Characteristics of Major Air Pollutants in China

Abstract

Following the rapid development of China’s economy, air pollution has become more and more serious. Air pollution in China presents complex pollution characterized by high PM_2.5 and O₃ concentration. This study presents an overview of the status of air quality and emission in China and discusses the temporal and spatial distribution of major pollutants (PM₁₀, PM_2.5, SO₂, NO_X, and O₃). The results show that the reduced emissions have improved the air quality in China. However, the Chinese National Ambient Air Quality Standard (CNAAQS) for PM₁₀ and PM_2.5 still be exceeded in many cities of China in 2015. A total of 77.5% (for PM_2.5) and 65.4% (for PM₁₀) of the monitoring cities were found to be exceeded CNAAQS. The average annual O₃ concentration was increasing during 2013–2015, and 16% of the total cities in 2015 did not meet the CNAAQS, indicating that O₃ pollution should be paid more attention. For NO₂ and SO₂, the exceedances of CNAAQS are rare. PM_2.5, PM₁₀, and SO₂ concentrations are higher in northern than in southern regions. High NO₂ occurred in Beijing-Tianjin-Hebei and Yangtze River delta region. Secondary particles formation and motor vehicle exhaust were the main sources of PM_2.5 in megacities. Dust was the main source for PM₁₀. The formation of O₃ is VOC-limited in urban areas of China and NO_X-limited in nonurban areas.

2.1 Introduction

In recent years, China’s economy has developed rapidly, the process of urbanization and industrialization has been speeded up, and energy consumption has increased. The statistics shows that China’s gross domestic product (GDP) annual growth rate was 6.9–9.5% during 2011–2015 [17]. According to China Vehicle Environmental Management Annual Report, the vehicle population in 2015 was 279 million [11,12,15]. Since the late 1970s, the total energy consumption has greatly increased from 571 million tonnes of coal equivalence (Mtce) in 1978 to 4300 Mtce in 2015 [18]. Coal is the major fraction of energy consumption, accounting for 70% of China’s energy consumption. Coal burning is the major source of ambient sulfur dioxide (SO₂), nitrogen oxides (NO_X), and soot.

With the development of economy, regional complex air pollution (characterized as a complex status of ozone (O₃) and fine particle (PM_2.5)) is one of the major environmental problems. Since 2013, heavy pollution events occurred frequently in China with 75% of cities and eight million people suffering from haze pollution, which constrains sustainable development of society and economy and threatens human health [8]. Air quality in China ranked the second to last in 180 countries, only better than Bangladesh [6].

Air pollutants can cause a variety of health problems. Exposure to high concentration of particulate matters can increase mortality or morbidity; excessive O₃ can cause breathing problems, trigger asthma, reduce lung function, and cause lung diseases; long-term and peak exposures to high NO_X can increase symptoms of bronchitis in asthmatic children; and SO₂ can affect the respiratory system and the functions of the lungs and causes irritation of the eyes [27]. The 1948 Donora smog caused by SO₂ and its oxides killed 20 people and sickened 5911 people [4]. The 1952 Los Angeles photochemical smog episode killed 400 people and a great many of people with red eyes, swollen throat, inflammation, and other respiratory diseases [23]. Therefore, it is both important and valuable to study the atmospheric pollution status and the variation characteristics of air pollution and its influential factors.

In this study, based on the data of air pollutants (PM₁₀, PM_2.5, SO₂, NO_X, and O₃) obtained from the national air pollution monitoring network in China, we will present an overview and analysis of air quality in China, analyze emission and pollution characteristics of major atmospheric pollutants, and then discuss temporal and spatial distributions of these pollutants. The study results will provide basic information for studying the health effects of air pollutants.

2.2 Chinese National Ambient Air Quality Standard

The Chinese National Ambient Air Quality Standard was issued firstly in 1982, when concentration limits for total suspended particulates (TSP), SO₂, NO₂, lead, and BaP were set. This standard was both strengthened and expanded in 1996. In 2000, the standard was updated with less stringent limits for certain pollutants. In February 2012, China released a new ambient air quality standard, GB 3095-2012, which set limits for the first time on PM_2.5 and Maxium daily 8-h ozone (Table 2.1). Meanwhile, the standard threshold of PM₁₀ and oxynitride has also been tightened up.

