Heterogeneous Oxidation Of SO2 Research Articles

Effect of NOX, O3 and NH3 on sulfur isotope composition during heterogeneous oxidation of SO2: a laboratory investigation

Effect of NOX, O3 and NH3 on sulfur isotope composition during heterogeneous oxidation of SO2: a laboratory investigation

Atmospheric environment characteristic of severe dust storms and its impact on sulfate formation in downstream city

This study comprehensively investigated the impact of dust storms (DSs) on downstream cities, by selecting representative DS events. In this paper, we discussed the characteristics of meteorological conditions, air pollutants, PM2.5 components, and their influence on sulfate formation mechanisms. During DSs, strong winds, reaching speeds of up to 10 m/s, led to significant increases in PM10 and PM2.5, with maximum concentrations of 2684.5 and 429 μg/m3, respectively. Primary gaseous pollutants experienced substantial reductions, with decline rates of 48.1, 34.9, 36.8, and 9.0 % for SO2, NO2, NH3, and CO, respectively. Despite a notable increase in PM2.5 concentrations, only 7.6 % of the total mass of PM2.5 was attributed to ionic and carbonaceous components, a much lower value than observed before the DSs (77.3 %). Concentrations of Fe, Ti, and Mn exhibited increases by factors of 6.5–14.1, 10.4–17.0, and 1.6–4.7, respectively. In contrast to the significant decrease of >76.2 % in nitrogen oxidation ratio (NOR), sulfur oxidation ratio (SOR) remained at a relatively high level, displaying a strong positive correlation with high concentrations of Fe, Mn, and Ti. Quantitative analysis revealed an average increase of 0.187 and 0.045 μg/m3 in sulfate from natural sources and heterogeneous generation, respectively. The heterogeneous reaction on mineral dust was closely linked to atmospheric humidity, radiation intensity, the form of metal existence, and concentrations of it. High concentrations of titanium dioxide and iron‑manganese oxides in mineral dust promoted heterogeneous oxidation of SO2 through photocatalysis during the daytime and metal ion catalysis during the nighttime. This study establishes that the metal components in mineral dust promote heterogeneous sulfate formation, quantifies the yield of sulfate generated as a result, and provides possible mechanisms for heterogeneous sulfate formation.

Unveiling the Role of Carbonate Radical Anions in Dust‐Driven SO2 Oxidation

AbstractCarbonate radical anion () is generally overlooked in atmospheric chemistry. Our recent work emphasizes the important role of carbonate radicals produced on mineral dust surfaces in fast sulfate production under solar irradiation in the presence of CO2 at specifically low RH and light intensity. Yet so far how involves and affects secondary sulfate production under diverse RH, light intensity, and complex constituent matrix remains unknown, which essentially limits our comprehensive knowledge of initiated SO2 oxidation scheme in the atmosphere. Herein, we explored the heterogeneous SO2 oxidation over both model and authentic dust and clays in the presence of CO2 at atmospheric relevant RHs and light intensities. Interestingly, we observe that CO2 promotes sulfate yield over authentic dust and clays at atmospheric‐relevant RH and light intensity. This observation relates to the favorable kinetic between SO2 oxidation and while auto‐quenching of these radical ions is largely minimized due to the sufficient sites of crustal constituents. Furthermore, employing a suite of authentic dust and machine learning strategies, we evaluated the relative importance of each constituent within airborne minerals or clays as well as environmental conditions including relative humidity, light intensity, and CO2 concentration in affecting SO2 uptake capability. On this basis, sulfate formation mediated by dust‐driven pathway, accounting for nearly ∼20.9% of overall contribution by the end of this century during some pollution episodes, even higher than gas‐phase (∼16.9%), will be increased by 163% if CO2‐initiated SO2 oxidation scheme is incorporated.

