Research Progress on Heterogeneous Reactions of Pollutant Gases on the Surface of Atmospheric Mineral Particulate Matter in China

Abstract

Haze is the phenomenon of visibility degradation caused by extinction effects related to the physicochemical properties of atmospheric particulate matter (APM). Atmosphere heterogeneous reactions can alter the physicochemical properties of APM. Therefore, it is important to understand the atmospheric heterogeneous reactions of APM in order to reveal the cause of haze. Herein, the current situation, developmental trend, source, and composition of APM pollution in China are reviewed. Additionally, we introduce the reaction characteristics and key chemical processes of common inorganic, organic, and mixed pollutant gases on the surface of mineral particles. The effects of mineral particulate matter on aggregation, regulation, and catalysis in the formation of atmospheric aerosols and the synergistic reaction mechanism of SO₂, NO₂, O₃, and VOCs on the surfaces of different mineral particles are summarized. The problems existing in the current research on heterogeneous reactions on the surfaces of mineral particles are also evaluated. This paper aims to gain a deep understanding of the mechanism of mineral particulate matter promoting the formation of secondary aerosols and attempts to provide theoretical support for effective haze control.

1. Introduction

In the 21st century, rapid socioeconomic and industrial developments have caused serious air quality deterioration, and related health problems have gradually attracted significant attention. With the progress of science and technology, the seriousness of the harm to human health caused by haze has become evident, and global efforts have been made to implement relevant systems for haze control. As an example, China has implemented a series of relatively strict pollutant emission standards and pollution control measures since the promulgation and implementation of the Air Pollution Prevention and Control Action Plan in 2013 [1]. Currently, China’s ambient air quality is considered to be significantly improved. The proportion of excessive days with the primary pollutant at a level of PM_2.5, as well as the number of days with heavy air pollution, have been significantly reduced, while the number of days with no pollution has significantly increased [2]. However, according to national air quality monitoring data (Figure 1a), despite the fact that the concentrations of SO₂, NO₂, CO, PM₁₀, and PM_2.5 in the air are decreasing significantly, a large proportion of cities still exceed the PM_2.5 and PM₁₀ pollution standards (47.2% and 32%, respectively) [2]. Therefore, atmospheric particulate matter (PM_2.5, PM₁₀) remains the main air pollutant in cities. Moreover, as the concentrations of other pollutants decline, the pollution of O₃ shows an upward trend (Figure 1b). Studies have shown that PM_2.5 and O₃ generation share a complex link, having common precursors and influencing one another through multiple pathways in the atmosphere [3,4]. Therefore, atmospheric particulate matter (APM) and O₃ are important pollutants affecting ambient air quality and have become the major factors restricting its further improvement [5].

APM is a general term for all kinds of solid and liquid particulate matter in the atmosphere. All kinds of APM are evenly dispersed in the air to form a relatively stable suspension system, that is, the aerosol system [6,7]. APM can enter the human respiratory system through inhalation, causing a variety of respiratory and cardiovascular diseases, thus causing harm to human health, especially in the case of PM_2.5 [8]. Furthermore, aerosols are able to absorb and scatter solar light, acting as cloud condensation nuclei that alter cloud formation and the atmospheric lifetime. The resulting direct and indirect climate effects are some of the most uncertain factors in global climate change predictions [6,9,10]. Therefore, understanding the migration and transformation of aerosol particles in the atmosphere and their physicochemical properties is of great significance for assessing the health risks, environmental impacts, and climate effects of aerosols.

APM has a very complex composition, with an aerodynamic equivalent diameter of 0.001–100 μm, and approximately 3000–5000 Tg of APM is emitted into the atmosphere every year [11,12]. Its main components include mineral dust, sea salt, organic aerosols, sulfate, nitrate, ammonium salt, and black carbon [13,14]. Owing to the small particle size, large specific surface area, and strong adsorption capacity of APM, heterogeneous reactions tend to occur on its surface or interface [1]. Heterogeneous reactions on the surface of APM in the atmosphere not only involve the physicochemical characteristics of APM, but are also closely related to the type, concentration, and environment of trace gases in the atmosphere [7,15]. The interactions between APM and photo-oxidants such as O₃, •OH, and NO₂ make the atmospheric chemical processes extremely complex. The reaction of atmospheric photo-oxidants with SO₂, NO_x, and organic compounds, such as olefins and aromatic hydrocarbons, leads to an increase in the concentration of particulate matter in the atmosphere [16,17,18]. Volatile organic compounds (VOCs) and inorganic gases in the atmosphere form secondary particles by homogeneous reactions and heterogeneous reactions on the surfaces of particles, and affect the toxicity, hygroscopicity, and radiation characteristics of particles [10,19,20,21]. Hence, the transformation of polluted gases in the atmosphere to secondary particles has attracted extensive attention. The formation of secondary particles is related to the precursors, atmospheric oxidizability, adsorption, catalytic capacity, and acid and alkalinity of the particle surface, whereas in heterogeneous reactions, photo-oxidant depletion reactions on the particle surface and photocatalytic oxidation of transitional metals occur, and this also affects the atmospheric oxidizability. These reactions simultaneously make the chemical composition of particulate matter more complex and may have greater effects on human health and climate change [22]. Therefore, the study of atmospheric heterogeneous reactions is of great significance for understanding regional air quality improvements and global climate change. Taking mineral particles as an example, this paper reviews the recent progress in the study of the surface heterogeneous reactions of atmospheric mineral particles.

2. Source and Composition of APM

APM can be divided into primary and secondary particles [12]. Primary particulate matter is directly released into the atmosphere by natural and anthropogenic pollution sources, such as soil particles, sea salt particles, and burning soot. Secondary particulate matter refers to finer particulate matter, including primary gaseous pollutants (e.g., SO₂, NO_x, VOCs, and NH₃) discharged by fuel combustion and industrial and automobile exhaust gas, which are converted into fine particulate matter through gas-particle transformation in the atmosphere [6,12,23]. Based on the composition and morphology of single particles in the atmosphere, APM can also be divided into six types: soot, fly ash, complex secondary, mineral, organic, and metal particles [24,25]. Owing to the different sources and formation processes of APM, its composition can also vary significantly, especially within urban atmospheres, which are affected by a variety of pollution sources. As shown in Figure 2, the chemical composition of PM_2.5 is extremely complex and usually contains inorganic substances, such as mineral dust particles and sulfate, nitrate, and ammonium salts; organic matter, such as organic acids, aromatic hydrocarbons, and aerobic organic matter; and trace elements [26].