Table 2.1 Chinese National Ambient Air Quality Standard (GB 3095-2012) vs. WHO AQG

Current air quality standards include two grades of limit values. Grade I standards apply to special regions such as national parks. Grade II standards apply to all other areas, including urban and industrial areas.

The 24-h and annual PM_2.5 limit values are set at 75 μg/m³ and 35 μg/m³ (Table 2.1). WHO AQG is stricter than the Chinese National Ambient Air Quality Standard. The recommended WHO AQG short-term (24-h) and long-term (annual average) values were 25 μg/m³ and 10 μg/m³ for PM_2.5. The United States published the National Ambient Air Quality Standard for PM_2.5 in 1997 (24-h average, 65µg/m³; annual average, 15 μg/m³), but the Ministry of Environmental Protection of the People’s Republic of China did not published the National Ambient Air Quality Standard for PM_2.5 until 2012.

The 24-h and annual PM₁₀ limit values are set at 150 μg/m³ and 70 μg/m³. WHO AQG for PM₁₀ is lower than the Chinese National Ambient Air Quality Standard (Table 2.1). The recommended WHO AQG short-term (24-h) and long-term (annual average) values were 50 μg/m³ and 20 μg/m³ for PM₁₀.

The Chinese National Ambient Air Quality Standard (GB 3095-2012) set by the Ministry of Environmental Protection of the People’s Republic of China (MEP) for SO₂, as well as WHO guideline, is shown in Table 2.1. The limit value for the annual and 24-h mean SO₂ concentration are set at 60 μg/m³ and 150 μg/m³. Unlike the usual 24-h and annual mean levels, WHO recommends that SO₂ follows a more stringent 10-min and 24-h intervals based on recommendations resulting from epidemiological studies. The yearly guideline is not needed since the 24-h guideline would be sufficient in assuring low annual average level.

The limit value for the annual, 24-h, and 1-h mean NO₂ concentrations is set at 40 μg/m³, 80 μg/m³, and 200 μg/m³, respectively, which was identical to WHO AGQ. A maximum daily 8-h O₃ mean concentration was set at 160 μg/m³ and 1-h mean was 200 μg/m³ in China. The WHO AQG for O₃ is a daily maximum 8-h mean concentration of 100 μg/m³, as shown in Table 2.1. This recommended limit was reduced from the previous level of 120 μg/m³, based on recent conclusive associations between daily mortality and lower O₃ concentrations [27].

2.3 Characteristics of Major Air Pollutants

2.3.1 Fine Particle (PM_2.5)

2.3.1.1 Characteristics of PM_2.5 Pollution

China is one of the countries worst hit by PM_2.5 pollution. According to the global map of PM_2.5 published by NASA [16], the PM_2.5 pollution in north and east of China is the most serious, which was higher than that in India (Fig. 2.1). Recently, following the change of energy consumption structure, the pollution characteristics of particulate matters in China has changed from “coal smoke pollution” to “complex pollution,” which is characterized by high PM_2.5 and O₃ concentration.

Fig. 2.1

Global satellite-derived map of PM_2.5 averaged over 2001–2006 [16]

According to the report of the China air quality database, the CNAAQS limit value for PM_2.5 was exceeded in many cities of China in 2015. PM_2.5 has the highest percentage of exceedance among monitored pollutants. In 2015, the annual PM_2.5 concentrations in 338 cities were 11–125 μg/m³ with an average of 50 μg/m³, which was 0.43 times higher than the Chinese National Ambient Air Quality Standard (35 μg/m³). The exceedances occurred in 77.5% of the case in all the monitoring cities (Fig. 2.2). None of the cities were compliant with annual PM_2.5 World Health Organization (WHO) air quality guideline (10 μg/m³). The proportion of the number of days exceeded the Chinese National Ambient Air Quality Standard was about 17.5% [13].

Fig. 2.2

Percentage of cities at different PM_2.5 concentration levels in 338 cities, 2015 (Data sources: MEP [13])

PM_2.5 has obvious spatial and temporal distributions related to the patterns of source emissions, chemical reaction mechanism, regional transport, and other meteorological conditions (such as dry and wet deposition). Figure 2.3 shows the spatial distribution of PM_2.5 in 2015 in China. As shown in Fig. 2.3, higher PM_2.5 concentration is mainly concentrated in Beijing-Tianjin-Hebei region, the north and middle part of Shandong province, the south and middle part of Henan province, and most of Hubei province. Generally, PM_2.5 annual concentration in the northern region was much higher than in the southern region. A number of studies have revealed that the higher concentrations in the northern region were related to the emissions from fossil fuel combustion and biomass burning. The colder north burns much more coal for winter heating and has more heavy industry, which emits a large amount of particulate matter [5].