A study on the influence of inorganic ions, organic carbon and microstructure on the hygroscopic property of soot

Abstract. Soot is a crucial component of aerosols in the atmosphere. Understanding the hygroscopicity of soot particles is important for studying their role as cloud condensation nuclei (CCN) as well as their chemical behavior and atmospheric lifetime. However, there is still a lack of comprehensive understanding regarding the factors that determine the hygroscopic properties of soot. In this work, the hygroscopic behavior of soot particles generated from different types of fuel combustion and aged with SO2 for varying durations was measured by a vapor sorption analyzer. Various characterizations of soot were conducted to understand the key factors that influence the hygroscopic properties of soot. It was found that water-soluble substances in soot facilitate the completion of monolayer water adsorption at low relative humidity and increase the number of water adsorption layers at high relative humidity. On the other hand, soot prepared from fuel burning typically lacks water-soluble inorganic ions, and their hygroscopicity is primarily influenced by organic carbon (OC) and microstructure. Furthermore, the hygroscopicity of soot can be enhanced by the formation of sulfate due to heterogeneous oxidation of SO2. These finding sheds light on the critical factors that affect soot hygroscopicity during water adsorption and allows for estimating the interaction between water molecules and soot particles in a humid atmosphere.

Elucidating the Mechanism on the Transition-Metal Ion-Synergetic-Catalyzed Oxidation of SO2 with Implications for Sulfate Formation in Beijing Haze.

Currently, atmospheric sulfate aerosols cannot be predicted reliably by numerical models because the pathways and kinetics of sulfate formation are unclear. Here, we systematically investigated the synergetic catalyzing role of transition-metal ions (TMIs, Fe3+/Mn2+) in the oxidation of SO2 by O2 on aerosols using chamber experiments. Our results showed that the synergetic effect of TMIs is critically dependent on aerosol pH due to the solubility of Fe(III) species sensitive to the aqueous phase acidity, which is effective only under pH < 3 conditions. The sulfate formation rate on aerosols is 2 orders of magnitude larger than that in bulk solution and increases significantly on smaller aerosols, suggesting that such a synergetic-catalyzed oxidation occurs on the aerosol surface. The kinetic reaction rate can be described as R = k*[H+]-2.95[Mn(II)][Fe(III)][S(IV)] (pH ≤ 3.0). We found that TMI-synergetic-catalyzed oxidation is the dominant pathway of sulfate formation in Beijing when haze particles are very acidic, while heterogeneous oxidation of SO2 by NO2 is the most important pathway when haze particles are weakly acidic. Our work for the first time clarified the role and kinetics of TMI-synergetic-catalyzed oxidation of SO2 by O2 in haze periods, which can be parameterized into models for future studies of sulfate formation.

Critical Roles of Surface-Enhanced Heterogeneous Oxidation of SO2 in Haze Chemistry: Review of Extended Pathways for Complex Air Pollution

Critical Roles of Surface-Enhanced Heterogeneous Oxidation of SO2 in Haze Chemistry: Review of Extended Pathways for Complex Air Pollution

Effects of SO2 on NH4NO3 Photolysis: The Role of Reducibility and Acidic Products.

Nitrate photolysis is a vital process in secondary NOx release into the atmosphere. The heterogeneous oxidation of SO2 due to nitrate photolysis has been widely reported, while the influence of SO2 on nitrate photolysis has rarely been investigated. In this study, the photolysis of nitrate on different substrates was investigated in the absence and presence of SO2. In the photolysis of NH4NO3 on the membrane without mineral oxides, NO, NO2, HONO, and NH3 decreased by 17.1, 6.0, 12.6, and 57.1% due to the presence of SO2, respectively. In the photolysis of NH4NO3 on the surface of mineral oxides, SO2 also exhibited an inhibitory effect on the production of NOx, HONO, and NH3 due to its reducibility and acidic products, while the increase in surface acidity due to the accumulation of abundant sulfate on TiO2 and MgO promoted the release of HONO. On the photoactive oxide TiO2, HSO3-, generated by the uptake of SO2, could compete for holes with nitrate to block nitrate photolysis. This study highlights the interaction between the heterogeneous oxidation of SO2 and nitrate photolysis and provides a new perspective on how SO2 affects the photolysis of nitrate absorbed on the photoactive oxides.

