Heterogeneous Reactions Of SO2 Research Articles

Heterogeneous photochemical uptake of SO2 on typical brown carbon species: A significant sulfate source

The heterogeneous reactions of SO2 on organic aerosols (OA) have been proposed as a significant source of sulfates. The photochemical conversion of SO2 to sulfates on the surface of light-absorbing OA, i.e., brown carbon (BrC), remained slight understanding. Herein, the photochemical uptake of SO2 on typical BrC species, including aromatic carbonyl compounds (ACs), polycyclic aromatic hydrocarbons (PAHs) and phenols (PHs), was systematically investigated by a flow tube reactor. Light can significantly promote the uptake of SO2 on all BrC species used here, especially for ACs. The initial and steady-state uptake coefficients (γi and γss) of SO2 exhibited positive relationships with the light intensity, whereas they negatively correlated with the initial SO2 concentration. Significant sulfate formation was clearly observed with in situ attenuated total internal reflection infrared (ATR-IR) spectrometer, and H2O and O2 had obvious enhancement effects on the sulfate production. Photoinduced •OH, which primarily originated from •O2− that was generated through an electron transfer process on BrC under irradiation, was determined to be a key driver for the oxidation of SO2 to sulfates. Finally, SO2 lifetime and sulfate formation rate were evaluated on the basis of experimental results, highlighting potential contributions of BrC photochemistry to atmospheric SO2 loss and sulfate sources.

The influence of phenanthrene on the adsorption and conversion of SO2 on the hydroxylated {001} surface of α-quartz: A DFT study

One of the major challenges in environmental and atmospheric science is understanding the effects of mineral dust’s surface physiochemical properties and pre-adsorbed volatile organic compounds on the conversion of atmospheric pollutants. However, the process of adsorption and conversion of polluting gases on typical mineral dust surface is not fully understood. In this study, we systematically investigated the impact of surficial hydroxyl groups on the α-quartz {001} surface, oxidizers, and adsorbed phenanthrene (Phe) on the adsorption and oxidation process of SO2 by using density functional theory. Our findings indicate that the presence of hydroxyl groups on the α-quartz {001} surface increases SO2 adsorption whereas the presence of Phe decreases the SO2 capture capabilities. On pristine α-quartz {001} surfaces, SO2 tends to form dissociative SO42- groups and adsorb SO3. While surficial hydroxylation of α-quartz {001} facilitates the desorption of SO3, resulting in fewer dissociative SO42- groups and HSO3- salt. These results are consistent with our heterogeneous reaction of SO2 and α-quartz. In addition, O3 is more effective than O2 in the oxidation of SO2. Meanwhile, Phe inhibits the adsorption of HSO3- salt on hydroxylated α-quartz {001} surfaces. This study not only highlights the process surficial hydroxyl groups and adsorbed Phe regulate the evolution of adsorbed SO2 on α-quartz {001}, but also revealed the formation mechanism of the sulfur-containing aerosols.

Study on the characteristics of sulfate ion in condensable particulate matter from ultra-low emission coal-fired power plants

The sulfate ion (SO42−) in condensable particulate matters (CPM) discharged by ultra-low emission coal-fired power plants has caused a series of environmental problems. In this work, condensation temperature, H2O, NOx and NH3 were investigated to elucidate the characteristics of SO42− under fast condensation. The results demonstrated that SO3 was a crucial component for the formation of SO42− in CPM, and the SO42− concentration increased with the increase of proportion of deposited H2SO4. The SO42− produced by heterogeneous reaction of SO2 could be enhanced in low temperature, and SO2 and SO3 could jointly promote the formation of SO42−. Furthermore, H2O vapor could also strengthen the formation of SO42−, which was caused by the Stefan flow. NH3 acted as the core, and the formation of SO42− was promoted. NOx could provide an acidic environment, which would interfere with the state of SO42− and even suppress the aqueous-phase oxidation of SO2. Finally, the formation of fine particles would aggravate the capture difficulty of CPM, which might help to explain the decrease of SO42− concentration in real flue gas. The results could provide basic guidance for improving the control of SO42− in CPM emitted from coal-fired power plants.