Fig. 2.3

Spatial distribution of PM_2.5 annual average concentrations in 2015 (Data sources: National air pollution monitoring network in China)

With respect to seasonal variation, PM_2.5 has higher concentration in winter than that in other seasons and the lowest appeared in the summer (Fig. 2.4). The highest seasonal average concentrations were less than twice the lowest average values. Although the low temperature in the winter limited the secondary formation of particles, more frequent occurrences of the stagnant weather conditions caused the accumulation of atmospheric particles and high concentration episodes. Lower concentrations were observed in summer as particulate matters are washed out due to wet deposition.

Fig. 2.4

Monthly variations of PM_2.5 in 2013, 2014, and 2015 (Data sources: National air pollution monitoring network in china)

Because many effective measures have been carried out to improve the air quality, PM_2.5 annual concentration in China has decreasing trend according to observation data in the recent 3 years (see Fig. 2.5). The annual average of PM_2.5 in 74 key cities was 72 μg/m³ in 2013, and it decreased to 55 μg/m³ in 2015. The government has taken measures and PM_2.5 has decreased over the recent years; however, PM_2.5 in most Chinese cities is still far above the Chinese National Ambient Air Quality Standard (GB 3095-2012). In China, the PM_2.5 has large portion of PM₁₀ with 50–85%.

Fig. 2.5

Annual variation trends of PM_2.5 in 74 key cities in the recent 3 years (Data sources: MEP [9,10,13])

2.3.1.2 Chemical Composition and Source Apportionment of PM_2.5

Particulate matter originated from both primary emission sources and reaction of precursor gases, such as sulfur dioxide (SO₂), nitrogen oxides (NO_X), ammonia (NH₃), and volatile organic compounds (VOCs). The main precursor gases NH₃, SO₂, and NOx react in the atmosphere to form ammonium, sulfate, and nitrate compounds. These compounds form new particles in the air or condense onto preexisting ones and form so-called secondary inorganic aerosols. Figure 2.6 shows the chemical composition of PM_2.5 in most cities of China. The chemical composition of PM_2.5 is varied at different cities, which is related with pollution sources and meteorological conditions. In general, the crustal elements and organic matter are major species of PM_2.5. Secondary particles, such as sulfate, nitrate, and ammonium salt, have higher fractions in the eastern cities. Yang et al. [30] and He et al. [5] also find secondary ions, organic carbons, and crustal material that are the main components in urban and rural sites of China. This result indicated that there are more local formation/production and regional transport of the secondary aerosols in the eastern region, thus more intensive characteristic of “complex atmospheric pollution” compared to the western region.

Fig. 2.6

PM_2.5 speciation in China [1]

PM_2.5 can be emitted directly from selected sources (primary PM), such as combustion and industry, or generated by gas-to-particle conversion in the atmosphere (secondary PM). Figure 2.7 shows the major sources of PM_2.5 in many Chinese cities. From it we can see that source contribution rates are varied in different cities. In generally, secondary particles formation and motor vehicle exhaust were the main sources of PM_2.5 in megacities (such as Beijing, Wuhan, and Chongqing). PM_2.5 in western cities (such as Xining) was influenced mainly by dust. The contribution of stationary sources, including coal combustion and industrial emissions, shows a downward trend from north to south. During the haze pollution events, a large fraction of PM_2.5 was secondary species, that is, secondary organic aerosol (SOA) and secondary inorganic aerosol (SIA, sulfate, nitrate, and ammonium). The contribution of primary particulate to PM_2.5 was small [8].

Fig. 2.7

The major sources of PM_2.5 in many Chinese cities [1]

2.3.2 Inhalable Particulate Matter (PM₁₀)

2.3.2.1 Characteristics of PM₁₀ Pollution

In China, PM₁₀ remains an important pollutant. In 2015, PM₁₀ concentrations at 65.4% of the monitoring cities were found to be exceeded than the CNAAQS (Fig. 2.8). The annual PM₁₀ concentrations in 338 cities were 24–357 μg/m³ with an average of 87 μg/m³, which exceeded the Chinese National Ambient Air Quality Standard (70 μg/m³). Days of daily concentrations exceeding the air standard was about 12.1% of all monitoring days.