The combined effects of heterogeneous chemistry and aerosol-radiation interaction on severe haze simulation by atmospheric chemistry model in Middle-Eastern China

Heterogeneous chemistry and aerosol-radiation interaction (ARI) are considered two of the key mechanisms affecting haze formation and their reasonable parameterizations in atmospherically chemical models play an important role in accurate haze or air quality prediction. However, until the present, their relative contributions to severe haze have rarely been evaluated. Using the double-way atmospherically chemical model GRAPES_Meso5.1/CUACE CW, the relative contributions of heterogeneous chemistry and ARI to PM2.5 are investigated under different pollution levels in Middle-Eastern China (MEC) during December 2016. Study results show that the combined effects of heterogeneous chemistry and ARI generate 29.7% of total PM2.5 mass in MEC of December 2016, of which ARI contributes 19.7%, much higher than heterogeneous chemistry (6.1%). However, heterogeneous chemistry is important for reasonable reproduction of the proportion of secondary inorganic aerosols (SIA) of PM2.5, especially during heavy haze. Both the contributions of heterogeneous chemistry and ARI to PM2.5 increase with the haze pollution aggravation but to different degrees. The contribution of heterogeneous chemistry to modeled PM2.5 concentration rises from 4.0% on clean days to 6.6% under moderate pollution and 7.8% under heavy haze. And that of ARI increases rapidly from 9.1% on clean days to 21.0% under moderate pollution and 29.1% during heavy pollution. It is worth noticing that there is an additional contribution to PM2.5 caused by the combination of heterogeneous chemistry and ARI except for their respective contributions, which reaches up to 7.3% of PM2.5 mass and 18.5% of SIA generation under heavily polluted conditions. This is because ARI further promotes heterogeneous oxidation of SO2 and NOx by increasing the aerosol surface area concentration (SAC), causing additional SIA production via heterogeneous pathways. This work indicates the contribution of ARI to severe haze formation not only by altering meteorological conditions but also by reforming the chemical processes in the model.

Insight into the Mechanism and Kinetics of the Heterogeneous Reaction between SO2 and NO2 on Diesel Black Carbon under Light Irradiation.

The heterogeneous oxidation of SO2 by NO2 has been extensively proposed as an important pathway of sulfate production during haze events in China. However, the kinetics and mechanism of oxidation of SO2 by NO2 on the surface of complex particles remain poorly understood. Here, we systematically explore the mechanism and kinetics of the reaction between SO2 and NO2 on diesel black carbon (DBC) under light irradiation. The experimental results prove that DBC photochemistry can not only significantly promote the heterogeneous reduction of NO2 to produce HONO via transferring photoinduced electrons but also indirectly promote OH radical formation. These reduction products of NO2 as well as NO2 itself greatly promote the heterogeneous oxidation of SO2 on DBC. NO2 oxidation, HONO oxidation, and the surface photo-oxidation process are proven to be three major surface oxidation pathways of SO2. The kinetics results indicate that the surface photooxidation pathway accounts for the majority of the total SO2 uptake (∼63%), followed by the HONO oxidation pathway (∼27%) and direct oxidation by NO2 (∼10%). This work highlights the significant synergistic roles of DBC, NO2, and light irradiation in enhancing the atmospheric oxidation capacity and promoting the heterogeneous formation of sulfate.

Titanium Dioxide Promotes New Particle Formation: A Smog Chamber Study.

TiO2 is a widely used material in building coatings. Many studies have revealed that TiO2 promotes the heterogeneous oxidation of SO2 and the subsequent sulfate formation. However, whether and how much TiO2 contributes to the gaseous H2SO4 and subsequent new particle formation (NPF) still remains unclear. Herein, we used a 1 m3 quartz smog chamber to investigate NPF in the presence of TiO2. The experimental results indicated that TiO2 could greatly promote NPF. The increases in particle formation rate (J) and growth rate due to the presence of TiO2 were quantified, and the promotion effect was attributed to the production of gaseous H2SO4. The promotion effect of TiO2 on SO2 oxidation and subsequent NPF decreased gradually due to the formation of surface sulfate but did not disappear completely, instead partly recovering after washing with water. Moreover, the promotion effect of TiO2 on NPF was observed regardless of differences in RH, and the most significant promotion effect of TiO2 associated with the strongest NPF occurred at an RH of 20%. Based on the experimental evidence, the environmental impact of TiO2 on gaseous H2SO4 and particle pollution in urban areas was estimated.