Atmospheric mineral particulate matter (AMPM) is the most important component of atmospheric aerosols, accounting for approximately 30–60% of the mass concentration of tropospheric atmospheric particles [12,20,27]. These particles mainly originate from ground dust in arid and semi-arid desert areas, and their annual emissions are approximately 1500–4400 Tg [19,28]. Studies on the composition and characteristics of inhalable particulate matter in Beijing, Shanghai, Zhengzhou, Wuhan, and other large cities in China have shown that mineral particles are an important component of urban atmospheric particles, and AMPM accounts for approximately 50% of the mass concentration of APM in dry areas [29]. For example, AMPM in Beijing accounts for approximately 30–70% of the total particulate matter, and while AMPM in Chengdu is lower than in Beijing and some cities in the northwest, where it is still between 34–40% [30]. Clay and diagenetic minerals are the main components of urban particulate matter in dust paths in China, with aluminosilicate particles being the most common [31]. Among them, sodium feldspar, illite, potassium feldspar, anorthosite, hornblende, and chlorite account for 61.59% of the total particulate matter, and calcite and quartz particles account for 13.59%. Dolomite, gypsum, unformed amorphous substances, and other mineral components are also present [31]. Dong et al. [32] found that quartz, clay minerals, and amorphous materials accounted for 24.1%, 28.5%, and 20% of inhalable particles in northern China during dusty weather, respectively. Wang et al. [33] studied the composition of APM during two extremely large sandstorms in Beijing in 2015 and found that AMPM accounted for 85.3% and 95.4% of APM, respectively, among which the clay mineral content was the highest, being more than 50%, followed by quartz, feldspar, and carbonate particles (Figure 3). In India, Spain, Italy, and North Africa, AMPM account for 30–70% of the total particulate matter, and the main mineral phase is the same as in China. However, because of the different geographical locations and pollution situations, the proportion of each mineral phase is different [34].

The inorganic salts in APM are mainly sulfate, nitrate, and ammonium salts from the homogeneous and heterogeneous reactions of SO₂, NO_x, and NH₃ in the atmosphere [7,12,35,36]. The amount of inorganic salts in particulate matter varies depending on source variations, meteorological conditions, and varying atmospheric transformations [15]. Gao et al. [37] measured the concentration of water-soluble ions in Jinan PM_2.5, among which the highest SO₄²⁻ average concentration was 38.33 ± 26.20 μg/m³, accounting for 44.65 ± 11.30% of the total water-soluble ions, while the NH₄⁺ and NO₃⁻ average concentrations were 21.16 ± 16.28 and 15.77 ± 12.06 μg/m³, accounting for 17.63 ± 7.61% and 23.07 ± 5.85% of the total ions, respectively. Lai et al. [38] measured water-soluble ions in particulate matter in Guangzhou, Shenzhen, Zhuhai, and Hong Kong and found that SO₄²⁻, NH₄⁺, and NO₃⁻ together accounted for 59.3–77.7% and 59.3–77.1% of PM_2.5 and PM₁₀ water-soluble ions in winter, respectively. In summer, they accounted for 56.5–84.5% and 46.3–79.2% of PM_2.5 and PM₁₀, respectively. In particular, the concentration of SO₄²⁻, at 6.0–22.0 μg/m³, was higher than all other ions. The contents of sulfate and nitrate in the particulates in Chengdu were 21.55% and 11.20%, respectively, which were lower than those in Beijing, Shanghai, and Guangzhou [39]. Huang et al. [26] measured the chemical composition of PM_2.5 in Beijing, Shanghai, Guangzhou, and Xi’an during high pollution events in 2013, among which SO₄²⁻, NO₃⁻, and NH₄⁺ accounted for approximately 8–18%, 7–14%, and 5–10% of the total mass of PM_2.5, respectively. Kim et al. showed that the water-soluble ions NH₄⁺, NO₃⁻, and SO₄²⁻ in particulate matter detected at five stations on St. Nicholas Island in the United States accounted for 8–9%, 23–26%, and 6–11% of the total mass of PM₁₀, and 14–17%, 28–41%, and 9–18% of PM_2.5, respectively [40]. Rajeev et al. [41] measured the chemical composition of PM_2.5 and rainwater in India during the El Niño and Pacific Decadal Oscillation (PDO). Among them, SO₄²⁻, NH₄⁺, and NO₃⁻ accounted for 33.6%, 23%, and 8.8% of the total amount of water-soluble ions in PM_2.5, and 6.9%, 4.8%, and 7.5% in rainwater, respectively. Weagle et al. [42] interpreted the chemical composition and source of global PM_2.5 through global chemical transport model (GEOS-Chem) simulation and surface particulate matter network (SPARTAN) site observation. It was found that the secondary inorganic aerosols (SIA, the sum of SO₄²⁻, NO₃^-, and NH₄⁺) in each observation site accounted for 15–40% of the total mass of PM_2.5, and the content of SO₄²⁻ was the highest, accounting for 50–80% of the total SIA (

Table 1. PM_2.5 Mass and Composition at SPARTAN Sites from Measurements (obs) and GEOS-Chem Simulation (GC). Values Are Reported in μg/m⁻³ for a Laboratory RH of 30−40% and Simulated RH of 35%. (Reprinted with permission from Ref. [42]. Copyright 2018, American Chemical Society.).

Site	PM2.5		SIA		SO42−		NH4+		NO3−
	obs	GC	obs	GC	obs	GC	obs	GC	obs	GC
Beijing, China	67.1 ± 9.9	75.0	19.7 ± 2.3	36.3	11.2 ± 1.4	13.3	3.6 ± 0.6	9.0	4.9 ± 1.4	1.4
Bandung, Indonesia	30.8 ± 4.5	20.0	7.6 ± 0.8	9.9	5.6 ± 0.7	7.2	1.4 ± 0.3	2.6	0.6 ± 0.2	0.1
Manila, Philippines	19.2 ± 2.8	24.0	3.0 ± 0.3	12.0	2.1 ± 0.3	9.1	0.5 ± 0.1	2.9	0.4 ± 0.	0.0
Rehovot, Israel	17.5 ± 2.6	23.0	6.4 ± 0.7	7.7	4.7 ± 0.6	5.6	0.9 ± 0.1	2.0	0.8 ± 0.2	0.1
Dhaka, Bangladesh	49.9 ± 7.3	79.0	11.3 ± 1.2	28.0	7.1 ± 0.9	15.1	2.2 ± 0.4	7.2	2.0 ± 0.6	5.7
Buenos Aires, Argentina	10.7 ± 1.6	15.0	2.5 ± 0.3	6.2	1.3 ± 0.2	4.4	0.4 ± 0.1	1.5	0.8 ± 0.2	0.3
Ilorin, Nigeria	15.8 ± 2.3	17.5	2.4 ± 0.2	1.9	1.7 ± 0.2	1.3	0.5 ± 0.1	0.5	0.2 ± 0.1	0.1
Singapore, Vietnam	15.8 ± 2.4	15.6	4.0 ± 0.4	3.5	3.2 ± 0.4	2.2	0.6 ± 0.1	0.9	0.2 ± 0.1	0.4
Kanpur, India	71.9 ± 10.6	94.0	18.6 ± 1.9	29.2	10.2 ± 1.3	16.6	4.6 ± 0.1	7.6	3.8 ± 1.1	5.0
Hanoi, Vietnam	50.9 ± 7.5	45.0	17.2 ± 1.8	17.1	10.1 ± 1.3	10.0	3.4 ± 0.6	4.5	3.7 ± 1.1	2.6
Pretoria, South Africa	17.5 ± 2.6	30.6	7.3 ± 0.7	15.7	5.3 ± 0.7	11.3	1.4 ± 0.2	3.7	0.6 ± 0.2	0.7

). Moreover, they found that Beijing, Kanpur, and Dhaka all had much higher levels of PM_2.5 and SIA than other cities.