Fig. 2.8

Percentage of cities at different PM₁₀ concentration levels in 338 cities, 2015 (Data sources: MEP [13])

Figure 2.9 shows the spatial distribution of PM₁₀ in 2015. As shown in Fig. 2.9, PM₁₀ annual concentration in the northern region was much higher than that in the southern region. The higher PM₁₀ concentrations in the northern region were related to the influence of dust-sand.

Fig. 2.9

Spatial distribution of PM₁₀ annual average concentrations in 2015 (Data sources: National air pollution monitoring network in China)

The trends of PM₁₀ in the recent 3 years in 74 key cities were calculated based on the officially reported data (Fig. 2.10). Although PM₁₀ annual concentration also was decreasing trend, it was still far above the Chinese National Ambient Air Quality Standard (GB 3095-2012). The annual average of PM₁₀ in 74 key cities was 118 µg/m³ in 2013, and it decreased to 93 µg/m³ in 2015 which was about 33% higher than the Chinese grade II standards.

Fig. 2.10

Annual variation tendency of PM₁₀ in 74 key cities of China (Data sources: MEP [9,10,13])

PM₁₀ has also obvious seasonal variation, showing the concentrations in winter were higher than that in other seasons (Fig. 2.11). The highest concentrations appeared in December and January. Lower concentrations were observed in July and August, which was related with the frequency of rain.

Fig. 2.11

Monthly variations of PM₁₀ in 2013, 2014, and 2015 (Data sources: National air pollution monitoring network in china)

2.3.2.2 Chemical Composition and Source Apportionment of PM₁₀

Soil dust was the first abundant component for PM₁₀ in most cities of China. And the secondary aerosol was the second important component. Carbonaceous matter has also important contribution to PM₁₀ mass concentration [2, 21].

Figure 2.12 shows the major sources of PM₁₀ in some Chinese cities. It can be found that dust was the main source for PM₁₀. Stationary source and mobile source also have important contribution to PM₁₀ in the northern cities (such as Dalian, Shenyang, and Harbin).

Fig. 2.12

The major sources of PM₁₀ in many Chinese cities

2.3.3 Ozone (O₃)

2.3.3.1 Characteristics of O₃ Pollution

O₃ is a strong oxidant, formed from the reactions of precursors (VOCs, NO_X, and so on) and sunlight. The major health effect of O₃ is its effect on the respiratory systems. O₃ is the main component of photochemical smog. In 1974, the first photochemical smog events in China appeared in the Xigu Industrial District of Lanzhou City. Photochemical smog events have also appeared in some suburban regions. Photochemical smog, high O₃, and NO_X concentrations have gradually emerged into China’s three city clusters (Beijing-Tianjin-Hebei region, the Yangtze River Delta, and Pearl River Delta).

Although O₃ annual concentrations in the recent 3 years were lower than the CNAAQS, the average annual O₃ concentrations were increasing during 2013–2015 (as Fig. 2.13 shows), indicating that O₃ pollution should be paid more attention. Year-to-year differences in the O₃ levels are also induced by meteorological variations.

Fig. 2.13

Annual variation tendency of O₃ in 74 key cities of China (Data sources: MEP [11, 12, 13]

In 2015, the 90% of O₃ maximum daily 8-h mean concentrations in 338 cities were 62–203 μg/m³ with an average of 134 μg/m³. Sixteen percent of the total cities did not meet the CNAAQS (Fig. 2.14). Days of daily concentrations exceeding the air standard was about 4.6% of all monitoring days.

Fig. 2.14

Percentage of cities at different O₃ concentration levels in 338 cities, 2015 (Data source: MEP [13])

Differences in the distribution of O₃ precursor emission sources and climatic conditions in Europe result in considerable regional differences in O₃ concentrations. Higher ozone concentrations are observed, in general, in summer months as it is formed by photochemical reactions of NO_x and VOCs. Ozone concentrations tend to peak in early to midafternoon in areas where there is strong photochemical activity. The values indicate that ozone levels are within CNAAQS.