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.

Exploring dust heterogeneous chemistry over China: Insights from field observation and GEOS-Chem simulation

Dust heterogeneous chemistry plays an important role in tropospheric chemistry, but its parameterization in numerical models is often quite simplified, which hampers accurate prediction of particulate matter and its chemical component. In this study, we investigate the evolution of dust heterogeneous chemical process and its potential impacts on gaseous and aerosol components during a dust pollution episode from March 27 to April 2, 2015 over North China. Based on field measurements, the significant role of relative humidity (RH) in dust heterogeneous chemistry is found and a RH-dependent parameterization for uptake coefficients of HNO3 and SO2 is incorporated in GEOS-Chem to reproduce the dust heterogeneous chemical process. During the study period, observed dust sulfate (DSO4) and dust nitrate (DNIT) exhibit maximum concentrations of 9.1 and 22.8 μg m-3 respectively, accompanied by high RH and gaseous precursor concentrations. DSO4 concentrations are positively related to RH. The observed dust sulfate oxidation ratio (DSOR) is elevated evidently with increased RH, especially when RH is higher than ~40%, implying that enhanced RH could promote heterogeneous oxidation of SO2 to DSO4. Model simulation shows that when incorporating the RH-dependent parameterization, DNIT and DSO4 are generally well captured and the model performance of total sulfate oxidation ratio (TSOR) and total nitrate oxidation ratio (TNOR) are improved. High contribution of DNIT and DSO4 are found to be located over the regions close to source areas (>60%) and downwind regions (>40%), respectively. Sensitivity results show that SO2 and HNO3 reduce by 2-24 μg m-3 and 1-18 μg m-3 when considering dust heterogeneous impacts, thus leading to reduction in non-dust sulfate and non-dust nitrate concentrations. As a result, simulated NH3 increases and ammonium reduces by more than 20%. Our study indicates that the contribution of heterogeneous reactions to sulfate formation is 20-30% over North China.

Aqueous phase oxidation of bisulfite influenced by nitrate and its photolysis

Nitrate aerosol is ubiquitous in the atmosphere. Nitrate in the particulate and aqueous phase can affect various atmospheric chemical processes through its hygroscopicity and photolysis. The impacts of nitrate photolysis on the heterogeneous oxidation of SO2 have been attracting attention. However, the influence of nitrate on heterogeneous aqueous phase formation of atmospheric sulfate aerosol is still not very clear. In this study, the effects of nitrate on aqueous phase oxidation of bisulfite under different conditions were investigated. Results show that nitrate photolysis can promote the oxidation of bisulfite to sulfate, especially in the presence of O2. It is found that pH plays a significant role in the reaction, and ammonium sulfate has significant impacts on the enhancement of aqueous phase sulfate production through regulating the pH of solution. An apparent synergism is found among halogen chemistry, nitrate and its photochemistry and S (IV) aqueous oxidation, especially the oxidation of halide ions by nitrate and its photolysis and by the intermediate products produced by the free radical chain oxidation of S (IV) in acidic solution, leading to the coupling of the redox cycle of halogen with the oxidation of bisulfite, which promotes the continuous aqueous oxidation of bisulfite and the formation of sulfate. In addition, the role of nitrate itself in the aqueous phase oxidation of bisulfite is revealed. These results provide a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO2 in cloud and fog droplets and haze particles.