Organic pollutants form the main component of APM, accounting for approximately 20–50% of the particulate matter [43,44]. In heavy pollution events, organic matter contributes to more than 50% of atmospheric particulate matter. Due to the influence of molecular weight and saturated vapor pressure, organic pollutants are more likely to attach to fine particles [45]. The organic matter in particulate matter mainly comes from VOCs, of which olefins, aromatic hydrocarbons, alkanes, and other anthropogenic emissions come from industry, agriculture, and automobile exhausts, and fossil, biomass, and solid waste combustion [46]. Isoprene, monoterpenes, and sesquiterpenes are mainly derived from volcanic activity, plant emissions, biological hydrocarbon emissions, and other natural sources [43,47]. VOCs can be produced by photolysis or through reactions with oxidizing agents (such as •OH, •NO₃, and O₃) in the atmosphere, which can form new free radicals (•HO₂, •RO₂, •RO) and peroxides on the mineral surface to facilitate VOCs as photochemical smoke promoters [14,48,49,50]. In addition, they promote the formation of organic sulfate, organic nitrate, and other secondary organic aerosol (SOA) owing to their oxygen-containing active functional groups (C=O, COH, COOH) with high reactivity [46,51]. Studies have found that the spatial distributions and temporal variations of typical organic matter concentration levels in APM in China are significantly varied [52]. The concentration levels of organic pollutants in APM in different regions are not only related to energy structure, but also to climatic conditions (such as temperature, humidity, wind speed, and rainfall) and local circulation. The difference in spatial distribution shows that the level of organic pollution in northern cities is much higher than that in southern cities, and that in eastern inland and western cities it is higher than that in eastern coastal cities. The temporal variations in autumn and winter are higher than those in spring and summer [52].

More than 70 types of metal elements exist in APM [53,54,55,56]. Aluminum, Si, Ti, Ca, Mn, Fe, and other crustal elements mainly originate from natural sources and exist as coarse particles, which are related to geological and surface conditions, whereas pollutants such as Pb, As, Cr, Ni, Cu, and Cd are mainly derived from anthropogenic sources and are related to human activities, such as industrial production and fuel combustion [56,57,58]. Zou et al. [59] analyzed air heavy metal pollution in 53 cities in 29 provinces and found that As, Cr, Cd, Ni, Mn, Pb and other heavy metals in China’s atmosphere mainly originate from fossil fuel combustion, metal smelting, and traffic exhaust emissions. Tan et al. [60] compared the differences in heavy metal pollution levels in the atmosphere across northern and southern cities in China and found that the concentrations of Pb, Cr, and Cd in the atmosphere of southern cities were higher than those of northern cities, while the concentrations of V, As, Mn, and Ni in the atmosphere of northern cities were higher than those of southern cities. These results showed that the differences in heavy metal pollution between the northern and southern cities may be related to coal burning, climate characteristics, and the industrial structure in winter. In addition, current studies have found that the volume concentration of heavy metals in coarse particles is higher than that in fine particles, but the mass concentration of heavy metals in fine particles is higher than that in coarse particles, indicating that heavy metals tend to be enriched in fine particles [54]. Most heavy metals present on the surface of APM have different chemical valence states, which act as catalysts in atmospheric chemical reactions, affecting the surface reaction activity of particles and the generation of free radicals, thus accelerating the migration and transformation of pollutants [61].

3. Surface Heterogeneous Reactions of AMPM

Atmospheric heterogeneous reactions refer to gas–solid–liquid three-phase reactions occurring on the surfaces/interfaces of micro-nano atmospheric particles [15]. These surface/interface reactions are involved in all aspects of atmospheric chemical processes and are of great significance for climate change, human health, and ecological balance [62,63]. In the troposphere, aerosol particles and gas composition are complicated by human activities and natural emissions [23]. Heterogeneous chemical reactions can easily occur between pollutant gases and particulate matter, which are coupled with environmental conditions, such as temperature, relative humidity (RH), illumination, and pressure, making the atmospheric environment more complex and affecting a wider area of this environment [7]. Mineral particles usually manifest with a heteromorphic and porous structure, along with a large surface area, strong adsorption properties, and high reactivity, features which enable them to provide a carrier for the adsorption, catalysis, oxidation, and hydrolysis of a variety of gaseous pollutants [12]. Field observation and laboratory simulation studies have confirmed that the heterogeneous reactions on the surfaces of mineral particles have an important impact on the removal of common gas pollutants in the atmosphere [36,50]. Mineral particles are an important source and sink of various gas pollutants, and also produce new secondary components, thus changing the chemical composition of atmospheric particles [44,47,49]. To understand the contribution of heterogeneous reactions involving mineral particulate matter and polluting gases to haze, a large number of laboratory simulations and external field observations have been conducted to reveal the heterogeneous reaction mechanisms, kinetic parameters, and other information related to haze formation. Therefore, the heterogeneous reaction of AMPM surfaces has become an increasingly important research field in atmospheric and environmental science [64].

Research on atmospheric heterogeneous reactions usually adopts the combination technology of the reaction device (Knudsen cell [65,66,67,68] or flow tube [69,70]) and the in situ detection equipment [71,72,73,74,75,76]; diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS); time of flight mass spectrometer (TOF); gas chromatograph–mass spectrometer (GC-MS); micro-Raman spectroscopy) to explore the kinetic parameters (uptake coefficient (γ)) of the reactions of atmospheric particles and atmospheric trace gases that change with humidity, temperature, and time. Thus, the activation energy, entropy change, enthalpy change, and other important physicochemical parameters of the reactions can be obtained. Then, the physicochemical characteristics of the intermediate and final products are characterized by ion chromatography (IC), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Finally, based on the abovementioned research results, the mechanisms of the heterogeneous reactions are inferred. However, the samples usually used in the abovementioned technologies are stacked particles, which are very different from the suspended particles that do not come into contact with one another in the actual atmospheric environment. The close contact of stacked samples brings substantial uncertainty to the experiment. Therefore, smog chamber [77,78,79] and individual particle analysis methods [80,81,82,83,84] have become popular in atmospheric heterogeneous reaction research in recent years. Smog chamber and individual particle analysis methods utilize non-contacting or suspended particles, which can truly reflect the heterogeneous reaction process on the surface of particles in the actual atmospheric environment, to obtain accurate kinetic parameters that reflect the real-life conditions of the atmosphere. At the same time, smog chamber and individual particle research, combined with optical microscope or electron microscopy technology, can observe the size, morphology, and phase state changes of particles during the reaction process, which plays an important role in clarifying the reaction process and mechanism.

