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][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][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][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][11,12]. Its main components include mineral dust, sea salt, organic aerosols, sulfate, nitrate, ammonium salt, and black carbon ^[13][14][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][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][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]}[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][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][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][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. 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][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][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. 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]}[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). Moreover, they found that Beijing, Kanpur, and Dhaka all had much higher levels of PM_2.5 and SIA than other cities.

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

Organic pollutants form the main component of APM, accounting for approximately 20–50% of the particulate matter ^[43][44][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][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]}[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][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]}[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][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.