2.3.3.2 Sources and Formation of O₃

VOCs and NO_X emissions from motor vehicle were the major precursor gases of O₃ formation. Shao et al. [20] found that alkenes contribute a large fraction of VOC activity with 75%. The formation of O₃ is VOC-limited in urban areas of China and NO_X-limited in nonurban areas [7, 24, 31]. The influence of biogenic VOCs on O₃ formation was minor [19]. Heterogeneous NO₂ could increase the concentration. The influence of the reaction of NO₃ and N₂O₅ on O₃ was unimportant [29]. CH₄ and CO also play a role in O₃ formation in certain environments. Tie et al. [24] reported that oxidation of CO contributed to 54% of the total O₃ production in eastern region of China.

Both local formation and regional transport contributed to O₃ concentrations. Wang et al. [25] reported that the contribution from regional transport was about 17.8% of O₃ concentration in PRD. Tang [22] found that 40% of O₃ concentration in Beijing was from southern and southeastern regions.

Meteorological conditions have also a major influence on O₃ formation. High O₃ concentration was related to the occurrence of high-pressure synoptic systems [28].

2.3.4 Sulfur Dioxide (SO₂)

SO₂ has greatly contributed to acid rain and has adverse effects on ecosystems and the respiratory system [26]. It is also the main precursor to formation of particulate matter.

Global SO₂ emissions have been dramatically reduced from 121 Tg to 103 Tg during the period of 1990–2010. Figure 2.11 gives the emission trend of SO₂ from 2011 to 2015 in China. Following the emission control legislations, SO₂ emissions have been decreasing dramatically. National emission of SO₂ in 2011 was about 22.17 Mt/year and it decreased to 18.59 Mt/year in 2015. In the period 2011–2015, SO₂ emission decreased by 16% (Fig. 2.15).

Fig. 2.15

Emission of SO₂ in China from 2011 to 2015 (Data sources: MEP [9–13])

SO₂ is emitted primarily from fuels containing sulfur burning. The main anthropogenic emissions of SO₂ in China are derived from industrial sources (including power plant, domestic heating, and industrial production processes), and contribution from urban life source was little. As reported in “Annual Report of Environmental Statistics” [11], SO₂ emission in 2014 was about 19.7 Mt, and the contribution of industrial source and urban life source was about 88% and 12%, respectively.

With respect to the spatial distribution of SO₂ emission, Cao et al. (2010) found that the SO₂ emission in Shandong province, Hebei province, and Shanxi province was the highest, which was related to large consumption of coal in these regions [3]. The SO₂ emission in the western region (Qinghai, Xizang, and Gansu provinces) was relatively little.

In 2015, the annual SO₂ concentrations in 338 cities were 3–87 μg/m³ with an average of 25 μg/m³. The average concentration was lower than the Chinese National Ambient Air Quality grade II standard (60 μg/m³). SO₂ concentrations at 3.3% of the monitoring cities were found to exceed the CNAAQS (Fig. 2.16). Days of daily concentrations exceeding the air quality standard was about 0.7% of all monitoring days.

Fig. 2.16

Percentage of cities at different SO₂ concentration levels in 338 cities, 2015 (Data sources source: MEP 11)

Figure 2.17 shows the spatial distribution of SO₂ annual concentration in 2015. Maximum SO₂ annual concentrations were found in northern regions, especially in North China and Inner Mongolia region. It may be related to coal heating in China. The SO₂ concentration in southern regions was relatively lower.

Fig. 2.17

Spatial distribution of SO₂ annual average concentrations in 2015 (Data sources: National air pollution monitoring network in China)

Figure 2.18 shows the annual variation of SO₂ concentrations in 74 key cities of China. It is clear that the average annual values for 74 key cities show a decline trend in the recent 3 years, and all annual SO₂ concentrations stayed below the grade II standard value, indicating that the measures taken to control SO₂ pollution were effective.

Fig. 2.18

Annual variation tendency of SO₂ in 74 key cities of China (Data sources: MEP [9,10,13])

SO₂ shows the highest concentration in the winter and the lowest in the summer, which is due to the effects of emission sources and meteorological conditions. The energy structure was based on coal in China. Energy for heating is mainly coal in winter and these coals contain high-sulfur fraction over 0.5%, which cause the higher emission in winter. In addition, slow winds and shallow mixing layers occur more frequently in winter, trapping the pollutants near the surface and leading to high concentrations.

2.3.5 Nitrogen Oxides (NO_X)

Vehicle is the main contributor of NO_X. Although the vehicle population increased by about 14.9% per year during 2007–2015 in China, the NO_X emissions have decreased obviously by about 23% during 2011–2015. This indicated that the control measure for NO_X was effective (Fig. 2.19).