3.1. Heterogeneous Reactions of Inorganic Gases on the Surface of AMPM

3.1.1. Heterogeneous Oxidation Reaction of SO₂ on the Surface of AMPM

The inorganic salts in APM are mainly sulfate, nitrate, and ammonium salts, of which sulfate is the most abundant [77,85,86,87]. Ueda et al. [80] researched the internal structure and composition of single atmospheric dust particles after the Asian dust outflow event in February 2012 using focusing ion beam (FIB) slicing technology and TEM. Detailed microscopic analyses revealed that the dust particles were composed mainly of amorphous silica and calcite, whereas Fe-rich domains were found inside the particles as submicrometer-diameter grains (Figure 4). Interestingly, a sulfur-containing domain that co-exists with calcium was found in a small domain near the particle surface. The authors speculated that the sulfur-containing domains may be the result of the interaction between calcite and SO₂ or H₂SO₄ during the atmospheric transport of mineral dust. However, this mechanism requires further study using actual atmospheric dust particles. To date, a series of advances have been made in the study of the heterogeneous reaction process of SO₂ on mineral oxide surfaces. SO₂ has been found to form sulfite only at alkaline oxygen vacancies or hydroxyl sites on the mineral oxide surfaces of A1₂O₃, MgO, TiO₂, and CaO, and these sulfite species transform into sulfate species only when heated under aerobic conditions or in the presence of other oxidants [63,64,65,66,86,88,89,90]. Using Fourier transform infrared spectroscopy (FTIR), Goodman et al. [65] found that H₂O absorbed on the surface of MgO could transform its surface sulfite into sulfate, yet was unable to do so on the surface of A1₂O₃. In contrast to other mineral oxides, Fe(III) oxides can directly convert SO₂ to SO₄²⁻ because of their strong oxidizability. Baltrusaitis et al. [91] used XPS to study the heterogeneous reaction process of SO₂ on the surfaces of goethite and hematite under ultra-high vacuum conditions and found that molecular oxygen could be activated on the surfaces of these minerals and convert SO₂ to sulfate. Fu et al. [71] further demonstrated by in situ diffuse reflectance infrared spectroscopy and XPS that surface hydroxyl groups and adsorbed oxygen molecules are the main factors for the oxidation of SO₂ to sulfate on the iron oxide surface. This conclusion was confirmed by the heterogeneous reaction of SO₂ on other typical oxide surfaces observed by Zhang et al. [88]. Dupart et al. [92] studied the reaction of Arizona test dust (ATD) and iron oxide particles with SO₂ in aerosol flow tubes and observed the formation of new particles under ultraviolet (UV) irradiation. They believed that light causes the formation of hydroxyl radicals on the surfaces of metal oxides in mineral particles, which then promotes the conversion of SO₂ to sulfate. Park and Yu [93,94] measured the kinetic uptake coefficient of SO₂ on the surfaces of the Gobi Desert (GDD) and ATD particles and found that the uptake coefficient of SO₂ under UV irradiation was significantly higher than that in darkness, and they verified this concept through simulation calculations.

3.1.2. Heterogeneous Reaction of SO₂ and O₃ on the Surfaces of AMPM

Oxidants such as O₃, NO_x, and H₂O₂ in the atmospheric environment have important effects on the heterogeneous reaction process of SO₂ on the surfaces of mineral particles. Ozone is the third-largest greenhouse gas after CO₂ and CH₄. It is an important photo-oxidant in the troposphere and has a significant impact on atmospheric oxidation [63]. Usher et al. [66] studied the uptake process of SO₂ on the surfaces of common mineral particles (mineral oxide, CaCO₃, and China loess) using FTIR and Knudsen cell. They found that SO₂ was reversibly adsorbed on the SiO₂ surface, but irreversible adsorption occurred on other particle surfaces, and sulfite species were generated, which could be further oxidized to sulfate by O₃. Ullerstam et al. [73] investigated the kinetic process and mechanism of SO₂ oxidation by O₃ on the Saharan dust surface and demonstrated that, in the absence of an oxidant, only sulfite was produced; however, O₃ was able to further convert sulfite to sulfate. Li et al. [74] found that SO₂ existed in the form of sulfite on the surface of CaCO₃ and was rapidly converted to sulfate by O₃. Han et al. [95] investigated the influence of various environmental factors on the formation of sulfate aerosols on the surface of MgO using a technique that combined simulation experiments with long-term real-time field measurements. The results showed that the main product of the SO₂ reaction on the surface of MgO was sulfite, and O₃ and UV light could promote the conversion of sulfite to sulfate. Under the UV light conditions, the effects of temperature and RH on the surface sulfate formation of MgO exhibited a single-peak mode. The sensitivities of the factors to the sulfate formation followed the order of RH > UV light > temperature > O₃ > SO₂. Ozone has been widely studied as an oxidant enabling SO₂ to produce sulfate, and the roles of other oxidants have gradually attracted attention.

3.1.3. Heterogeneous Reactions of SO₂ and NO₂ on the Surfaces of AMPM

Nitrogen dioxide is a polluting gas that exists with SO₂ in the atmosphere and can participate in a variety of atmospheric chemical reactions, directly or indirectly changing atmospheric oxidation, which has an important impact on air quality [10,72,75]. The results of field observations have shown that mineral particles co-existed with both sulfate and nitrate [11,96]. Moreover, Chen et al. [84] employed the non-destructive surface-enhanced Raman scattering (SERS) technique to investigate the morphology, chemical composition, and mixing state of multiple components of the individual fine particles during a haze episode in Beijing. The results showed that the atmospheric particles were mainly opaque soot during the cleaning period, while during the pollution period, the atmospheric particles were a mixture of opaque (soot) and transparent particles (nitrate and sulfate) and had a core-shell structure. In addition, the abundance of soot was negatively correlated with the PM_2.5 mass concentration because, on hazy days, the former declined, while secondary inorganic aerosol (SNA) concentrations increased. This indicates that the formation of sulfate species on the surface of APM is correlated with NO₂. Wang et al. [81] studied the heterogeneous reaction of the atmospheric individual particle CaCO₃ using micro-Raman spectroscopy. They found that NO_x can react with CaCO₃ to form Ca(NO₃)₂, and that NO_x and its product Ca(NO₃)₂ can further react with SO₂ and oxidants to form CaSO₄. The heterogeneous reaction pathways of CaMg(CO₃)₂ particles in the atmosphere were similar to those of CaCO₃ particles. Ma et al. [75] found that SO₂ alone could only exist in the form of sulfite on the γ-Al₂O₃ surface, while co-existence with NO₂ could promote the transformation of sulfite to sulfate. Nitrogen dioxide alone was converted to nitrate through the surface nitrite intermediate, and, when co-existing with SO₂, it could oxidize sulfite on the surface of γ-Al₂O₃, thus forming sulfate. Liu et al. [76] systematically studied the influence of NO₂ on the heterogeneous reaction of SO₂ on the surfaces of other mineral dust particles and found that the same conclusion was also applicable to other atmospheric particles. They proposed that NO₂ can activate surface oxygen to produce N₂O₄ intermediates, thus promoting the formation of SO₄²⁻ (Figure 5). The abilities of O₃ and NO₂ to oxidize SO₂ on the surface of α-A1₂O₃ were compared under different humidity conditions by Liu et al. [27]. The sulfate formation rate and uptake coefficient (γ) of SO₂ increased with increasing humidity in the two systems. Under the same humidity conditions, the γ of the SO₂/O₃ system on α-A1₂O₃ particles was higher than that of SO₂/NO₂, and O₃ oxidized SO₂ to sulfate more easily than NO₂. In addition, the heterogeneous reaction pathway of SO₂/NO₂ on α-A1₂O₃ varied with humidity. Furthermore, N₂O₄ replaced nitrite as intermediate in the presence of SO₂ under dry conditions. Under high RH conditions, the intermediate was HONO (Figure 6).