Oxygen vacancies promote sulfur species accumulation on TiO2 mineral particles

In this study, we investigated the heterogeneous SO2 oxidation catalyzed by defective TiO2 mineral particles at molecular scale. Theoretical DFT calculations revealed that O2 favorably attached to oxygen vacancies, while SO2 adsorption more likely took place over the unsaturated Ti5c and O2c or hydroxyl sites of TiO2 bearing surface oxygen vacancies (TiO2-OV). In the dark, the amount of sulfite species formed on TiO2-OV surface via the SO2 adsorption was 8 times that on pristine TiO2 due to the increased charges of the undercoordinated Ti and O sites around OVs. Although solar light irradiation could promote the sulfate accumulation on both pristine TiO2 and TiO2-OV, OVs facilitated molecular oxygen activation to generate O2− capable of photochemically converting interfacial SO2, accounting for higher sulfate accumulation of TiO2-OV than that of pristine TiO2. These findings provide in-depth understanding of sulfur species identification and sulfate aerosol formation on defective metal oxide minerals in atmosphere.

Sulfur isotope composition during heterogeneous oxidation of SO2 on mineral dust: The effect of temperature, relative humidity, and light intensity

Sulfate as the major constituent of atmospheric aerosol plays a key role in aerosol formation and aging processes. However, little attention has been paid to the impacts of mineral dust on the oxidation of SO2, along with the effects of temperature, moisture, and light intensity. In this study, the influences of these parameters on the formation of sulfate and sulfur isotope composition were investigated in detail. Temperature promoted the heterogeneous oxidation of SO2 on the surface of hematite (α-Fe2O3), while excessive temperature was unfavorable for sulfate formation. Besides, the effect of temperature on sulfur isotope fractionation was not apparent. In addition, relative humidity (RH, < 40%) boosted the oxidation of SO2 with the concurrent enrichment of heavy isotope. Meanwhile, light with high intensity inhibited the formation of sulfate due to the photo-induced reductive dissolution of hematite, and had little effects on the sulfur isotope compositions. Furthermore, compared with other minerals, α-Fe2O3 exhibited greater uptake capacity of SO2, favoring the enrichment of light sulfur isotopes, whereas the enrichment of heavy sulfur isotopes existed on the surface of MgO. This study contributes to the investigation of the heterogeneous oxidation of SO2 and provides useful parameters for atmospheric modeling studies.

Rapid sulfate formation from synergetic oxidation of SO2 by O3 and NO2 under ammonia-rich conditions: Implications for the explosive growth of atmospheric PM2.5 during haze events in China

Extremely high levels of atmospheric sulfate aerosols have still frequently occurred in China especially in winter haze periods and often been underestimated by models due to some missing formation mechanisms. Here we investigated the heterogeneous reaction dynamics of SO2 oxidation by the abundantly co-existing O3 and NO2 in the urban atmosphere of China by using a laboratory smog chamber simulation technique. Our results showed that with an increase of NH3 concentrations from 0.05 ppm to 1.5 ppm, SO2 oxidation by O3 can be greatly promoted and lead to an exponential increase of diameter growth factor (GF) of particles in the chamber from 1.29 to 1.98 for NaCl seeds and from 1.20 to 1.60 for (NH4)2SO4 seeds, along with an increasing uptake coefficient (γ) of SO2 from 4.47 × 10−5 to 1.52 × 10−4 on NaCl seeds and from 2.32 × 10−5 to 5.74 × 10−5 on (NH4)2SO4 seeds, respectively. The heterogeneous production of sulfate from oxidation of SO2 under NH3-rich conditions by O3 and NO2 mixture in the chamber was 2.0–3.5 times the sum of sulfate from SO2 oxidations by O3 and NO2, suggesting a strongly synergetic effect of the mixed oxidants on the heterogeneous oxidation of SO2, which can cause rapid formation of (NH4)2SO4 and NH4NO3 and is responsible for the explosive growth of PM2.5 in the winter haze period of China. Our chamber results further showed that such synergetic process is only efficient under NH3-rich conditions, clearly indicating that the combined controls on O3, NOx and NH3 are necessary for further mitigating the PM2.5 pollution in China.

