Assessment of urban air quality in China using air pollution indices (APIs)

Abstract

This study gathered and processed the available air quality daily reports in 86 cities throughout China in 2001–2011. Urban air quality was assessed in terms of the evolution of the key pollutants, the pollution level, and the PM₁₀ (particulate matter with an aerodynamic diameter <10 μm) concentrations. The authors conclude that PM₁₀ is the most important pollutant in Chinese cities, especially after the national sulfur dioxide (SO₂) controls during the 11th Five Year Plan (FYP; 2006–2010). A notable advance was the reduction of extremely heavily polluted days with air pollution index (API) above 150 from 7% in 2001 to 1% in 2011 in the all-city average. In addition, the average API-derived PM₁₀ concentrations continually decreased during the past 11 yr. Additionally, the pollution pattern of “more severe from south to north” in China became less obvious due to the decline of PM₁₀ concentrations in the northern cities and the more obvious regional characteristics of air pollution. Nevertheless, more pollutants should be included in the API system to fully reflect the air quality status and guide future air pollution controls in Chinese cities.

Implications:

Air quality daily report, the only publicly accessible observation database in the past decade, provides valuable insight into the air quality in Chinese cities. Using this data set, this paper assesses the status and change of urban air quality in China in 2001–2011, during which great effort was made to mitigate urban air pollution. It is valuable for the further refinement of national air quality control strategies, and the needs of updating the present daily report system are implicated.

Introduction

As a developing country, China began the development of an air quality monitoring network in the 1980s. In January 1997, the Committee for Environmental Conservation of the State Council required the 46 key environmental protection cities to start issuing weekly air quality reports to the public as early as possible. The 46 key cities included all the municipalities, provincial capitals, special economic cities, and some major tourist cities (nos. 1–47 in Figure 1, except Lhasa). By June 1998, all the key cities had begun issuing their air quality weekly reports based on their monitoring network. The pollutants involved were total suspended particulates (TSPs), sulfur dioxide (SO₂), and nitrogen oxide (NO_x).

Figure 1. The 86 cities with API reported in 2001–2011 in mainland China.

In 2000, the number of key environmental protection cities increased to 47 (nos. 1–47 in Figure 1). The State Environmental Protection Agency of China (SEPA) requested that the 47 cities change their weekly reports to daily reports and replace the criteria of TSP to coarse particulate matter (aerodynamic diameter <10 μm; PM₁₀) and NO_x to nitrogen dioxide (NO₂), step by step. On June 5, 2000, 42 cities began to release their daily air quality report. On June 5, 2001, all the key cities were involved.

In 2003, SEPA expanded the number of key environmental protection cities to 113 and required the newly joined cities to begin their daily reports gradually. More cities began publishing their air quality reports through various media. In 2004, the number of reporting cities increased to 84 (nos. 1–84 in Figure 1). In 2006, the number expanded to 86 (Figure 1).

The daily air quality is reported using air pollution index (API), a nondimensional number calculated according to the urban daily average concentrations of SO₂, NO₂, and PM₁₀. First, the API for each pollutant is computed. Then, the maximum API out of the three is published as the city's API, and the corresponding pollutant is reported as the “key pollutant.” This process is interpreted in detail in the section Data and Process.

The API provided the only publicly accessible data on urban air quality in China until the real-time hourly concentrations of SO₂, NO₂, and PM₁₀ of each national site in all the key environmental protection cities were published on the National Monitoring Centers Web site in 2011. Despite the database's simplicity, dealing with only three pollutants, it is still a valuable database because it is long term (more than 10 yr), covering tens of cities all over China. Not only are megacities such as Beijing, Shanghai, and Guangzhou included, but also many less developed medium- and small-sized cities are also involved. Because of the lack of available air quality observation data, many modeling studies over China have used this database in their model evaluations (Streets et al., 2006; Fu et al., 2009; Wang et al., 2010; Liu et al., 2010; Xing et al., 2011). However, very few researchers have used it to obtain a better understanding of the general status and change of the urban air quality in China in recent years. Gao et al. (2011) gathered API data for 81 cities for June 2004 to June 2007 to analyze the different pollution regimes in these cities. They found that most of the cities could be categorized into seven clusters according to their pollution characteristics. The three northern clusters have different seasonal variations to the four southern clusters. Pan et al. (2008) analyzed the spatial and seasonal distribution of the APIs using 1-yr data of 2006 in 86 cities and concluded that the air pollution in Chinese cities showed an obvious seasonal variation, with worsening quality from south to north and from the coast to inland. Yu et al. (2011) performed a chaotic analysis of the APIs in Lanzhou City for 2001–2010 to interpret its temporal evolution and find the major causes of air pollution. They found that Lanzhou's special valley basin topography, pollutant emissions, weather, and energy consumption structure were the four major dynamic variables to interpreting the API time series. These studies are either long-term analyses (10 yr) for one city or, for most cities, only 1- to 3-yr analyses. Another limitation is that all of these studies used the original API in their analysis. As we know, API is the maximum index of the three pollutants, so the same API may indicate quite different key pollutants. Two cities having the same API level may have different pollution problems.