3.1.4. Heterogeneous Reactions of SO₂ and H₂O₂ on the Surfaces of AMPM

Moreover, H₂O₂, as an important oxidant in the atmosphere, has potential effects on the heterogeneous reaction of SO₂ on the surfaces of mineral particles. Huang et al. [89] investigated the uptake kinetics of SO₂ on α-A1₂O₃ in the absence and presence of H₂O₂ at different RH values. The results showed that the saturation coverage of SO₂ increased with the increase in RH in the absence of H₂O₂, because the adsorption of water on the surfaces of particles is beneficial to the hydrolysis of SO₂ and mitigates the increase in surface acidity. However, the reaction ultimately reached saturation because the produced sulfite/bisulfite could not be further converted to sulfate/bisulfate in the absence of oxidants. In the presence of H₂O₂, the saturation coverage of SO₂, as well as the time required for the reaction to reach saturation, increased with increasing RH. A similar phenomenon has been observed for other types of particulate matter, such as chemically inactive SiO₂. Under the same conditions, Huang et al. [69] also investigated the uptake kinetics of SO₂ on three authentic mineral dust samples (Asian mineral dust (AMD), Tengger desert dust (TDD), and ATD). It was found that, due to the different mineralogic compositions and aging extents of these dust samples, with an increase in RH, the corrected uptake coefficient (γ_c) decreased for the AMD particles but increased for the ATD and TDD particles. It was determined that the presence of H₂O₂ can promote the absorption of SO₂ by mineral powders under different humidity conditions and generate •OH radicals on the mineral dust surface through heterogeneous decomposition to promote the formation of sulfate.

3.1.5. Heterogeneous Reactions of NH₃/CO₂ and SO₂ on the Surfaces of AMPM

In addition to these oxidizing gases, Yang et al. [85,86] found that NH₃ could coordinate with SO₂ to generate sulfate and ammonium in typical mineral dust (α-Fe₂O₃, MgO, γ-Al₂O₃, and TiO₂). Hydroxyl and oxygen on the surface of mineral dust played major roles in the conversion of SO₂ to sulfate. However, NH₃ was adsorbed on the Lewis acid site and the hydroxyl groups on the mineral dust surface, enhancing the surface Lewis basicity and leading to more SO₂ molecules being adsorbed on the surface of the mineral dust. Surface-adsorbed SO₂ interacted with the surface hydroxyl and oxygen atoms and was oxidized to sulfate. In turn, the formation of sulfate contributed to the adsorption of NH₃, mainly as NH₄⁺, owing to the enhanced Brønsted acid sites. The synergistic effect between SO₂ and NH₃ was more significant for acid mineral oxides, such as γ-Al₂O₃ and α-Fe₂O₃, than for basic oxides, such as MgO. Further studies have shown that, due to the different affinities of mineral dusts to water, adsorbed water can inhibit the adsorption and transformation of SO₂ on α-Fe₂O₃, despite the presence of NH₃. In γ-Al₂O₃, the increase in RH slightly promoted the formation of sulfate but inhibited the formation of ammonium. Kebede et al. [70] found that surface water from TiO₂ particles could catalyze the conversion of surface NH₃ into NO_x, and that the photo-oxidation of NH₃ on the surface of mineral oxides may be one of the sources of NO_x in the atmosphere. Carbon dioxide, the most prominent greenhouse gas, is 4–5 orders of magnitude higher than the above trace gas. Liu et al. [97] found that CO₂ competed with SO₂ on α-A1₂O₃ for adsorption sites and blocked the active groups, thus inhibiting the heterogeneous oxidation of SO₂ on α-A1₂O₃. These findings suggest that atmospheric CO₂ has a negative effect on sulfate formation, which potentially decreases solar radiation scattering and exacerbates global warming.

3.2. Heterogeneous Reactions of Organic Gases on the Surface of AMPM

As an important component of APM, under different atmospheric temperatures and RH levels, organic matter mainly undergoes heterogeneous chemical reactions with O₃, NO₃, and •OH in the atmosphere [14,98]. This aging process affects a series of physicochemical properties of the atmospheric particles, such as the chemical composition, phase state, particle size, hygroscopicity, density, and optical properties, thus affecting human health, air quality, and climate change [16,48]. Studies have shown that SOA has multiple formation mechanisms, with three main pathways [6,63]: (1) in the gas phase, VOCs generate semi-volatile organic compounds (SVOCs) through chemical reactions, and the SVOCs are distributed in the gas/particle phase during the nucleation and growth of new aerosol particles [47,78,99]; (2) VOCs and SVOCs enter existing aerosols or cloud droplets via adsorption or absorption [7,43,50]; and (3) heterogeneous reactions. Different reactions can occur on the surface of aerosols or particles, the interface of the gas–liquid phase, and in the interior of aerosols to produce different low and non-volatile organic compounds (NVOCs) and further enrich the composition of aerosols [12,100]. Further study of these three formation mechanisms has shown that the formation of SOA is complex and diverse, causing it to be considered the greatest aspect of uncertainty in global climate model constructions [6]. Therefore, it is important to explore the SOA formation mechanisms.

Xu et al. [82] used TEM and EDS to characterize the morphology, composition, and mixing state of individual aerosol particles in Lin’an, the surrounding area of the Yangtze River Delta (Figure 7). They classified individual aerosol particles into seven types, including sulfur-rich, potassium-rich, organic matter (OM), soot, fly ash, metal, and mineral particles, and found that the fraction of organic-coated S-rich inorganic particles was proportional to sunshine intensity. These findings imply that light can promote the formation of SOA through the heterogeneous reactions of organic matter on the surface of S-rich inorganic particles. They also imply that organic matter can form SOA through heterogeneous reactions on the surfaces of S-containing inorganic particles. However, there is a great deal of uncertainty regarding the origin and formation mechanisms of SOA. This is mainly due to the complexity of organic reactions, the variety of species, and the variability of reaction conditions. However, the adsorption or catalytic oxidation of VOCs on the particulate surface are the two heterogeneous reaction pathways to the formation of SOA.

3.2.1. Heterogeneous Uptake of VOCs on AMPM

1. Effects of humidity and temperature on the heterogeneous reactions

Owing to the different surface properties of minerals and their different ways of interacting with H₂O molecules, RH (the surface adsorption of water) affects the adsorption capacity of different minerals to form VOCs. For example, the adsorption of mineral dust on organic acids increases with increasing humidity, whereas the adsorption of some mineral dust decreases with increasing humidity [101,102,103]. Tang et al. [104] investigated the heterogeneous uptake of gaseous acetic acid by different oxides, including γ-Al₂O₃, CaO, and SiO₂, under a range of RH conditions. Under dry conditions, the uptake of acetic acid led to the formation of both acetate and molecularly adsorbed acetic acid on γ-Al₂O₃ and CaO, and molecularly adsorbed acetic acid alone on SiO₂. When the humidity increased from 0% to 15%, the adsorption rate of the gaseous acetic acid molecules by γ-Al₂O₃ increased by three to five times, whereas for SiO₂ particles, acetic acid and water were found to compete for the surface adsorption sites. Fang et al. [105] showed that both HCOOH and HNO₃ could reversibly adsorb onto the SiO₂ surface under dry conditions. At elevated RH levels, the adsorbed water competed with these acids for surface adsorption sites and promoted their dissociation into hydronium ions and the corresponding anions. The competitive adsorption mechanism of acetic acid and water on kaolinite has been explored in detail by Alstadt et al. [106]. Four chemisorbed acetate species were found on kaolinite after exposure to acetic acid, where acetate bonded through a monodentate or bidentate bridging linkage with an aluminum atom. These species exhibited varying levels of stability after the introduction of water, indicating that water vapor affects the adsorption of organic acids. Specifically, the type of −OH on the surface of mineral dust determines the binding of carboxylic acids to the surface and the stability of these interactions with the addition of water vapor. In atmospheric environments, the ability of H₂O to replace chemisorbed molecules may affect the efficiency of mineral dust as a sink for these compounds, thus affecting the atmospheric composition. Romanias et al. [107] studied the heterogeneous interaction between natural Gobi mineral dust and isopropanol at different humidity levels and temperatures. The initial uptake coefficient (γ) was found to be independent of temperature and was negatively correlated with RH. In addition, under UV light, trace TiO₂ in natural Gobi dust could oxidize isopropanol to form small amounts of oxygenated VOCs (acetone, formaldehyde, acetic acid, and acetaldehyde).