Impact of flow velocity on the heterogeneous reaction of SO2 over Fe2O3

Flow velocity, potentially linked to dry deposition velocity and wind speed parameters, show non-marginal significance in the heterogeneous oxidation of SO2 on α-Fe2O3 particles. Our uptake measurement results produce lab-based evidence that an increase in flow velocity can potentially strengthen SO2 uptake capability over α-Fe2O3 by nearly one order of magnitude within the range of considered reaction conditions. Additionally, specimen analysis of surface S-containing products indicates a distinct SO2 oxidation regime, shifting from the heterogeneous reaction mediated by active sites under low flow velocity and low RH to an aqueous-like multiphase-dominated pathway under high flow velocity and high RH. An attempt was further made to link this experimental parameter to dry deposition velocity regulated by wind force in the atmosphere. This work highlights the significance of flow velocity in triggering fast sulfate production concerning dust chemistry.

Heterogeneous reaction of SO2 on CaCO3 particles: Different impacts of NO2 and acetic acid on the sulfite and sulfate formation

Despite the heterogeneous reaction of sulfur dioxide (SO2) on mineral dust particles significantly affects the atmospheric environment, the effect of acidic gases on the formation of sulfite and sulfate from this reaction is not particularly clear. In this work, using the in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique, we employed a mineral dust particle model (CaCO3) combined with NO2 and acetic acid to investigate their effects on the heterogeneous reaction of SO2 on CaCO3 particles. It was found that water vapor can promote the formation of sulfite and simulated radiation can facilitate the oxidation of sulfite to sulfate. The addition of NO2 or acetic acid to the reaction system altered the production of sulfate and sulfite accordingly. There was a synergistic effect between NO2 and SO2 that promoted the oxidation of sulfite to sulfate, and a competitive effect between acetic acid and SO2 that inhibited the formation of sulfite. Moreover, light and water vapor can also affect the heterogeneous reaction of SO2 with the coexistence of multiple gases. These findings improve our understanding of the effects of organic and inorganic gases and environmental factors on the formation of sulfite and sulfate in heterogeneous reactions.

Effects of relative humidity on heterogeneous reaction of SO2 with CaCO3 particles and formation of CaSO4·2H2O crystal as secondary aerosol

Haze related air pollution has become one of major environmental concerns in some Asian countries. Sulfate is a main component of ambient particulate matter (PM) in the urban environment during haze episodes. Among the pollutants, sulfur dioxide (SO2) is an important precursor of sulfate and new atmospheric particle formation. However, under different atmospheric conditions its underlying formation mechanisms are not clearly elucidated. In the current work, we collected ambient PM in two cities: Zurich (Switzerland) and Beijing (China). The PM morphology and the composition were investigated by scanning electron microscopy, energy dispersive X-Ray spectroscopy and Raman micro-spectroscopy. In addition, a series of lab controlled experiments were also performed to study the SO2 reaction with CaCO3 aerosol. We found different types of CaSO4·2H2O crystals in the Chinese urban samples, with clearly different compositions than those from Zurich. The experimental data showed that the relative humidity played a significant role on the new CaSO4·2H2O crystal formation including its size, morphology and composition. Relative humidity (RH) above 80% can significantly promote SO2 oxidation on the CaCO3 particles and form the CaSO4·2H2O crystals. In contrast, at relative humidity below 40%, only few CaCO3 particles can be converted to CaSO4 particles. The results of this study facilitate the understanding of secondary inorganic aerosol formation by the reaction of CaCO3 particles with SO2 with different RHs in different city environments, and provide useful information for air pollution control.

Photocatalytic Oxidation of SO2 by TiO2: Aerosol Formation and the Key Role of Gaseous Reactive Oxygen Species.

Photocatalytic materials are proved to effectively eliminate gaseous pollutants and are widely used in the environment. However, as one of the rare experiments focusing on their influence on secondary aerosol formation generated in the gas phase (SAg), our study demonstrated the high-yield SAg formation in the photocatalysis process. In this study, the photodegradation of SO2 by TiO2 under various relative humidity (RH) conditions was deeply explored with multiple methods. Unexpectedly, H2SO4 aerosols (SAg-H2SO4) in yields of 10.10-32.64% were observed under the studied RH conditions for the first time. Gaseous •OH and H2O2 generated from the oxidation of H2O and reduction of O2 by TiO2 were directly detected in the photocatalysis process, and they were identified as the determining factor for SAg-H2SO4 formation. The formation of SAg-H2SO4 was also influenced by RH, the heterogeneous reaction of SO2, and the uptake of H2SO4. The role of the released gaseous •OH and H2O2 on atmospheric chemistry was proved to be unignorable by adopting the obtained parameters into the real environment. These findings provided direct experimental evidence of secondary pollution in the photocatalysis process and are of great significance to the field of atmospheric environment and photocatalytic materials.