Therefore, we gathered the API data for all the reported cities, from 2001, the first year having API data throughout the year, to 2011. Based on these data, the urban air quality during the 11 yr was assessed, including the evolution of the key pollutants, the pollution grade distribution, and the concentration level. The back-calculated concentrations of the key pollutants are used instead of the original API to avoid confusion from the same API number being induced by different pollutants. This study is informative for understanding the trend of the urban air quality in China during the past 11 yr, and it may provide useful information for the assessment of the national control policies in the past, revealing valuable implications for the formulation of future control strategies.

Data and Process

The API of each pollutant is calculated using the equation:

(1)

where I_x is the API of pollutant x, and C_x is the urban daily average concentration of pollutant x. C_x,j is the concentration breakpoint (in Table 1) that is ≤C_x , C_{x,j+ 1} is the concentration breakpoint that is ≥C_x , I_x,j is the index breakpoint corresponding to C_x,j , and I_x,j+1 is the index breakpoint corresponding to C_{x,j+ 1} (see Table 1).

Table 1.  Concentration limits for each pollutant in the API calculation

	Pollutant Concentrations (mg m^−3)
API	SO2 (Daily)	NO2 (Daily)	PM10 (Daily)
50	0.050	0.080	0.050
100	0.150	0.120	0.150
200	0.800	0.280	0.350
300	1.600	0.565	0.420
400	2.100	0.750	0.500
500	2.620	0.940	0.600

The highest API of the three pollutants is then reported as the city's API, and the corresponding pollutant is the “key pollutant.” The pollution level and status will be published according to the API number, as shown in Table 2. Thus, the air quality daily report for one city generally contains four parameters: API, key pollutant, pollution level, and pollution status. It should be noted that if the API is lower than 50, the pollution status is “very good,” and no key pollutant will be reported.

Table 2.   API, pollution level, and pollution status

API	Pollution Level	Pollution Status
0–50	I	Very good
51–100	II	Good
101–150	III1	Slightly polluted
151–200	III2	Lightly polluted
201–250	IV1	Moderately polluted
251–300	IV2	Heavily polluted
300+	V	Severely polluted

Table 3 presents the concentration limits for major air pollutants in the China National Ambient Air Quality Standard (CNAAQS) (SEPA, 1996). It can be found that the Grade I and II limits are applied as the concentration limits for API numbers of 50 and 100, respectively. The Grade I standard is set for the natural reserves, national parks, and other protected areas. The Grade II standard is for urban residential, commerce-traffic-resident mixed, common industrial, and rural areas. Thus, if the city API is below 100, it meets the national air quality standard in terms of SO₂, NO₂, and PM₁₀; if it is below 50, the city's air quality is “very good,” meeting the requirements for national parks.

Table 3.  Concentration limits for some pollutants in CNAAQS (1996)

Pollutants	Averaging Time	Grade I	Grade II	Grade III
SO2	Annual	0.02	0.06	0.10
	Daily	0.05	0.15	0.25
	Hourly	0.15	0.50	0.70
TSP	Annual	0.08	0.20	0.30
	Daily	0.12	0.30	0.50
PM10	Annual	0.04	0.10	0.15
	Daily	0.05	0.15	0.25
NO2	Annual	0.04	0.08	0.08
	Daily	0.08	0.12	0.12
	Hourly	0.12	0.24	0.24
CO	Daily	4	4	6
	Hourly	10	10	20
O3	Hourly	0.16	0.20	0.20
Notes: Unit: mg m^−3. The Grade I standard is for natural reserves, national parks, and other protected areas. The Grade II standard is for urban residential, commerce-traffic-resident mixed, common industrial, and rural areas. The Grade III standard is for special industrial areas.