2. Effects of different mineral dust and VOCs on the heterogeneous reactions

Wu et al. [68] found that α-Al₂O₃ and CaCO₃ could effectively heterogeneously uptake representative unsaturated organic acids (acrylic and methacrylic acids) in the atmosphere. FTIR characterization indicated that these organic acids could interact with the surface OH groups on α-Al₂O₃, leading to the formation of carboxylate. Conversely, during the heterogeneous uptake of CaCO₃, the protons in acrylic acid could react with the HCO₃⁻ ions released from Ca(OH)(HCO₃) to form carbonic acid. These results suggested that heterogeneous reactions on α-Al₂O₃ and CaCO₃ can act as important sinks for acrylic acid and methacrylic acid, as well as possible contributors to the organic coating found on atmospheric aerosols, especially during high pollution events. Zhang et al. [108] found that the adsorption of VOCs (butyl propionate, 2-heptanone, 1,2,4-trimethylbenzene, and 2-butoxyethanol) on the surface of natural minerals (diatomite, stellerite, and vitric tuff) was positively correlated with the polarity and boiling point of the VOCs, as well as the specific surface area and pore volume of the natural minerals, but negatively correlated with environmental temperature and RH. Romanias et al. [109] studied the heterogeneous interactions of limonene and toluene with dust in six Saharan regions. The results showed that the metal oxides in Saharan dust may affect the dust acidity to enhance the adsorption of limonene, and the SiO₂ adsorption activity of toluene was low; therefore, the higher the SiO₂ content was, the smaller was the ability of the Saharan sand to adsorb toluene. Because of the influence of SiO₂, the six regional mineral dusts could adsorb more limonene than toluene (Figure 8). In addition, the desorption fraction and desorption rate coefficient of the dust samples with a high SiO₂ content were significantly reduced, the adsorption of dust samples with a high SiO₂ content was mainly irreversible, and the adsorption of dust samples with a low SiO₂ content was mainly reversible (Figure 8). Dourtoglou et al. [110,111] removed the metal oxides in natural mordenite and montmorillonite by acid treatment, confirming that the composition of the metal oxides in minerals can regulate their acidity, thus affecting their adsorption and transformation of limonene. Niu et al. [112] also confirmed that the interaction between toluene and SiO₂ was very weak at room temperature, and the adsorption of SiO₂ to form toluene was much smaller than that of α-Fe₂O₃ and BS (butlerite and szmolnokite). Wang et al. [113] studied the heterogeneous uptake of formic acid and acetic acid on mineral dust and fly ash in a Knudsen cell reactor in the temperature range of 263–298 K. The study showed that the uptake mechanisms of formic acid and acetic acid in the Inner Mongolian desert sand, ATD, and coal fly ash were mainly ones of physical adsorption, while the uptake process in Xinjiang sierozem was a combination of physical and chemical adsorption, as it contained a considerable amount of calcite. The uptake coefficients of formic and acetic acid on mineral dust increased with the content of elemental Si, aluminosilicate, and/or quartz in the mineralogy, but the uptake coefficients of coal fly ash were smaller than those of mineral dust, which was controlled by the chemical components and microstructure. Compared with formic acid, the uptake coefficients of acetic acid were higher for mineral dust and coal fly ash, especially in low-temperature regions. Iuga et al. [62] studied the adsorption of a single formic acid molecule on different silicate surface models (molecular clusters and a periodic crystal model of the (001) pyrophyllite surface) using methods of quantum mechanics. The silanol groups were found to be the most reactive formic acid adsorption sites. In the case of a periodic system, the silanol groups at the crystal edges were favored. However, the OH groups on the phyllosilicate octahedral sheet were also reactive sites through the tetrahedral cavities. Zeineddine et al. [49] further found that the isoprene uptake in natural Gobi dust was also negatively correlated with ambient temperature and RH. However, unlike isoprene, isopropanol has hydroxyl groups in its structure; therefore, the lifetime of the isopropanol removal by its heterogeneous interaction with the same Gobi dust sample was much longer than that of isoprene. The considerable difference in the lifetime values of the two compounds on the same natural sample indicates that the interactions between VOCs and mineral dust cannot be predicted or inferred from one VOC to another, and they should be determined experimentally to assess the individual importance of VOCs in the atmosphere through different heterogeneous pathways. Therefore, the structure of VOCs limits their interaction mode and pathway on the mineral surface, thus determining their fate and reactivity in heterogeneous reactions, rather than being directly related to their atmospheric abundance.

The aforementioned research has shown that the adsorption of VOCs on the surface of atmospheric mineral dust acts as an important sink of atmospheric VOCs and an important means of forming SOA. In other words, atmospheric mineral dust can be regarded as a reservoir for atmospheric organic matter. In addition to the impacts of environmental temperature and humidity, the physicochemical properties of VOCs (molecular size, group type, polarity, and boiling point), as well as mineral dust types, chemical composition, specific surface area, surface group type, and content, are critical influencing factors.

3.2.2. Heterogeneous Catalytic Oxidation of VOCs on AMPM

The catalytic oxidation of VOCs on the surfaces of mineral particles is another means of forming SOA, which may also cause changes to the atmospheric oxidation and the health risks of atmospheric particles. Styler et al. [114] found that both natural minerals (Mauritanian sand and Icelandic volcanic ash) and metal oxides (Fe₂O₃ and TiO₂) could oxidize oxalate to CO₂ under light conditions. The relationship between the oxidation of oxalic acid and the iron and titanium on the surfaces of four types of mineral dust to produce CO₂ was studied in the presence and absence of oxygen. Part of this reaction was mediated by iron, and titanium played an important role. Chen et al. [115] studied the oxidation pathways of toluene on TiO₂ by combining theoretical calculations and experiments. In general, the homogeneous atmospheric oxidation of xylenes was initiated by •OH, leading to minor H-abstraction and major •OH addition pathways. However, theoretical calculations demonstrated a favorable H-abstraction of the methyl group of xylenes by surface •OH with large exothermic energies, because TiO₂ was more easily connected to the methyl in the xylene by hydrogen bonds than it was to the benzene ring (Figure 9). The experimental results highlighted major H-extraction (87.18%) and minor OH-addition (12.82%) pathways to the OH-initiated heterogeneous oxidation of the three xylenes on TiO₂ under UV irradiation. Furthermore, the acute risk assessment results indicated that H-extraction products significantly promoted the formation of APM, with a significantly increased risk to human health.