Impact of greenhouse gas CO2 on the heterogeneous reaction of SO2 on alpha-Al2O3

The heterogeneous reaction of SO2 on mineral dust surfaces is generally considered as an important chemical pathway for secondary sulfate formation in the troposphere. To this day, there are no reported studies that assess the impact of atmospheric CO2 in sulfate production on mineral dust surfaces. In this work, we investigate the impact of CO2 on SO2 uptake on dust proxy aluminum oxide particles using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). CO2 is demonstrated to suppress the heterogeneous oxidation of SO2 on alpha-Al2O3. Compared to that measured in the CO2-free case, the uptake coefficient is decreased by nearly 57% when Al2O3 particles are exposed to the gas flow with atmospheric CO2 at a relative humidity (RH) of 25%. It is also found that there is a balance between the yield of active moiety −OH provided by Al(OH)3(CO)(OH)2 clusters and the loss of basic hydroxyl group on aluminum oxide surfaces blocked by CO2-derived (bi)carbonate species. This work, for the first time, reveals a negative effect of atmospheric CO2 on the sulfate formation, which potentially decreases solar-radiation scattering and further exacerbates global warming.

Effects of NO<sub>2</sub> and C<sub>3</sub>H<sub>6</sub> on the heterogeneous oxidation of SO<sub>2</sub> on TiO<sub>2</sub> in the presence or absence of UV–Vis irradiation

Abstract. The heterogeneous reactions of SO2 in the presence of NO2 and C3H6 on TiO2 were investigated with the aid of in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) under dark conditions or with UV–Vis irradiation. Sulfate formation with or without the coexistence of NO2 and/or C3H6 was analyzed with ion chromatography (IC). Under dark conditions, SO2 reacting alone resulted in sulfite formation on TiO2, while the presence of parts per billion (ppb) levels of NO2 promoted the oxidation of SO2 to sulfate. The presence of C3H6 had little effect on sulfate formation in the heterogeneous reaction of SO2 but suppressed sulfate formation in the heterogeneous reaction of SO2 and NO2. UV–Vis irradiation could significantly enhance the heterogeneous oxidation of SO2 on TiO2, leading to copious generation of sulfate, while the coexistence of NO2 and/or C3H6 significantly suppressed sulfate formation in experiments with UV–Vis lights. Step-by-step exposure experiments indicated that C3H6 mainly competes for reactive oxygen species (ROS), while NO2 competes with SO2 for both surface active sites and ROS. Meanwhile, the coexistence of NO2 with C3H6 further resulted in less sulfate formation compared to introducing either one of them separately to the SO2–TiO2 reaction system. The results of this study highlighted the complex heterogeneous reaction processes that take place due to the ubiquitous interactions between organic and inorganic species and the need to consider the influence of coexisting volatile organic compounds (VOCs) and other inorganic gases in the heterogeneous oxidation kinetics of SO2.

Competitive reactions of SO2 and acetic acid on α-Al2O3 and CaCO3 particles

Heterogeneous reactions between gaseous pollutants and mineral particles have gradually become a research hotspot in the field of atmospheric chemistry. In this paper, competitive reactions between SO2 and acetic acid on the surface of α-Al2O3 and CaCO3 particles were studied by the diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) technique in dark and dry conditions. At the same time, the temporary evolution of the integrated absorbance of acetate and sulfite was investigated to further understand the interaction of SO2 and acetic acid on the mineral particles. On the surface of α-Al2O3 particles, acetate and sulfite can compete for surface-active sites, resulting in a decrease in the total amount of acetates. In dark and dry conditions, the effect of acetic acid on SO2 cannot be obtained by the DRIFTS method. On the surface of CaCO3 particles, SO2 can have a competitive impact on acetic acid by grabbing active sites, leading to a slight decrease of the amount of acetates. The heterogeneous reaction of SO2 can be impeded by coexisting acetic acid, resulting in a drastic reduction of the number of sulfites. It can be seen that the formation mechanisms of acetate and sulfite on the surface of different mineral particles in the atmosphere are different, which provides a variety of ideas and possibilities for the formation of related inorganic and organic salts in the atmosphere.

Contrary Role of H2O and O2 in the Kinetics of Heterogeneous Photochemical Reactions of SO2 on TiO2.