Therefore, we can back-calculate the daily concentrations of the key pollutants according to the daily report using the eq 2 below. It should be noted that for any day, we can only calculate one pollutant. The concentrations of a specific pollutant for all the days of one city may not be available if the key pollutant varies or it has “very good” days with API below 50.

(2)

In this study, all the available API data in the 86 cities from 2001 to 2011 were gathered from the data center of the China Ministry of Environmental Protection (MEP) (http://datacenter.mep.gov.cn) (in 2008, the subministerial SEPA was promoted to the ministry level and became the MEP). The API reports have been available for city nos. 1–42 since 2001 and for the other 44 cities since 2006 (see Figure 1). The key pollutant concentrations for all days were back-calculated except for the “very good” days. Statistically, 211,528 APIs were processed in this study. Based on this data set, we pursued an assessment of the urban air quality in China during the past 11 yr, as discussed below.

Results and Discussion

Key pollutants

First, we analyzed the frequencies of the three criteria pollutants as the “key pollutant” during the past 11 yr. Three representative years, 2001, 2006, and 2011, were chosen to present the results, as shown in Figure 2. These years are, respectively, the beginning year of China's 10th (2001–2005), 11th (2006–2010), and 12th (2011–2015) Five Year Plans (FYPs). The FYP program, initiated in 1953, has been one the most important guidelines for the social, economic, and environmental development in China. The FYP consists of a series of initiatives on social and economic aspects, and the environmental aspect has been included since the 6th FYP (1981–1985). By choosing these three years, we can also assess the environmental progress achieved in each FYP.

Figure 2. The frequencies of three key pollutants and “very good” days in 2001, 2006, and 2011.

Figure 2 presents the appearance frequencies of the three key pollutants and the “very good” days. In 2001, PM₁₀ was the most important pollutant according to our statistics, with 31 of the reported 42 cities having more than 70% of PM₁₀ days. The average number was 76.1%. Three cities, Haerbin, Wuhan, and Xi'an, even had 100% days of PM₁₀ pollution. Two regions had relatively more obvious SO₂ pollution. One, in northern China, includes Beijing, Tianjin, Shijiazhuang, Taiyuan, Yinchuan, and Urumqi, with 12.1% (in Tianjin) to 30.4% (in Shijiazhuang) days of SO₂ as the key pollutant. These cities are either large urban centers (Beijing, Tianjin), or have coal mining and industry (Taiyuan, Shijiazhuang, Yinchuan, Urumqi). The other is the southwestern area, another coal base in China, including Guiyang, Kunming, and Chongqing, which had 23.8% (in Chongqing) to 39.5% (in Guiyang) of SO₂ days. The all-city average SO₂ frequency was 7.1%. Only five cities had NO₂ key pollution reports: Guangzhou, Zhuhai, and Shenzhen in the Pearl River Delta (PRD) and Shanghai and Hohhot. High frequencies of “very good” days appeared in the eastern and southern coastal cities, such as Haikou (91.5%), Zhanjiang (70.1%), Zhuhai (49.6%), Xiamen (42.5%), Shantou (39.2%), Shenzhen (38.4%), Nanning (38.1%), and Kunming (29.6%). The 42-city average was 16.2%.