Ponczek et al. [116] found that the adsorption capacity of butanol on ATD was inversely proportional to the environmental temperature and humidity, and the larger the atmospheric mineral dust load was, the smaller was the lifetime of butanol in the atmosphere. Under light conditions, trace TiO₂ in the ATD reacted with H₂O molecules to produce •OH radicals, thus oxidizing butanol into smaller VOCs. Lv et al. [17] found that isoprene (C₅H₈) in the atmospheric environment was directly photo-oxidized by iron oxide clusters to generate formaldehyde (CH₂O), without the participation of H₂O to generate •OH radicals. Under illumination, the photo-oxidation and desorption of isoprene on iron oxide clusters were competitive. Interestingly, both the photo-oxidation and desorption of isoprene on the iron oxide clusters increased with increasing cluster size. However, in the gas-particle interaction system, the desorbed C₅H₈ molecules were re-adsorbed by the iron oxide clusters, and the large iron oxide clusters could absorb photons and C₅H₈ molecules more efficiently, thus promoting the formation of CH₂O. Theoretical studies have shown that the photo-oxidation of C₅H₈ to CH₂O must overcome a significant barrier, and that light irradiation can accelerate this process. Moreover, compared with UV irradiation, visible irradiation can selectively increase the yield of CH₂O.

These studies have shown that atmospheric mineral dust is not inert but acts as a reservoir of organic matter in the atmosphere. Trace metals such as Fe, Ti, and Al in mineral dust can oxidize the adsorbed organic matter on the mineral dust surface, either directly or through •OH radicals generated by the reaction of these metals with water molecules. Therefore, atmospheric mineral dust can not only act as a sink for organic pollutants in the atmosphere, but also transports them into the atmosphere and changes the oxidation and toxicity of atmospheric particles during the migration.

3.3. Heterogeneous Reactions of Mixed Gas on the Surface of AMPM

3.3.1. Heterogeneous Reaction of VOCs and SO₂ on the Surfaces of AMPM

Currently, there are few studies on the heterogeneous reactions of organic and inorganic mixed pollution gases on the surface of AMPM. Shao et al. [83] used TEM-EDX to analyze the morphology, composition, and mixing state of single particles collected after the implementation of the “Action Plan for Comprehensive Prevention and Control of Autumn and Winter Air Pollution in Beijing–Tianjin–Hebei and Surrounding Areas 2017–2018” (the Action Plan). After comparing the relative proportions of different particle types and the main pollution sources before and after the implementation of the Action Plan, it was speculated that SO₂ and organic matter can react on atmospheric particles (fly ash, mineral dust, and black carbon) to form mixed particles, which may act as an important sink of SO₂ and atmospheric organic matter. In the atmospheric environment, VOCs are oxidized to oxygenated volatile organic compounds (OVOCs) by atmospheric oxidants (such as O₃, NO₃, and •OH) via gas and heterogeneous reactions. OVOCs can form SVOCs and NVOCs through free-radical reactions, thus forming SOA [43]. OVOCs can also be further oxidized and decomposed into small molecules, such as alcohols, aldehydes, ketones, ethers, or acids, and finally oxidized to CO₂ [6,117]. There are three main ways by which the effects of small-molecule organic matter on atmospheric heterogeneous reactions are produced:

1. Small-molecule organic matter competes with other polluting gases for reactive oxygen species (ROS) to form aloxy radicals or peroxy alkyl radicals, thereby promoting or inhibiting the reaction of other polluting gases. Chu et al. [118] found that pre-adsorbed propene (C₃H₆) on TiO₂ had no effect on sulfuric acid formation compared to the SO₂ reaction on TiO₂ alone. This is because formic acid, acetic acid, and other substances produced by the heterogeneous oxidation of C₃H₆ could be easily removed from the surface of TiO₂. However, by introducing both C₃H₆ and SO₂ simultaneously, C₃H₆ competed with SO₂ for ROS, thus inhibiting sulfate formation. In addition, when TiO₂ was exposed to NO₂ and C₃H₆ simultaneously, the reaction products occupied the active sites of TiO₂, thus suppressing sulfate formation during heterogeneous photo-oxidation.

2. Small-molecule organics inhibit or promote the reactions by binding to or acting as a bridge between the particulate surface and the pollutant gas. Wu et al. [119] found that formic acid and SO₂ competed for adsorption on α-Fe₂O₃, which has a higher affinity to SO₂, and only a small amount of formic acid was absorbed on its surface to form formate. However, a small amount of formate on the surface of α-Fe₂O₃ was catalyzed by SO₂ in the gas phase to form CO₂, which further promoted the conversion of SO₂ in the gas phase to sulfate (Figure 10). This indicates that formic acid and SO₂ had a synergistic effect on α-Fe₂O₃. Li et al. [120] further systematically investigated the effects of dicarboxylic acid salts (including oxalate, malonate, and succinate) on the heterogeneous SO₂ transformation with hematite nanoparticles, using a film flow reaction and in situ DRIFTs experiments. It was observed that trace amounts of dicarboxylic acid salts significantly accelerated the sulfate formation under solar light. In addition, the absorption of SO₂ increased with the increase in light intensity, and the Fe-oxalate complexation under light irradiation also enhanced the solubility of iron.

3. Small molecule organics compete with other pollutants for reaction sites on the particle surface, thus inhibiting the reaction. Yang et al. [96] found that HAc and SO₂ competed for the active sites on α-Al₂O₃ and CaCO₃ to form acetate and sulfite, respectively. However, the gaseous state of HAc was more likely to occupy the -OH and Lewis acid active sites, adsorb to the particulate matter surface, and eventually form acetate. Sulfites could still be generated in the presence of water because water provides abundant -OH active sites, sufficient for HAc and SO₂ to be used on the surface of the CaCO₃ particles. Wang et al. [72] investigated the influence of NO₂ and acetic acid on the heterogeneous reaction of SO₂ on the surface of CaCO₃ using DRIFTS. It was further confirmed that there was a synergistic interaction between NO₂ and SO₂, and a competitive interaction between acetic acid and SO₂ on the surface of CaCO₃ particles. NO₂ could promote sulfite oxidation to sulfate, while acetic acid inhibited sulfite generation. Additionally, Zhao et al. [121] found that the pre-adsorption of SO₂ on the α-Fe₂O₃ surface significantly hindered the subsequent heterogeneous oxidation of CH₃CHO to produce acetate, whereas the pre-adsorption of CH₃CHO significantly inhibited the heterogeneous oxidation reaction of SO₂ on the α-Fe₂O₃ surface.