Although the heterogeneous conversion of SO2 on TiO2 has been considered a potential source of sulfate in the troposphere, little is known about the kinetics of SO2 uptake on TiO2 under ambient conditions. In this study, heterogeneous reactions of SO2 on a TiO2 surface at 298 K were investigated by a coated-wall flow tube reactor and in situ diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). The effect of UV irradiation, relative humidity (RH), and O2 on the uptake coefficients and adsorption amount of SO2 have been analyzed comprehensively. UV irradiation exhibits a slight effect on the initial uptake coefficient (γBET), which increases from 1.23 × 10-6 to 1.48 × 10-6 in air conditions and 5.89 × 10-7 to 6.42 × 10-7 in a N2 stream under dry conditions. Surface water exhibits a competition effect on the adsorption of SO2 on TiO2. An increase in RH (in the range of 0-75%) led to a decrease in the γBET and adsorption amount of SO2 both in the dark and the light reaction. In contrast, oxygen plays an important role in the uptake of SO2. Both the uptake coefficients and adsorption amount of SO2 on TiO2 decreased in the N2 stream compared to those in air conditions. Only sulfite was formed during the adsorption of SO2 in dark and dry conditions. The presence of UV irradiation was found to promote the conversion of SO2 to sulfate and the adsorption amount of sulfur species on the TiO2 surface. These results suggest that heterogeneous reaction on TiO2 as a sink for SO2 is not important in the atmosphere.

The effect of water on the heterogeneous reactions of SO2 and NH3 on the surfaces of α-Fe2O3 and γ-Al2O3

In this study, wall-coated flow tube, in situ diffuse reflectance infrared Fourier transform spectroscopy and ion chromatography were used to investigate the effect of water on the heterogeneous reactions of SO2 and NH3 on α-Fe2O3 and γ-Al2O3.

Heterogeneous reactions of SO2 on the hematite(0001) surface.

Heterogeneous reactions at the surfaces of mineral dusts represent a key process in the formation of atmospheric aerosols. To quantify the rate of aerosol formation in climate modeling as well as combat hazardous aerosols, a deep understanding of the mechanisms of these reactions is essential. In the present work, density functional theory calculations, including a Hubbard-like +U correction, were employed to elucidate the reaction between SO2 and the hematite(0001) surface. Three reaction conditions are considered: dry, wet, and aerobic. In the absence of water and oxygen, adsorption energies of SO2 on the clean Fe-O3-Fe-termination were found to be about -0.8 to -1.0 eV and resulted in the formation of an adsorbed SO3-like species. The addition of water leads to surface hydroxylation and has little effect on promoting the SO2 adsorption. Under such circumstances, an HSO3-like species was formed with a smaller adsorption energy of about -0.5 eV. By contrast, the presence of molecular oxygen enhances the SO2 adsorption significantly as the two species combine to form sulfate SO4 2-, with adsorption energies of -1.31 to -1.64 eV. The calculated vibrational frequencies of the adsorbate species provide insight into the surface bonding and a useful spectral fingerprinting for experimental measurements. These results elucidate the atomistic mechanism of the reaction between SO2 and hematite and highlight the important role of atmospheric O2 in the formation of sulfates.

Adsorption of SO2 on mineral dust particles influenced by atmospheric moisture

Moisture plays a crucial role in the heterogeneous formation of sulfur compounds on mineral dust particles. The heterogeneous uptake of SO2 under various humidity conditions, however, is not well explained. In this study, heterogeneous reaction of SO2 on hematite particles are investigated using an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The interaction of SO2 with H2O is divided into the initial moisture absorption stage and the subsequent heterogeneous oxidation stage, indicating physical adsorption and chemical conversion of SO2, respectively. Uptake coefficients were estimated by Brunauer-Emmett-Teller (BET) surface area and geometric surface area. For unreacted (fresh) particles, the coefficients peak at 33% RH and then decline with increasing RH in both stages, implying the competition between SO2 and large amount of adsorbed H2O besides the promoting effect of water. For hematite after in-situ exposure to SO2 (sulfated particles), the coefficients increase as a function of RH in moisture absorption stage, indicating SO2 adsorption with particle hygroscopic growth. Nevertheless, the coefficients exhibit similar variation with that for fresh particles in heterogeneous oxidation stage, highlighting the competition effect after H2O accumulation. Generally, H2O plays both positive and negative roles in the adsorption of SO2. Reaction on sulfated hematite is significantly influenced by previously formed sulfate compounds with low hygroscopicity. Moreover, sulfate products may influence particle acidity after H2O introduction, and further result in diverse existence forms of S(IV) species. The extremely great uptake capacity of sulfated particles in moisture absorption stage may offer opportunities to explain the severe haze in high RH.