In 2006, the reported cities increased to 86. PM₁₀ was still the dominant pollutant, being the key pollutant for more than 70% of the days in 58 cities. The 42- and 86-city average frequencies were the same at 73.4%, a slight decrease from 76.1% in 2001. The frequency of SO₂ as the key pollutant was 8.3% in the all-city average, with the 42-city average being 6.0%, lower than the 7.1% recorded in 2001. This result may indicate a progress in SO₂ controls in the first group of key environmental control cities. Ten cities, including Shijiazhuang, Beijing, Guiyang, and Kunming, showed an increase in PM₁₀-dominant days with a simultaneous decrease in SO₂. In Shijiazhuang City, for example, 69.6% of the days in 2001 were PM₁₀ polluted. This number rapidly increased to 97.0% in 2006, along with a decline from 30.4% to 0.8% of the key SO₂-polluted days. The average PM₁₀ percentage in these 10 cities increased by 11% and that of SO₂ decreased by −11.2%. These two numbers nearly balance out each other, which may indicate that the decrease of SO₂ frequencies may be partially attributable to the increase in PM₁₀ pollution in some cities.

It should be noted that on the national level, although the 10% reduction in SO₂ emissions from 2001 to 2005 was one of the major goals of 10th FYP, China's national SO₂ emission increased, instead of decreased, by approximately 27% during the 5 yr according to the statistics of SEPA, due mostly to the unexpectedly rapid growth in energy consumption (55.2% from 2001 to 2005) (SEPA, 2006). According to the Report on National Environmental Statistics (MEP, 2001–2011), the national SO₂ emissions increased by as much as 33%, from 19.48 Mt in 2001 to 25.59 Mt in 2006 (see Figure 3). The growth is similar to that of 36% from 2001 to 2006 in Zhang et al.’s (2009) estimation. The national emissions of fly ash and industrial dust are also presented in Figure 3. It can be observed that the emission of fly ash slightly increased by 2.8% and that industrial dust emissions decreased by 18.4% in 2006 from 2001.

Figure 3. National SO₂, flying ash, and industrial dust emissions in 2001–2011. The data are from the Report on National Environmental Statistics (MEP, 2001–2011).

NO₂ nearly disappeared in 2006; as shown in Figure 2, its levels were only detectable in Guangzhou and Shenzhen, at 6.6% and 2.2%, respectively. For the number of “very good” days, 32 cities showed an average increase of 7.1% days, including Hohhot, Yantai, Zhuhai, Lhasa, and Urumqi, and eight cities had an opposite average of −6.5%. The eight cities were mostly among the cleanest cities in 2001, including Haikou, Kunming, Xiamen, and Nanjing. The all-city average “very good” day percentage was 18.1%, slightly higher than the 16.2% recorded in 2001.

A notable difference between 2011 and 2006 is the obvious increase in the frequency of “very good” days, from an average of 18.1% to 23.5%. Sixty-three of the 86 cities had more “very good” days in 2011. These cities are concentrated in three regions: northern China, the Sichuan Basin, and the middle and lower reaches of the Yangtze River. The average increase was 10.9%. Fewer “very good” days appeared in 23 cities, which were mostly among the cleanest cities, in 2006. On the 86-city average, the PM₁₀-dominant days slightly decreased to 71.5% compared with 73.4% in 2006. Among the 86 cities, 51 had decreased PM₁₀-polluted days, at an average of −11.3%. The average increase of the other 35 cities was 11.6%. Twenty-four cities showed an increase in SO₂ frequencies at an average of 4.1% versus the decrease in 47 cities by an average of 8.4%. The all-city average of the SO₂ days was 4.9%, an evident decrease from 8.3% in 2006. One of the major goals of 11th FYP was a 10% reduction in the national SO₂ emissions in 2010 from 2005. This goal was exceeded, with a 14.29% reduction, by large-scale desulfurization in power plants, phasing out inferior facilities, among other factors (see Figure 3). Emission control is one of the most important factors in urban pollution mitigation. Figure 4 presents the evolution of SO₂ occurrence in the eight representative cities that had the highest SO₂ frequencies in 2001, out of the 42 cities, and in 2006 out of the other 44 cities. These cities are either in northern China (Shijiazhuang, Yangquan) or in the southwest. All these cities showed a visible decreasing trend in SO₂ frequency, except Nanchong City, which had a large increase in 2011. The most rapid decline appeared in Liuzhou, from 290 days in 2006 to 92 days in 2011. According to our statistics, however, the corresponding PM₁₀ frequencies increased from 16 days to 200 days. Thus, the SO₂ decrease may be partially attributable to the PM₁₀ increase. Kunming and Guilin showed the same status. In Shijiazhuang, Yangquan, and Changsha, the SO₂ frequencies evidently declined simultaneously with the relatively slight growth in PM₁₀-dominant days. This pattern may indicate that SO₂ and PM₁₀ are still the major pollution problems in most Chinese cities, especially in the medium- and small-sized cities.

