 2 pathway played a dominant role in wintertime secondary SO 4 2− production (71%), while S(IV)+Fe×Mn was the main pathway for secondary SO 4 2− production in summer (55%) [14].In addition, the contribution of the S(IV)+H 2 O 2 and S(IV)+NO 2 pathways to total secondary SO 4 2− production had the lowest percentages (17% in summer and 13% in winter).
Toxics 2024, 12, 105 13 of 18 SO4 2− production, but in summer, the S(IV)+Fe×Mn pathway was the largest contribu (54%).A study in Tianjin also reported that the S(IV)+H2O2 pathway played a domin role in wintertime secondary SO4 2− production (71%), while S(IV)+Fe×Mn was the m pathway for secondary SO4 2− production in summer (55%) [14].In addition, the contri tion of the S(IV)+H2O2 and S(IV)+NO2 pathways to total secondary SO4 2− production h the lowest percentages (17% in summer and 13% in winter).To understand the secondary SO4 2− aqueous-phase formation pathways in differ regions under different air pollution levels in China, we compared the published datas of secondary SO4 2− formation rates and their relative contributions in different cities a seasons (Table 1).During winter, under PM2.5 concentrations < 75 µg m −3 , our observati in Haikou showed that S(IV)+H2O2 was the main secondary SO4 2− formation pathw while a previous study in Beijing reported that S(IV)+Fe×Mn was the fastest pathway secondary SO4 2− production [15].The discrepancies between this study and the previ study in Beijing may be related to the selection of calculating parameters.In our obser tions, we measured the water-soluble concentrations of Fe and Mn and estimated the H concentrations using an empirical equation [14,30].However, in Song et al.'s study, authors measured H2O2 concentrations but cited the water-soluble concentrations of and Mn from other studies [15].By further comparing the calculation processes in diff ent studies, we found that the authors of these studies did not directly measure the wa soluble concentrations of Fe and Mn; instead, they assumed concentrations [11,15,59] addition, secondary SO4 2− formation in Guangzhou and Zhengzhou exhibited differ pathways, with these differences potentially being a result of the assumed parame [58,60,61].Thus, synchronous analyses of multiple chemical parameters (such as g phase NO2 and O3, as well as water-soluble ions) are an important basis for the accur calculation of secondary SO4 2− formation rates during winter haze periods.
Surprisingly, a limited number of summarized datasets have shown that S(IV)+Fe×Mn pathway is the highest contributor to secondary SO4 2− formation in summ in both southern and northern cities of China under different PM2.5 levels (Table 1).Th To understand the secondary SO 4 2− aqueous-phase formation pathways in different regions under different air pollution levels in China, we compared the published datasets of secondary SO 4 2− formation rates and their relative contributions in different cities and seasons (Table 1).During winter, under PM 2.5 concentrations < 75 µg m −3 , our observations in Haikou showed that S(IV)+H 2 O 2 was the main secondary SO 4 2− formation pathway, while a previous study in Beijing reported that S(IV)+Fe×Mn was the fastest pathway for secondary SO 4 2− production [15].The discrepancies between this study and the previous study in Beijing may be related to the selection of calculating parameters.In our observations, we measured the water-soluble concentrations of Fe and Mn and estimated the H 2 O 2 concentrations using an empirical equation [14,30].However, in Song et al.'s study, the authors measured H 2 O 2 concentrations but cited the water-soluble concentrations of Fe and Mn from other studies [15].By further comparing the calculation processes in different studies, we found that the authors of these studies did not directly measure the water-soluble concentrations of Fe and Mn; instead, they assumed concentrations [11,15,59].In addition, secondary SO 4 2− formation in Guangzhou and Zhengzhou exhibited different pathways, with these differences potentially being a result of the assumed parameters [58,60,61].Thus, synchronous analyses of multiple chemical parameters (such as gas-phase NO 2 and O 3 , as well as water-soluble ions) are an important basis for the accurate calculation of secondary SO 4 2− formation rates during winter haze periods.Surprisingly, a limited number of summarized datasets have shown that the S(IV)+Fe×Mn pathway is the highest contributor to secondary SO 4 2− formation in summer in both southern and northern cities of China under different PM 2.5 levels (Table 1).These results may be explained by the relatively high Fe(III) and Mn(II) concentrations due to the low pH, which promotes secondary SO 4 2− formation [14,51].In addition, a low AWC causes relatively higher water-soluble Fe(III) and Mn(II) levels, further enhancing the reactivity of the S(IV)+Fe×Mn pathway in summer [10,15].Although secondary SO 4 2− formation by the S(IV)+Fe×Mn pathway in Haikou had two to three orders of magnitude difference compared to other studies in Wangdu and Tianjin, the relative contributions of the S(IV)+Fe×Mn pathway to total secondary SO 4 2− formation are comparable with Tianjin.

Figure 1 .
Figure 1.The percentages of water-soluble inorganic ions in PM 2.5 in autumn (a), winter (b), spring (c), and summer (d) in Haikou.

Figure 2 .
Figure 2. Box plots of seasonal H 2 O 2 concentrations (a), AWC (b), pH (c), ionic strength (d), and Fe(III)×Mn(II) concentrations (e).The yellow, blue, green, and red boxes represent autumn, winter, spring, and summer, respectively.The large boxes indicate the interquartile range from the 25th to 75th percentile.The dashed line inside the box indicates the average value.The solid line indicates the median value and whiskers indicate the 10th and 90th percentiles.

Figure 3 .
Figure 3.Time series of secondary SO4 2− formation rates by six different SO4 2− aqueous-phase formation pathways, made on the first day of a month.

Figure 3 .
Figure 3.Time series of secondary SO 4 2− formation rates by six different SO 4 2− aqueous-phase formation pathways, made on the first day of a month.

Figure 5 .
Figure 5.A scatter plot of aerosol pH with secondary SO 4 2− formation rates by the S(IV)+H 2 O 2 (a) and S(IV)+Fe×Mn (b) pathways.The solid circles represent pH < 2 (a) and pH < 2.4 (b), and the open circles indicate pH > 2 (a) and pH > 2.4 (b).