Fig. 2.19

Emission of NO_X in China from 2011 to 2015 (Data sources: MEP [9–13])

In many developed counties, the main emission sources of NO_X are mobile vehicles. In the United States, 57.5% of NO_X emission was from mobile sources, with fuel combustion and industrial processes only account for 24.2% and 8.4%, respectively. However, in China, the main NO_X emissions sources are industrial sources (including power plants and industrial production) and motor vehicles. As reported in the “Annual Report of Environmental Statistics” [11], NO₂ emission in 2014 in China was about 20.8 Mt, and the emissions from industrial source accounted for 67% and motor vehicle accounted for 30%. Urban life source only accounted for 3%.

Emissions of NO_X vary significantly by province owing to factors such as population, energy sources, and economic base. In China, NO₂ emission in Shandong province, Jiangsu province, Hebei province, and Guangdong province was the highest, which was related to large consumption of coal in these regions [9].

Figure 2.20 gives the annual variation tendency of NO₂ in the recent 3 years. It can be observed that NO₂ was decreasing during 2013–2015. The NO₂ concentrations in 2013 and 2014 were 44 μg/m³ and 42 μg/m³, respectively. It decreased to 39 μg/m³ in 2015 and was lower than the CNAAQS grade II standard.

Fig. 2.20

Annual variation tendency of NO₂ in 74 key cities of China (Data sources: MEP [9,10,13])

In 2015, the annual NO₂ concentrations in 338 cities were 8–63 μg/m³ with an average of 30 μg/m³. The average of NO₂ annual concentration did not exceeded the Chinese National Ambient Air Quality Standard and WHO AQG (40 μg/m³). NO₂ concentrations at 81.7% of the monitoring cities were found to be lower than the CNAAQS (Fig. 2.21), and only 19.3% of the monitoring cities exceeded the CNAAQS. The number of days with NO₂ concentrations exceeding the CNAAQS occupied only 1.6% of all monitoring days.

Fig. 2.21

Percentage of cities at different NO₂ concentration levels in 338 cities, 2015 (Data source: MEP [13])

Spatial distribution of NO₂ showed that the concentration in northern regions was higher than that in southern regions (Fig. 2.22). The NO₂ concentrations in North China, Pearl River Delta, and Urumqi City were the highest.

Fig. 2.22

Spatial distribution of NO₂ annual average concentrations in 2015 (Data sources: National air pollution monitoring network in China)

2.4 Conclusion

With the development of economy and industries, air pollution is getting more and more serious in China. Although reduced emissions have improved air quality in China, heavy pollution events occurred frequently.

PM_2.5 was the major pollutant in China. Although PM_2.5 annual concentration in China has decreasing trend, the CNAAQS limit value for PM_2.5 exceeded in large parts of China. The exceedances occurred in 77.5% of the case in all the monitoring cities in 2015. Beijing-Tianjin-Hebei region, the north and middle part of Shandong province, the south and middle part of Henan province, and most of Hubei province have higher PM_2.5 concentrations. The crustal elements and organic matter are major species of PM_2.5. Secondary particles formation and motor vehicle exhaust were the main sources of PM_2.5 in megacities.

PM₁₀ also is an important pollutant in China. In 2015, PM₁₀ concentrations at 65.4% of the monitoring cities were found to be exceeded the CNAAQS. PM₁₀ annual concentration in the northern region was much higher than that in the southern region, which related to the influence of dust on northern cities. Soil dust was the first abundant component for PM₁₀ in most cities of China.

Although O₃ annual concentrations in the recent 3 years were lower than the CNAAQS, the average annual O₃ concentrations were increasing during 2013–2015. In 2015, the 90% of O₃ maximum daily 8-h mean concentrations in 338 cities were 62–203 μg/m³ with an average of 134 μg/m³. VOCs and NO_X emissions from motor vehicle was the major precursor gases of O₃ formation. The formation of O₃ is VOC-limited in urban areas of China and NO_X-limited in nonurban areas.

The average annual values of SO₂ for 74 key cities shows a decline trend in the recent 3 years, and all annual SO₂ concentrations stayed below the grade II standard value. NO_X also has decreasing trend and it decreased to 39 μg/m³ in 2015, which was lower than the CNAAQS grade II standard. These results indicated that the measures taken to control SO₂ and NO_X pollution were effective.