Figure 4. The number of days of SO₂ as the key pollutant for some typical cities in 2001–2011.

NO₂ was nearly absent in 2011. Some recent studies have indicated that during 2001–2010, the NO_x emissions in China had a rapid, significant increase (Zhang et al., 2009, 2012), which was neither reflected nor warned of by the present API system. This failure may indicate that the present NO₂ standard is insufficient to guide the NO_x controls in Chinese cities for two main reasons: the present NO₂ limit is relatively looser, and a large fraction of NO_x exists as NO in the urban environment (Zhou et al., 2008). Thus, in the newest revised standard (MEP, 2012), which will be implemented in 2016, the NO₂ limit will be strengthened and the NO_x limit will be added as a reference restriction.

Pollution level

Figure 5 shows the distribution of the API levels in the cities in 2001, 2006, and 2011. White and blue represent API levels below and equal to 50 and 51–100, respectively. As discussed above, an API below and equal to 100 indicates that the city's air quality meets the CNAAQS. The figure shows that in 2006 the API percentage within 51–100 distinctly increased at the same time the API within 101–150 decreased in most cities located in northern China and in the middle and lower reaches of the Yangtze River. In 2001, the reported 42 cities had an average of 273 days reaching the air quality standard, accounting for 75% of the 365 days. In 2006, this number dramatically grew to 322, or 88% of the year, in the 86-city average. Considering that most of the newly included cities in 2006 were medium or small and had relatively better air quality, the 42-city average was also calculated. The results show no large difference (321 days). In the all-city average, this increase mainly came from the growth in days with an API within 51–100 (59–70%), instead of “very good” days (16–18%). In the meantime, the days with an API between 101 and 150 declined, on average, from 67 (18%) in 2001 to 35 (10%) in 2006, which indicates that during this period a large number of concentrations slightly higher than the standard were cut down to below the standard. The corresponding decreases for the API within 151–200 and above 200 were 17 (4.6%) to 6 (1.6%) days and 8 (2.2%) to 3 (0.8%) days, respectively.

Figure 5. API distribution in cities in 2001, 2006, and 2011.

In 2011, the days reaching the standard continued to increase. The 86-city average number grew to 336 days, or 92% of the year. The days with an API above 150 nearly disappeared, indicating most exceeding days were located within the API range of 101–150. The average number of days with an API above 100 was 28, accounting for 8% of a year, of which the API within 151–200 and above 200 were 3 days and 1 day on average, respectively, accounting for 0.8% and 0.3% of the year.

PM₁₀ concentrations

For an average of more than 70% days in the reported API number for PM₁₀, we plot the API-calculated annual PM₁₀ concentrations and the corresponding frequencies of PM₁₀-polluted days in 2001–2011, as shown in Figure 6. To distinguish the first group of 42 cities and the others that joined in 2005, with the data being averaged over all available cities, the 42 and remaining 44 cities are plotted separately. It is understandable that the 44 cities in the second group, which are mostly medium- and small-sized cities, have better air quality than the first 42 cities. The average difference is 5–10 μg m⁻³.

Figure 6. API-derived annual PM₁₀ concentrations and frequencies of PM₁₀ as the key pollutant during 2001–2011.

We found that, regardless of which group, the PM₁₀ concentrations continually decreased over the past 11 yr. In the former 42 cities, the average annual concentrations declined by 22.5%, from 130 to 100 μg m⁻³, in 2001–2011. The other 44 cities showed a decrease of 9.3%, from 106 to 96 μg m⁻³, in 2006–2011. The reduction in 2001–2005 was relatively fast. After 2006, the decrease slowed down, and in 2010 it even showed a slight increase. The frequencies of PM₁₀-dominant days were similar to PM₁₀ concentrations before 2005. In 2006, the frequency of these days exhibited a visible rise in all averages, after which it decreased until 2010 for the first 42 cities. For the other 44 cities, the turning point was 2008, after which values started to visibly increase. This general increase may be partially attributed to, as mentioned before, the less frequent SO₂-dominated days. SO₂ pollution was evidently mitigated by the effective national SO₂ emission controls during 2006–2010. Therefore, although the PM₁₀ concentrations continued to decrease, their frequencies increased by replacing SO₂ as the key pollutant in some days.

