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., SO
2, NO
x, VOCs, and NH
3) 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 SO
2, NO
x, and NH
3 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 SO
42− average concentration was 38.33 ± 26.20 μg/m
3, accounting for 44.65 ± 11.30% of the total water-soluble ions, while the NH
4+ and NO
3− average concentrations were 21.16 ± 16.28 and 15.77 ± 12.06 μg/m
3, 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 SO
42−, NH
4+, and NO
3− together accounted for 59.3–77.7% and 59.3–77.1% of PM
2.5 and PM
10 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 PM
10, respectively. In particular, the concentration of SO
42−, at 6.0–22.0 μg/m
3, 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 SO
42−, NO
3−, and NH
4+ 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 NH
4+, NO
3−, and SO
42− 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 PM
10, 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, SO
42−, NH
4+, and NO
3− 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 SO
42−, NO
3-, and NH
4+) in each observation site accounted for 15–40% of the total mass of PM
2.5, and the content of SO
42− 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.
Table 1. PM2.5 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 |
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, •NO
3, and O
3) in the atmosphere, which can form new free radicals (•HO
2, •RO
2, •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.