Heterogeneous oxidation mechanism of SO2 on γ-Al2O3 (110) catalyst by H2O2: A first-principle study

Here, we present a simulation research of the heterogeneous oxidation mechanism of SO2 on Al2O3 (mineral oxides) surface by H2O2, density functional theory (DFT) calculations was used to investigate the adsorption mechanism of SO2 and H2O2 on the perfect and Odefect γ-Al2O3 (110) surfaces. The results show that SO2 molecularly adsorbed on the prefect and Odefect surfaces, while H2O2 was adsorbed in the molecule form on the perfect surface. In particular, H2O2 dissociation only occurred on the Odefect γ-Al2O3 (110) surface. The oxygen defects not only enhanced the adsorption intensities of H2O2 and SO2, but also promoted the H2O2 decomposition (H2O2→OH + OH). Analysis of partial density of states, differential charge density and Mulliken population indicated that H2O2 decomposition followed the Haber-Weiss mechanism (formation of surface OH), and SO2 was oxidized by the OH radicals to form HOSO2 molecule when SO2 and H2O2 co-adsorbed on Odefect γ-Al2O3 (110) surface. Moreover, the lower energy barrier of H2O2 decomposition (32.69 kJ/mol) and SO2 oxidation (78.21 kJ/mol) demonstrated that SO2 to be oxidized easily by the H2O2 on the Odefect γ-Al2O3 (110) surfaces. These results can well explain the formation mechanism of OH radicals and the oxidation mechanism of SO2 by H2O2 on the mineral dust at the molecular level, which is of great significance for understanding the role of H2O2 in the heterogeneous oxidation of SO2 on the mineral dust and the formation mechanism of sulfate aerosols in the atmosphere.

The abatement of acid rain in Guizhou province, southwestern China: Implication from sulfur and oxygen isotopes

The high frequency of acid rain in southern China has captured public and official concern since 1980s. Subsequently, gas emission reduction measures have been implemented to improve the air quality. Variations in SO2 emission intensities can influence the sulfur and oxygen isotopic compositions of sulfate in rainwater, since atmospheric sulfate is mainly formed via the oxidation of sulfur gases from natural and anthropogenic sources. To evaluate the impacts of emission reduction measures on atmospheric sulfate, the seasonal and long-term trends in stable isotopic compositions of sulfate in rainwater in Guizhou province, southwestern China have been investigated based on rainwater samples collected from June 2016 to June 2018 and literature investigation (2000–2010).The results reveal that coal combustion remains a major contributor to sulfate in rainwater, although its SO2 emission has significantly decreased over the past two decades. The δ34Ssulfate and δ18Osulfate values in rainwater are negatively correlated and have significant seasonal changes. The seasonality in δ34Ssulfate has been interpreted as due to the changes in contributions of dimethyl sulfide and coal combustion, while the seasonal pattern of δ18Osulfate is consistent with that of δ18Owater values, indicating sulfate in rainwater is mainly formed by heterogeneous oxidation of SO2. Combined with the data from previous studies (Xiao and Liu, 2002; Liu, 2007; Xiao et al., 2009; Xiao et al., 2014), we found that the volume weighted mean δ34S values of sulfate in rainwater in Guizhou province show a marked increase between 2001 and 2018, indicating that the 34S-depleted SO2 emission from coal combustion has declined during this period. Furthermore, the synchronous changes in δ34S values, sulfate concentration and pH values of rainwater suggest that the frequency of acid rain in Guizhou province has dropped over the past two decades, which is likely to result from the emission reduction measures taken in Guizhou province.

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.

Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles.

Heterogeneous oxidation of SO2 is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO2 by in-particle oxidants (OH, NO2, and NO2-/HNO2) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO2 with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl-, Br-, and I-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl-, Br-, and I-, respectively. A linear relation was found between the nitrate photolysis rate constant, jNO3-, and the initial molar ratio of Cl- to NO3-, [Cl-]0/[NO3-]0, as jNO3- = 9.7 × 10-5[Cl-]0/[NO3-]0 + 1.9 × 10-5 at [Cl-]0/[NO3-]0 below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.