Elsevier

Science of The Total Environment

Volume 649, 1 February 2019, Pages 1393-1402
Science of The Total Environment

Impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 in the absence and presence of simulated solar irradiation

https://doi.org/10.1016/j.scitotenv.2018.08.295Get rights and content

Highlights

  • Important impacts of adsorbed nitrate on heterogeneous conversion of SO2

  • Significant roles of adsorbed H2O and light in formation of sulfate

  • Nitrate photolysis was coupled with SO2 oxidation.

  • New insights on formation pathway and mechanism of adsorbed N2O4

Abstract

Adsorbed nitrate is ubiquitous in the atmosphere, and it can undergo photolysis to produce oxidizing active radicals. Nitrate photolysis may be coupled with the oxidation conversions of atmospheric gaseous pollutants. However, the processes involved remain poorly understood. In this study, the impact of adsorbed nitrate on the heterogeneous oxidation of SO2 on α-Fe2O3 was investigated in the absence and presence of simulated solar irradiation by using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that for α-Fe2O3 particles with no adsorbed nitrate, the formation of adsorbed sulfate on humid particles is stronger than that on dry particles. Meanwhile, light can also promote the heterogeneous conversion of SO2 and the formation of sulfate on dry particles because α-Fe2O3 is a typical photocatalyst. However, the heterogeneous conversion of SO2 on humid α-Fe2O3 particles is somewhat suppressed under light, suggesting the occurrence of photoinduced reductive dissolution. For the heterogeneous conversion of SO2 on α-Fe2O3 particles with adsorbed nitrate, the formation of sulfate on humid particles is still higher than that on dry particles. For the dry α-Fe2O3 particles with adsorbed nitrate, light promotes the formation of adsorbed sulfate. For the humid α-Fe2O3 particles with adsorbed nitrate, the heterogeneous conversion of SO2 under light is stronger than that under no light, indicating that the photolysis of adsorbed nitrate is coupled with the oxidation of SO2 and the formation of sulfate. The consumption of adsorbed nitrate and the formation of adsorbed N2O4 are observed during the introduction of SO2. A possible mechanism for the impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 particles is proposed, and atmospheric implications based on these results are discussed.

Introduction

With the deepening of industrialization and urbanization in China, both the population of vehicles and energy consumption are increasing rapidly. These lead to considerable primary emissions of gaseous pollutants including SO2, NOX and VOCs. Heterogeneous reactions of SO2, NOX and VOCs are found to play significant roles in the formation of secondary aerosols in haze events in China (Fu and Chen, 2017; Huang et al., 2014; Wang et al., 2016). Therefore, their atmospheric heterogeneous reactions have attracted much attention. Recently, the interactions among these atmospheric pollutants during the atmospheric heterogeneous reactions have been revealed in several previous studies (Huang et al., 2017; Kong et al., 2014b; Liu et al., 2012; Qiao et al., 2015; Sun et al., 2016; Zhao et al., 2018), which can improve available atmospheric chemistry models for evaluating the formation of secondary aerosols (Barrie et al., 2001; Kasibhatla et al., 1997). However, there is still a large uncertainty about the mechanism of atmospheric secondary aerosol formation.

Sulfur dioxide is an important precursor of atmospheric sulfate aerosol. It can undergo several pathways to produce sulfate aerosol, including gas-phase oxidation, aqueous-phase oxidation in cloud and fog droplets, and the heterogeneous reactions on the surfaces of particles (e.g. TiO2, ZnO, Al2O3, Fe2O3, CaCO3 and mineral dust) (Gao and Chen, 2006; Li et al., 2011; Qiao et al., 2015; Shang et al., 2010; Zhang et al., 2006). Several atmospheric chemistry models have demonstrated that the gaseous oxidation by OH radical and the aqueous oxidation by ozone and hydrogen peroxide are insufficient to bridge the gap between field and modelling studies on a global scale (Barrie et al., 2001; Kasibhatla et al., 1997; Luria and Sievering, 1991) Therefore, heterogeneous conversion of SO2 has attracted wide attention. For example, laboratory studies have found that NO2 can promote the heterogeneous conversion of SO2 to sulfate (Liu et al., 2012; Ma et al., 2008). However, the heterogeneous conversion of SO2 on α-Fe2O3 can be suppressed by the presence of CH3CHO (Zhao et al., 2015), whereas the formation of sulfate is not affected by the presence of formic acid (Wu et al., 2013). These studies imply that the heterogeneous conversions of SO2 in the real atmosphere are complicated and susceptible to other species.

