Analysis of aerosol temporal dynamics
The quality of China's atmospheric environment is poor according to the bulletin of China's ecological environment from 2016 to 2018. In 2016, only 84 cities nationwide exhibited ambient air quality that reached the standard, comprising 24.9% of all cities. Further, heavy pollution occurred a total of 2,464 times in cities across the country, of which 80.3% occurred due to PM2.5 and 20.4% due to PM10. In 2017, 29.3% of China's urban ambient air quality measurements reached the standard, while the proportion of PM2.5 as the primary pollutant decreased to 74.2%, and the proportion of PM10 as the primary pollutant was the same as in 2016. The ambient air quality in 2018 improved compared with 2016 and 2017, with 35.8% of urban ambient air quality measurements reaching the standard, the proportion of PM2.5 as the primary pollutant decreasing to 60.0%, and the proportion of PM10 as the primary pollutant increasing to 37.2%.
The PM2.5 and PM10 concentration limits were the same as in the ambient air quality standard (GB3095-2012) (Table 1) and cities were divided into seven geographic regions (Table 2).
Table 1
PM2.5 and PM10 concentration limits for standards
Contaminants | Averaging period | Concentration limit | Unit |
One-level | Second-level |
PM2.5 | Annual average | 15 | 35 | µg/m3 |
24 -hour average | 35 | 75 |
PM10 | Annual average | 40 | 70 |
24 -hour average | 50 | 150 |
Table 2
Cities with data evaluated in this study and their corresponding geographic division
Geographic division | City |
Northeast China | Harbin, Jiamusi, Changchun, Baicheng, Shenyang, Chaoyang |
North China | Beijing, Tianjin, Hohhot, Alxa Left Banner, Hulunbuir, Shijiazhuang, Taiyuan, Datong, Changzhi |
East China | Shanghai, Jinan, Jining, Weihai, Nanjing, Nantong, Lianyungang, Hangzhou, Lishui, Zhoushan, Hefei, Anqing, Suzhou, Fuzhou, Xiamen, Nanping, Nanchang |
South China | Guangzhou, Shenzhen, Zhanjiang, Nanning, Beihai, Guilin, Haikou, Sanya |
Central China | Zhengzhou, Xinyang, Sanmenxia, Wuhan, Jingzhou, Shiyan, Changsha, Zhangjiajie |
Northwest China | Xi'an, Yan'an, Hanzhong, Urumqi, Kashgar, Altay, Haixi, Lanzhou, Jinchang, Yinchuan, Guyuan, Zhongwei |
Southwest China | Chongqing, Xining, Guiyang, Tongren, Liupanshui, Chengdu, Ganzi, Kunming, Lijiang, Jinghong, Lhasa |
Analysis of interannual PM2.5 and PM10 variation among seven geographic regions of China
The interannual variation characteristics of PM2.5 and PM10 across the country were determined using the hourly PM2.5 and PM10 data from 71 cities between 2016 and 2018. The PM2.5 and PM10 concentrations decreased year by year (Fig. 2). The national annual average concentration of PM2.5 reduced from 44.94 to 37.75 µg/m3, while the PM10 reduced from 80.42 to 70.14 µg/m3 from 2016 to 2018. Thus, China's atmospheric environment improved after the implementation of the Ambient Air Quality Standard (GB3095-2012) on January 1, 2016. Even so, the annual average concentrations of PM2.5 and PM10 could only barely reach the secondary concentration limit in 2018.
The data from each city were sorted and analyzed (Table 2) to assess the interannual changes in PM2.5 and PM10 in each major geographic region between 2016 to 2018. The average annual PM2.5 mass concentration from 2016 to 2018 (Fig. 3) followed the order of Central > North > Northwest > Northeast > East > Southwest > South China. In addition, the average annual PM10 mass concentration from 2016 to 2018 followed the order of Northwest > Central > North > Northeast > East > Southwest > South China. Querol et al.(2001; 2004) demonstrated that the PM2.5 to PM10 concentration ratios greatly vary among different regions, although the PM2.5 to PM10 concentration ratios in most areas are about 0.6–0.75 (Gehrig et al., 2003; Parkhurst et al., 1999). The ratio of PM2.5 to PM10 concentrations in Northwest China was smaller than in central and northern China. However, South China encompasses the southern coast of China. In this area, ocean winds and the monsoon climate can increase the horizontal migration of aerosol particles on the surface, while clean air input at sea level can reduce the mass concentrations of aerosol particles.
Seasonal variation of PM2.5 and PM10 in seven geographic regions of China
Statistical analysis of PM2.5 and PM10 concentrations in 71 cities across China showed that the variation characteristics were generally high in spring and winter, and low in summer and autumn (Fig. 4). The PM2.5 concentration was highest in winter, followed by spring, autumn, and summer, at 60.16, 41.95, 37.10, and 26.94 µg/m3, respectively. PM10 concentrations in spring and winter exhibited little difference at 89.50 and 92.30 µg/m3, respectively, which were higher than in autumn and summer.