The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles

Heterogeneous reactions of SO2 and O3 with CaCO3 particles were investigated at a series of relative humidity (RH, 1% to 90%) and 298K using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The uptake coefficients of SO2 on CaCO3 at different RHs were obtained for the first time. Our results proved that high RH could substantially promote the formation of sulfate, for which the highest concentration (80% RH and reaction time of 200min) and highest formation rate in stable stage (85% RH) were 14 times and 43 times that at 1% RH, respectively. The surface products, increment of concentration and formation rate of sulfate changed with RH which were due to the surface adsorbed water (SAW) on the particles. SAW could increase the reactive sites on the particles and thus accelerate the conversion of SO2 into sulfite, and sulfite could be oxidized rapidly. Liquid-like water layers formed on the particle surface could enhance the ion mobility and promote the aggregation of CaSO4 hydrates, which could expose more reactive sites and result in additional adsorption of SO2. Piecewise equations of uptake coefficient with RH were given and could be referred by model simulation. The results are of importance in understanding the explosive growth of sulfate during severe haze episodes accompanied with high RH.

Key Role of Nitrate in Phase Transitions of Urban Particles: Implications of Important Reactive Surfaces for Secondary Aerosol Formation

AbstractAmmonium sulfate (AS) and ammonium nitrate (AN) are key components of urban fine particles. Both field and model studies showed that heterogeneous reactions of SO2, NO2, and NH3 on wet aerosols accelerated the haze formation in northern China. However, little is known on phase transitions of AS‐AN containing haze particles. Here hygroscopic properties of laboratory‐generated AS‐AN particles and individual particles collected during haze events in an urban site were investigated using an individual particle hygroscopicity system. AS‐AN particles showed a two‐stage deliquescence at mutual deliquescence relative humidity (MDRH) and full deliquescence relative humidity (DRH) and three physical states: solid before MDRH, solid‐aqueous between MDRH and DRH, and aqueous after DRH. During hydration, urban haze particles displayed a solid core and aqueous shell at RH = 60–80% and aqueous phase at RH > 80%. Most particles were in aqueous phase at RH > 50% during dehydration. Our results show that AS content in individual particles determines their DRH and AN content determines their MDRH. AN content increase can reduce MDRH, which indicates occurrence of aqueous shell at lower RH. The humidity‐dependent phase transitions of nitrate‐abundant urban particles are important to provide reactive surfaces of secondary aerosol formation in the polluted air.

Synergistic effect among Cl2, SO2 and NO2 in their heterogeneous reactions on gamma-alumina

Severe haze in China has been a global concern in recent years. Most studies about the mechanism of haze formation mare only focused on the heterogeneous reactions of SO2 and NO2 on mineral aerosols. However, little is known about the role of molecular chlorine (Cl2) in those reactions. Here, we investigated the heterogeneous uptake of Cl2, SO2 and NO2 on γ-Al2O3 particles under different conditions using a quartz-based flow reactor. We found that the existence of γ-Al2O3 seed aerosols significantly promotes the formation of secondary chloride, sulfate and nitrate aerosols, and Cl2, NO2 and SO2 have synergistic effects when they react on γ-Al2O3 surface under humid condition. The results also shows that Cl2 can promote the formation of secondary sulfate and nitrate aerosols on γ-Al2O3 surface. Moreover, Cl2 is much easier to react with the surface of γ-Al2O3 and form secondary Cl− aerosol when comparing with NO2 and SO2, suggesting that Cl2 is of great importance in atmospheric chemistry, it has the potential to alter the surface properties (e.g., chemical composition and fraction) of mineral aerosol, enhance the production of secondary inorganic aerosols in the troposphere, and thus cause adverse effects on the climate and human health.