NOx emitted from natural and anthropogenic sources can be adsorbed on the surfaces of mineral particles to form adsorbed nitric acid, nitrite and nitrate (Anttila et al., 2011; Goodman et al., 1998; Underwood et al., 1999). Adsorbed nitrite and nitrate can undergo photolysis and produce OH and O (3P) (Chu and Anastasio, 2007; Goldstein and Rabani, 2007; Schuttlefield et al., 2008). These active species are important oxidants for the heterogeneous oxidations of atmospheric SO2 and organic species. Nitrate photolysis may be coupled with the oxidation conversions of atmospheric gaseous pollutants. However, the processes involved remain poorly understood. Moreover, the accumulation of generated adsorbed nitric acid, nitrite and nitrate on the particle surfaces will change the physicochemical properties of these particles, which will further affect their hygroscopicity, optical properties and heterogeneous reactivities.

In addition, field measurements showed that a linear correlation exists between nitrate and sulfate in PM2.5, indicating the mutual influence between the formation processes of sulfate and nitrate in the atmosphere (Kong et al., 2014a; Kong et al., 2014b). Wang et al. (2016) found that the oxidation of SO2 by NO2 occurs efficiently in aqueous media including in-cloud oxidation and on fine particulate matter with NH3 neutralization. Xie et al. (2015) found that the formation of sulfate can be enhanced by NO2 in mineral dust surface catalytic reactions of SO2 and in dust-induced photochemical heterogeneous reactions of SO2, as well as aqueous oxidations of S (IV) under foggy/cloudy conditions with high NH3 concentration. The role of NO2 in the production of sulfate during Chinese haze-aerosol episodes has also been observed (Li et al., 2018). These field observations indicate an important role of NO2 in SO2 oxidation in the real atmosphere. Several laboratory studies have focused on the synergistic effects between NO2 and SO2. For example, the synergistic reaction between SO2 and NO2 on different mineral oxides has been observed (Huang et al., 2017; Liu et al., 2012; Ma et al., 2008; Ma et al., 2017), and it is found that the presence of NO2 can promote the oxidation of SO2, whereas SO2 can alter the reaction pathways of NO2 and lead to the transformation of intermediate from nitrite to N2O4 (Ma et al., 2008). On the other hand, SO2 can also initiate the efficient conversion of NO2 to HONO on MgO surface (Ma et al., 2017). Zhao et al. (2018) found that the CaCO3 solid particle was first converted to the Ca(NO3)2 droplet by the reaction with NO2 and the deliquescence of Ca(NO3)2, and then NO2 oxidized SO2 in the Ca(NO3)2 droplet forming CaSO4 under the introduction of the SO2/NO2/H2O/N2 gas mixture. However, all the laboratory studies mentioned above have been performed in the absence of light, and few studies have focused on the interactions between NO2 and SO2 or between adsorbed nitrate and SO2 under light.

In this study, the impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 in the absence and presence of simulated solar irradiation was investigated using an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The roles of adsorbed nitrate itself and its photolysis in the heterogeneous reaction of SO2 have been revealed, and the possible heterogeneous reaction mechanism was discussed. This study will provide significant information about the roles of adsorbed water, light, and adsorbed nitrate itself and its photolysis in the atmospheric chemistry, and help us to have a better understanding of the formation of secondary inorganic aerosols in the real atmosphere.

Section snippets

Materials

α-Fe2O3 powder was prepared according to the method in the previous study (Kong et al., 2014b). Powder X-ray diffraction confirmed the prepared sample as pure hematite, and the Brunauer–Emmett–Teller (BET) area was 13.2 m2 g−1 (Micromeritics TriStar 3000, Micromeritics Instrument Co., USA). O2 (99.999% purity, Shanghai Qingkuan Chemical Co., Ltd.) and N2 (99.999% purity, Shanghai Qingkuan Chemical Co., Ltd.) were introduced into a reaction chamber through gas dryers before use. The selected

Results and discussion

To investigate the impact of adsorbed products from the uptake of NO2 on the heterogeneous conversion of SO2, the following aspects are involved: (1) Investigations of the heterogeneous uptakes of NO2 on dry and humid α-Fe2O3, respectively. (2) Investigations of the impacts of adsorbed products from NO2 preadsorption for 30 and 90 min on the heterogeneous reactions of SO2 on α-Fe2O3 particles with and without illumination, respectively.

Conclusion

The impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 in the absence and presence of simulated solar irradiation was investigated by using DRIFTS. For this purpose, the heterogeneous reactions of SO2 on the α-Fe2O3 particles with adsorbed nitrate from NO2 preadsorption for 30 and 90 min were studied under four different reaction conditions, including dry particles + no light, dry particles + light, humid particles + no light and humid particles + light, respectively.

Disclaimer

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Key R&D Program of China (2017YFC0209505) and the National Natural Science Foundation of China (Grant Nos. 21777027, 41475110 and 21277028).

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