The influence of temperature on the heterogeneous uptake of SO₂ on hematite particles

Highlights

• Heterogeneous uptake of SO₂ on mineral dust is investigated under various temperature and humidity conditions.

• Sulfite compounds occupy a great share among all the products at high temperature due to rapid H₂SO₃ ionization.

• Activated SO₂ molecules increase with increasing temperature in the initial reaction stage, implying positive temperature effects.

• Reaction rate decreases with increasing temperature in the latter reaction stages, resulting from decreased water adsorption on particles.

Abstract

Despite the increased awareness of heterogeneous reactions of SO₂ on mineral particles, the knowledge of how temperature influences the product species and kinetic parameters remain a crucially important part in atmospheric research. Here, we reported the formation of sulfur-containing species on hematite particles under various temperature and humidity conditions by mean of in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ion chromatography (IC). High temperature is helpful in the ionization of H₂SO₃, making sulfite compounds occupy a great share among total products. The whole reaction could be divided into three stages according to the formation rate of hydroxyl groups. High temperature brings about more activated SO₂ and then results in the increased uptake coefficients with increasing temperature in the initial reaction stage. On the contrary, moisture absorption on particles is inhibited by high temperature, leading to the decreased uptake coefficients with increasing temperature in the latter two stages. Observed enthalpy and entropy, as well as activation energy values for relevant reactions were calculated. Overall, the product specie and reaction rate vary with temperature and humidity conditions, as well as reaction stages. This work broadens the knowledge of heterogeneous reactions on mineral dust influenced by temperature, and consequently provides important opportunities for atmospheric model improvements.

Introduction

Sulfate aerosols, deriving from SO₂ oxidation processes, contributes greatly to fine particles during heavy pollution episodes (Cheng et al., 2016; Wang et al., 2016). The condition is extremely significant in developing countries (e.g. China and India) due, in part, to the large emission of SO₂ from excess coal consumption (C. Li et al., 2017). Global sulfate production behaves a key role in aerosol radiative forcing (Harris et al., 2013). It has been reported that sulfate may limit the formation of environmental persistent free radicals (Feld-Cook et al., 2017). With phases changing after particle hydration and dehydration in different humidity environments, sulfate compounds could even determine particle mixing structures (Li et al., 2017, Li et al., 2017, Li et al., 2017). Besides, sulfate components account for aerosol toxicity (Fang et al., 2017), and may lead to frequent occurrence of lung cancer (Raaschou-Nielsen et al., 2016). Mechanisms responsible for the sulfate enhancement on haze days, however, are still not well understood, hindering the forecast and mitigation of haze disaster (Dowd et al., 2017).

Mineral dust, constituting ~36% of total primary aerosol emissions, is the most widespread and concentrated tropospheric aerosol (Wu et al., 2013a, Wu et al., 2013b). Nano-sized particles have lifetimes of up to several days and can be transported over thousands of kilometers (Tang et al., 2016). In the global journey, these particles provide active sites for various atmospheric heterogeneous reactions and simultaneously serve as sinks for many trace gases (Tan et al., 2017). As chief constituents in the upper continental crust, metal oxides including SiO₂, Al₂O₃, Fe₂O₃, MnO₂ and MgO are constantly selected as model aerosols in laboratory research (Chen et al., 2012; Usher et al., 2003). Thereinto, transition metal ions [e.g. Fe(III) and Mn(II)] could lead to considerable oxidation processes by the help of oxygen. Influenced by steel smelting and coal combustion, enhanced iron concentrations were previously observed in many sites during haze episodes (Li et al., 2017, Li et al., 2017, Li et al., 2017). The heterogeneous reaction of gas pollutants on iron-rich particles is of worldwide concern at present (Guan et al., 2014).

Temperature is considered as an essential index when discussing various atmospheric processes (Tie et al., 1997). The experimental determination of kinetic parameters for important atmospheric reactions and how these data vary with temperature remain a crucially important issue in atmospheric research (Smith, 2003). However, most previous laboratory studies concerned heterogeneous reactions at room temperature while neglected the formation of secondary aerosols as a function of seasonal temperature. Previous studies concerning the heterogeneous adsorption of N₂O₅ (Griffiths and Cox, 2009), nitric acid (Hudson et al., 2002), gas-phase alcohol, and various organic acids (Jayne et al., 1991) on diverse medium surfaces found that the uptake capacity exhibited negative temperature dependence. In recent years, the Ge's research group have explored the heterogeneous uptake of SO₂ (Zhou et al., 2014), nitrogen dioxide (Wu et al., 2013a, Wu et al., 2013b), hydrogen peroxide (Zhou et al., 2012), formic acid (Wu et al., 2012), acetic acid (Hou et al., 2016), and arcylic acid (Liu et al., 2017) on model particles or authentic sands, and investigated the increased reactive uptake coefficients for reactant gas with decreasing temperature levels. Meanwhile, they carefully discussed the heterogeneous oxidation of SO₂ by the help of ozone in a wide temperature range and observed a turning point for sulfate formation rate at about 250 K (Wu et al., 2011). However, these instructive studies were conducted under dry condition and paid little attention to the influence of water vapor. To the best of our knowledge, a fundamental understanding of how temperature influences the formation of sulfur-containing species under various humidity conditions remains unclear.

By the help of in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ion chromatography (IC), this work for the first time presents the heterogeneous reaction of SO₂ on hematite particles varying with temperatures and humidity. Sulfur-containing species formed under diverse conditions were distinguished and discussed, along with the further calculations of uptake coefficients and thermodynamic indexes. Detailed reaction mechanism was summarized and atmospheric implications were carefully illuminated. This work could help understand the SO₂ adsorption on iron-rich mineral particles in various environments varying with temperature and relative humidity (RH), and simultaneously provide extremely useful parameters for atmospheric modelling studies.