Size-resolved aerosol water-soluble ions during the summer and winter seasons in Beijing: Formation mechanisms of secondary inorganic aerosols

Size-segregated water-soluble ionic species (WSIs) were measured using an Anderson cascade impactor from Jul. to Aug. 2008 and from Dec. 2009 to Feb. 2010 in urban Beijing. The results showed that fine particles (PM2.1, Dp < 2.1 μm) accounted for ∼49% (summer) and ∼34% (winter) of the total particulate mass, and WSIs accounted for 23–82% of the mass concentration of PM2.1. Secondary inorganic aerosols (SIAs, the sum of SO42−, NO3− and NH4+) accounted for more than 30% of the fine particles, which were greatly elevated during particle pollution events (PM events), thereby leading to an alteration of the size distributions of SO42− and NO3− to nearly single fine-mode distributions peaking at 0.65–2.1 μm. This finding suggests that heterogeneous aqueous reactions were enhanced at high RH values. SIAs also increased during dust events, particularly for coarse mode SO42−, which indicated enhanced heterogeneous reactions on the dust surface. The positive matrix factorization (PMF) model was used to resolve the bulk mass size distributions into condensation, droplet, and coarse modes, representing the three major sources of the particles. The formation of SO42− was attributed primarily to in-cloud or aerosol droplet processes during summer (45%), and the heterogeneous reaction of SO2 on mineral dust surfaces was an important formation pathway during winter (45%). The formation pathways of NO3− in fine particles were similar to those of SO42−, where over 30% were formed by in-cloud processes. This work provides important field measurement-based evidence for understanding the formation pathway of secondary inorganic aerosols in the megacity of Beijing.

Heterogeneous reaction of SO2 with soot: The roles of relative humidity and surface composition of soot in surface sulfate formation

The conversion of SO2 to sulfates on the surface of soot is still poorly understood. Soot samples with different fractions of unsaturated hydrocarbons and oxygen-containing groups were prepared by combusting n-hexane under well-controlled conditions. The heterogeneous reaction of SO2 with soot was investigated using in situ attenuated total internal reflection infrared (ATR-IR) spectroscopy, ion chromatography (IC) and a flow tube reactor at the ambient pressure and relative humidity (RH). Water promoted SO2 adsorption and sulfate formation at the RH range from 6% to 70%, while exceeded water condensed on soot was unfavorable for sulfate formation due to inhibition of SO2 adsorption when RH was higher than 80%. The surface composition of soot, which was governed by combustion conditions, also played an important role in the heterogeneous reaction of SO2 with soot. This effect was found to greatly depend on RH. At low RH of 6%, soot with the highest fuel/oxygen ratio of 0.162 exhibited a maximum uptake capacity for SO2 because it contained a large amount of aromatic CH groups, which acted as active sites for SO2 adsorption. At RH of 54%, soot produced with a fuel/oxygen ratio of 0.134 showed the highest reactivity toward SO2 because it contained appropriate amounts of aromatic CH groups and oxygen-containing groups, subsequently leading to the optimal surface concentrations of both SO2 and water. These results suggest that variation in the surface composition of soot from different sources and/or resulting from chemical aging in the atmosphere likely affects the conversion of SO2 to sulfates.

Hydrogen peroxide maintains the heterogeneous reaction of sulfur dioxide on mineral dust proxy particles

The heterogeneous oxidation of sulfur dioxide (SO2) on α-Al2O3 particles was investigated using a flow reactor coupled with a transmission-Fourier transform infrared (T-FTIR) spectrometer at different relative humidities (RH) in the absence or presence of hydrogen peroxide (H2O2), with an emphasis on the saturation coverage of SO2 and the timescale on which the reaction reaches saturation. It is found that the saturation coverage of SO2 in the absence of H2O2 increases with rising RH due to the hydrolysis of SO2 by surface adsorbed water. However, the reaction ultimately reaches saturation since the produced sulfite/bisulfite cannot be further converted to sulfate/bisulfate in the absence of oxidants. In addition, the presence of H2O2 can significantly increase the saturation coverage of SO2 by efficiently oxidizing sulfite/bisulfite to sulfate/bisulfate. Under humid conditions, adsorbed water facilitates the hydrolysis of SO2 and mitigates the increase of surface acidity, which can inhibit the hydrolysis of SO2. Hence, in the presence of H2O2, the saturation coverage of SO2 as well as the time of reaction reaching saturation increases with rising RH and the surface is not saturated on the timescale of the experiments (40 h) at 60% RH. Furthermore, the increase of saturation coverage of SO2 in the presence of H2O2 was observed on chemically inactive SiO2 particles, indicating that the hydrolysis of SO2 and subsequent oxidation by H2O2 likely occurs on other types of particles. Our findings are of importance for understanding the role of water vapor and trace gases (e.g., H2O2) in the heterogeneous reaction of SO2 in the atmosphere.