The API-calculated PM₁₀ concentrations and their changes in each city are plotted in Figure 7. Plots a–c are, respectively, the annual concentrations in 2001, 2006, and 2011. Plots e–f are the changes in percentage in 2001–2006, 2006–2011, and 2001–2011. In 2001, the calculated annual concentrations were within the range of 57–241 μg m⁻³. The PM₁₀ concentrations in 28 of the 42 cities exceeded 100 μg m⁻³, the annual standard for PM₁₀. It should be noted that this API-derived concentration may be slightly higher than the reality because the “very good” days were not included, as mentioned above. Figure 7a shows that the annual PM₁₀ concentrations in northern China were obviously higher than in the south in 2001. The highest 10 cities were Lanzhou (241 μg m⁻³), Shijiazhuang (224 μg m⁻³), Taiyuan (206 μg m⁻³), Urumqi (205 μg m⁻³), Xining (185 μg m⁻³), Changsha (183 μg m⁻³), Beijing (180 μg m⁻³), Tianjin (180 μg m⁻³), Shenyang (178 μg m⁻³), and Hohhot (166 μg m⁻³). All are located in northern China except Changsha. The five lowest concentrations appeared in Zhanjiang (57 μg m⁻³), Haikou (59 μg m⁻³), Kunming (76 μg m⁻³), Zhuhai (79 μg m⁻³), and Xiamen (80 μg m⁻³). Table 4 summarizes the best and worst 10 cities in terms of annual PM₁₀ concentrations in 2001, 2006, and 2010.

Table 4.  The 10 highest and lowest API-derived annual PM₁₀ concentrations of the cities in 2001, 2006, and 2011 (μg m^{− 3})

2001		2006		2011
Highest
Lanzhou	241	Lanzhou	191	Lanzhou	144
Shijiazhuang	224	Urumqi	176	Urumqi	144
Taiyuan	206	Beijing	172	Beijing	135
Urumqi	205	Datong	160	Xi'an	126
Xining	185	Taiyuan	155	Hefei	125
Changsha	183	Weinan	146	Chifeng	118
Beijing	180	Shijiazhuang	145	Wuhan	115
Tianjin	180	Pingdingshan	145	Jining	114
Shenyang	178	Chifeng	142	Anshan	113
Hohhot	166	Yangquan	140	Zhengzhou	113
Lowest
Zhanjiang	57	Zhanjiang	59	Zhanjiang	57
Haikou	59	Beihai	62	Haikou	65
Kunming	76	Guilin	65	Mianyang	70
Zhuhai	79	Haikou	68	Nanchong	70
Xiamen	80	Zhuhai	70	Yuxi	70
Shantou	81	Karamay	70	Lhasa	73
Shenzhen	82	Yantai	72	Zhuhai	74
Wenzhou	88	Liuzhou	72	Shantou	76
Guangzhou	89	Rizhao	75	Xiamen	77
Lhasa	89	Lhasa	77	Shenzhen	78

Figure 7. Annual average PM₁₀ concentrations and their changes during 2001–2011 for 86 cities. (a–c) Annual average concentrations in 2001, 2006, and 2011 (μg m⁻³). (d–f) Changes in percentage in 2001–2006, 2006–2011, and 2001–2011.

In 2006, many cities showed a decrease in PM₁₀ concentrations. The average number was 109 μg m⁻³, within the range of 59–191 μg m⁻³, 9.2% lower than in 2001. The highest five cities were Lanzhou (191 μg m⁻³), Urumqi (176 μg m⁻³), Beijing (172 μg m⁻³), Datong (160 μg m⁻³), and Taiyuan (155 μg m⁻³) (see Table 4). Compared with 2001, 31 cities had decreased concentrations in 2006. A larger decrease appeared in the northern cities, such as Shijiazhuang, Shenyang, Yinchuan, and Tianjin, with a greater than 30% reduction (Figure 7d). Ten cities had increasing concentrations within the range of 2.5–25.0%. These cities are mostly located in the coastal and southwest areas, such as Kunming, Dalian, Chengdu, Guangzhou, and Chengdu. In general, the northern cities still exhibited higher concentrations than the south, but the differences were not as clear as in 2001.