3.3.2. Heterogeneous Reactions of O₃ and VOCs on the Surfaces of AMPM

Ozone in the atmospheric environment also has an important influence on the adsorption and heterogeneous reactions of organic compounds on the surfaces of mineral particles. Wang et al. [122] conducted a comprehensive theoretical study on the heterogeneous reaction processes of four low-molecular-weight unsaturated ketones and O₃ on SiO₂. They found that not only did SiO₂ exhibit a better adsorption performance with these ketones, and a better sedimentation of low-molecular-weight oxygen-containing VOCs in the atmosphere, it also accelerated the reaction between the unsaturated ketones and O₃ in the atmosphere. At the same time, the heterogeneous reactions of these ketones on the surfaces of available mineral aerosols may compete with the corresponding gas-phase reactions, especially in arid and semi-arid areas with frequent dust storms. Lian et al. [123] studied the influence of humidity on the heterogeneous reactions of isoprene and O₃ on α-Al₂O₃. The results showed that the presence of O₃ led to the rapid transformation of isoprene into carboxylate (COO⁻) ions on the surface of α-Al₂O₃ particles, whereas the formation of carboxylate ions was significantly inhibited with an increase in humidity. Ma et al. [124] investigated the heterogeneous reaction of O₃ with anthracene adsorption on TiO₂ and Asian sandstorm particles in the presence and absence of light at room temperature. The adsorption of O₃ and anthracene on the Asian sandstorm particles followed the Langmuir–Hinshelwood mechanism. The consumption of anthracene on TiO₂ may be caused by volatilization and photodimerization in a flow of dry N₂, in the presence of light. In the dark, the anthracene on TiO₂ was oxidized to anthraquinone by O₃. Under light conditions, the rate constant of the heterogeneous reaction between O₃ and anthracene adsorbed on TiO₂ was 1.5 times higher than that under dark conditions, and anthraquinone was photo-oxidized further. Moreover, at extremely high O₃ concentrations, the anthracene degradation rate in the presence of light was three times higher than that under dark conditions. In addition, the k_1,obs values of the heterogeneous reactions of O₃ and anthracene adsorbed by the Asian dust storm particles were two orders of magnitude lower than those measured on SiO₂, α-Al₂O₃, and α-Fe₂O₃ under dark conditions.

3.3.3. Heterogeneous Reactions of NO₂ and VOCs on the Surfaces of AMPM

Similarly, Chen et al. [125] found that TiO₂, a photoactive material, could promote the reaction between m-xylene and NO_x, thus reducing the yield of SOA generated by its interaction in the gas phase. In particular, after the addition of (NH₄)₂SO₄ inorganic aerosol seeds in the gas phase, the yield of SOA decreased. Ma and Wang [126,127] found that the heterogeneous reaction between NO₂ and polycyclic aromatic hydrocarbons (PAHs) adsorbed on SiO₂, Al₂O₃, and TiO₂ led to the formation of NPAHs. Kameda et al. [21] studied the adsorption and transformation of PAHs on quartz surfaces and reported that the active acid sites on the mineral surfaces were mainly used as electron receptors to participate in the transformation reaction of PAHs and NO₂. Ji et al. [18] studied the effect of SiO₂ on the heterogeneous reaction between formaldehyde and NO₂ using a quantum chemistry method and found that formaldehyde was more easily adsorbed on the SiO₂ model than NO₂. The presence of SiO₂ reduced the reaction barrier and accelerated the reaction rate of formaldehyde and nitrogen dioxide on the SiO₂ surface. The reaction produced HONO and altered the oxidation capacity of the atmospheric environment. Chu et al. [128] systematically studied the interaction between NO₂, SO₂, and toluene in the system of Al₂O₃ as a seed aerosol. A synergistic effect was observed between NO₂, SO₂, and toluene, which was enhanced under high concentrations of NH₃.

Therefore, very complex heterogeneous reactions can occur under complex air pollution conditions because of the ubiquitous interactions between organic and inorganic species. The relationship between co-existing VOCs and inorganic gases on different mineral surfaces changes due to changes in the mineral and VOCs species and even in the external environment. This makes the oxidation and toxicity of particles more complex through the heterogeneous reaction processes. Therefore, to better explain the causes of haze and evaluate its toxicity, the interactions and influences of VOCs and other inorganic gases and minerals must be considered.

4. Conclusions and Outlook

The environmental and climatic effects of aerosols have received increasing attention, leading to more intensive studies of the heterogeneous reactions on the surface of APM. Although the impacts of mineral dust on the generation of secondary particles are complex, mineral particles may promote atmospheric oxidation and exacerbate haze formation. The exploration of the assemblages of mineral interface reaction products and the significance of their co-evolution can provide a scientific basis for identifying air pollutant sources and pollution control planning.

Firstly, we used China as an example to briefly introduce the current situation and developmental trend of APM pollution. The concentrations of APM in Chinese cities were relatively low before 2010 and then increased annually, with significant seasonal and regional characteristics. After the implementation of a series of air pollution prevention and control policies, especially since the implementation of the Air Pollution Prevention and Control Action Plan in 2013, the emissions of APM, SO₂, and NO₂ have ehibited a downward trend, the frequency of heavy pollution weather has declined, and the ambient air quality has been improved. However, compared with other countries, the APM concentrations in China remain high, with a large proportion of cities still exceeding the APM pollution standards, and O₃ pollution exhibiting an upward trend. APM and O₃ are currently important pollutants affecting the environmental air quality in China, and there are intricate correlations between them. Therefore, the coordinated control of APM and O₃ is key to the prevention and control of air pollution.

Secondly, we reviewed the sources and characteristics of APM and the surface reaction characteristics of polluted gas and mineral particulate matter. Additionally, we summarized the effects of mineral particulate matter on aggregation, regulation, and catalysis in the formation of atmospheric aerosols. The aim of our analysis was to provide guidance for future studies on the reactions between AMPM and pollutant gases in the atmosphere to form secondary aerosols, and their effects on the chemical composition of the atmosphere. Furthermore, this review is of environmental significance, since it facilitates future studies of both the micro-interface chemistry of the surface characteristics of AMPM in complex pollutants and the combined effects of APM-pollutant aerosol systems in fog-haze formation, transformation, particle production, and blocking behavior.

Finally, having reviewed the existing studies, it can be concluded that a few unresolved issues in the study of atmospheric heterogeneous reactions remain, which will be the focus of future studies. These include the following: (1) The correlation between the atmospheric fine particles and ozone. There is a complex relationship between the formation of atmospheric fine particulate matter and ozone. They not only share common precursors, but also influence one another in the atmosphere in various ways. Therefore, the coordinated control of atmospheric fine particulate matter and ozone is difficult to enact and will be a major challenge for China in its efforts to improve air quality in the next stage. (2) The function of natural mineral dust in haze formation. At present, the role and contribution of mineral dust aerosols to atmospheric aerosol pollution are understood to be mostly based on metal oxides, SiO₂, and other active particles. The structure, physics, and physiochemical characteristics of the natural mineral dust, which occupies the main body of the atmospheric particles, and their role in haze formation have been overlooked. Therefore, the synergistic evolution effect mechanism of mineral dust and polluted gas should be explained. (3) Compound effects in heterogeneous reactions. Most studies on the formation process of haze are simplified and involve single laboratory simulation reactions, and few involve the complex action system of multi-component gas. Research on the organic/inorganic mixed gas action system is especially lacking and does not reflect the migration or transformation of multi-component complex gases, nor does it focus on the synergistic or antagonistic effects among various complex or parallel reactions in the process of haze accumulation, which limits our understanding of the formation mechanisms of aerosols. Therefore, it is necessary to study heterogeneous reaction processes when multiple pollutants coexist in a more realistic atmosphere. (4) The role of micro-nano mineral particles. In the future, research should focus on the control mechanisms of the micro-nano mineral surface, micro-area curing, and catalytic transformation of trace pollution gas in the atmosphere from a mineralogical perspective, based on the multiple characteristics of micro-nano mineral particles in haze, such as pollutant sources, carriers, and reaction sites. Additionally, attention should be given to the combined effects of the micro-nano mineral-pollutant aerosol system on fog haze formation, transformation, particle production, and blocking behavior.