In 2011, the PM₁₀ concentrations in 66 cities continued declining. The overall average concentration was 98 μg m⁻³, within the range of 57–144 μg m⁻³. Still larger decreases occurred in the northern cities (Figure 7e), resulting in a more uniform concentration distribution over China, compared with 2001 (Figure 7c). The largest reduction appeared in three cities in Shanxi province: Datong, Yangquan, and Changzhi. The cities with increasing concentrations are mostly located in the Guangxi and Shandong provinces and the cities in the lower reaches of the Yangtze River. In 2011, the five highest cities were Lanzhou (144 μg m⁻³), Urumqi (144 μg m⁻³), Beijing (135 μg m⁻³), Xi'an (126 μg m⁻³), and Hefei (125 μg m⁻³).

In the 11-yr view, the PM₁₀ pollution in most cities was mitigated (Figure 7f), by an average of 21.4%. Only five cities, Wenzhou, Haikou, Hefei, Nanning, and Kunming, showed an increased concentration of 7.3% on average. These are either among the clean tourist cities (Haikou, Kunming, Nanning) or rapid economically developing cities (Wenzhou, Hefei).

Conclusion

In this study, we gathered and processed the available daily air quality reports for 86 cities throughout China for the past 11 yr. Based on this information, the urban air quality in China was assessed in terms of the evolution of the key pollutants, the pollution level, and the PM₁₀ concentrations.

It was found that PM₁₀ is undoubtedly the most important pollutant in Chinese cities. In the all-city average, the frequencies of days having PM₁₀ as the key pollutant are 76.1%, 73.4%, and 71.5% in 2001, 2006, and 2011, respectively. SO₂-dominated days decreased, on average, from 7.1% in 2001 to 4.1% in 2011, which may be attributable to the mitigation of SO₂ pollution in some cities and the more obvious domination of PM₁₀ pollution in other cities. A notable advance is the decrease of extremely heavily polluted days, where the frequencies of API above 150 declined from 7% to 1% from 2001 to 2011. The percentages of days meeting the CNAAQS are 75%, 88%, and 92% in 2001, 2006, and 2011, respectively.

The API-derived PM₁₀ concentrations continually decreased during the past 11 yr, with the all-city average concentrations declining from 130 to 98 μg m⁻³ from 2001 to 2011. Overall, the northern cities have higher PM₁₀ concentrations, but they became more uniform due to the more obvious regional characteristics of air pollution during the past 11 yr.

The recent severe winters and frequent haze occurrences over eastern China have attracted much attention from the public and from researchers. The severe haze pollution also reveals the limitation of the present API system, that is, only the three pollutants are involved, and NO₂ is nearly absent in the daily reports, which would guide NO_x controls. In February 2012, China's MEP released the new technical regulations on air quality daily reports, which will be officially implemented in 2016 (http://kjs.mep.gov.cn/hjbhbz/bzwb/dqhjbh/jcgfffbz/201203/t20120302_224166.htm). In this regulation, the API is replaced by the air quality index (AQI), and the daily concentrations of carbon monoxide (CO) and fine particulate matter (aerodynamic diameter ≤2.5 μm; PM_2.5) as well as 1-hr and 8-hr maximum concentrations of O₃ are added in the calculation. The NO₂ limits are strengthened to 0.080 and 0.180 mg m⁻³ for an API of 50 and 100, respectively. According to the seven pollution factors pursued and first published in the PRD area since March 2012, the number of days reaching the standard will be decreased by 10–30% by applying the new regulations. These changes will undoubtedly bring new challenges to the air pollution controls in Chinese cities. In January 2012, the MEP released the 12th FYP on environmental protection, in which the number of key environmentally protected cities was further expanded to 333. Better air quality monitoring networks and a publicly accessible database can be expected in the near future.

Acknowledgment

This study was sponsored by the National Natural Science Foundation of China (no. 41105105) and the Natural Science Foundation of Hebei Province (no. D2011402019).