Seasonal variations of PM2.5 and PM10 concentrations in major geographic regions were evaluated (Fig. 5). From 2016 to 2018, the PM2.5 concentration in winter was higher than in all other seasons, and even the concentration in winter was 2–3 times higher than that in summer. Wind speed, relative humidity, and precipitation in winter were lower than in other seasons. In addition, winter exhaust gas emissions from heating in northern China are high(Fan et al., 2020). Further, the coverage of green plants decreases in winter, and their consequent removal effects on pollutants decrease. The PM2.5 concentration in Central China was much higher than in other regions in winter, while in summer, the PM2.5 concentration in North China was the highest. Central, Northwest, and East China exhibited essentially the same trends, while Northeast, Southwest, and South China exhibited the lowest concentrations. Northeast and South China are relatively close to the ocean, and the prevailing hot and humid atmosphere blows inland from June to August, thereby greatly reducing PM2.5 concentrations in those regions. The PM2.5 concentration in spring and autumn had little difference among regions, while the PM2.5 concentration in the spring of Northwest China was above 25% higher than in autumn. From 2016 to 2018, PM10 concentrations were different from PM2.5, with no single season exhibiting higher values than the other seasons. North, Northwest, and Northeast China, the PM10 concentration was highest in the spring, followed by the winter, autumn, and summer, while the seasonal variations of the other four regions were similar to PM2.5.
Annual variation of PM2.5 and PM10 concentrations in seven geographic regions of China
The monthly average concentrations of PM2.5 and PM10 in 71 cities nationwide showed a downward trend from 2016 to 2018 (Fig. 6). The annual change trends for PM2.5 were relatively stable. From January to August, the monthly average PM2.5 concentration in 71 cities across the country gradually decreased, and then increased from September to December. The monthly average concentrations of PM10 were different from PM2.5, with two peaks that were observed yearly in March and December, respectively.
PM10 concentrations exhibited a sharp decreasing trend from March to June every year but rose sharply from October to December. The reasons underlying the sharp increase of PM10 and PM2.5 concentrations from October to December are two-fold. First, the temperature begins to decrease in October and the sinking of cold air decreases the diffusion of PM2.5 and PM10 in the atmosphere. Reduced rainfall also decreases wet deposition. Heating gradually increases in northern cities at this time and this fuel combustion is accompanied by the direct emission of particulate matter, while also producing SO2, NO2, and other noxious gases that provide a foundation for PM2.5 and PM10 generation, resulting in sharply increased concentrations of PM10 and PM2.5 from October to December(Hallquist et al., 2009).
The PM10 concentrations sharply decreased from March to June every year, while PM2.5 concentrations gradually decreased. National temperatures began to rise in March, coinciding with diminishing northern heating. The different trends of PM2.5 and PM10 concentration variations may be related to the rainy season that begins in March. Different rainfall, rainfall intensity, and rainfall lengths lead to different effects on the removal of atmospheric particulate matter of different particle sizes. Most of the rainfall between March to June is characterized by low rainfall intensity and long rainfall times.
Based on the above analyses, variations in the annual PM2.5 and PM10 concentrations or seven geographic regions were evaluated (Fig. 7). PM10 concentrations throughout the country increase to varying extents from October to December, and climate conditions considerably impact PM10 concentrations. However, increased PM10 concentrations between October to December were more obvious in North, Central, and Northwest China compared to within East, South, and Southwest China, reflecting that urban heating in the north influences PM10 concentrations across those cities. PM2.5 concentrations in Central China increased most significantly from October to December.
Aerosol spatial variation
Daily PM2.5 and PM10 data from 2016 to 2018 at an hourly resolution were averaged to obtain spatial distribution visualizations for PM2.5 and PM10 concentrations (Figs. 8 and 9). Areas with high annual average concentrations of PM2.5 and PM10 were mainly in the Beijing-Tianjin-Hebei, Jianghuai Plain, and Xinjiang regions, where annual average concentrations of PM2.5 were above 70 µg/m3.
High-pollution areas were primarily distributed in rapidly developing urban agglomerations such as the Central Plains, the Bohai Sea, and the Yangtze River Delta urban agglomerations. The rapid development of industries and urban construction in these areas have led to sharp increases in PM2.5 and PM10 concentrations. Meanwhile, these urban agglomerations are also densely populated areas in China. Human activities and the development of urban infrastructures including motor vehicles, heating in winter, and infrastructure construction have all contributed to increased air pollution(Li et al., 2022b).
The overall terrain of China is high in the west and low in the east, and the local topography is complex and diverse. Landforms such as plateaus, basins, and hills are not conducive to pollutant diffusions. The concentrations of PM2.5 and PM10 in Xinjiang are relatively high due to the effects of terrain factors, such as the flowing dunes in the Tarim Basin. When windy and dusty weather occurs, pollutants cannot be evacuated due to the terrain, thereby greatly increasing local PM2.5 and PM10 concentrations.
Areas with low PM2.5 and PM10 concentrations were primarily distributed in South China, the Qinghai-Tibet Plateau, and the Yunnan-Guizhou region. The Qinghai-Tibet Plateau is considered the pure land of China's environment. The plateau exhibits high altitudes and is also sparsely populated. The Yunnan-Guizhou region and South China exhibit considerable rainfall, lush green plants, and obvious monsoon climates that play an active part in the migration and purification of pollutants, so the air quality is better than in other regions.
Seasonal spatial characteristics
From 2016 to 2018, PM2.5 and PM10 concentrations in the spring, summer, autumn, and winter exhibit obvious seasonal changes that can be observed in Figs. 10 and 11. The concentrations of PM2.5 and PM10 were highest in the winter, followed by the spring and autumn, and then lowest in the summer. Seasonal variation of PM2.5 and PM10 concentrations are mainly affected by human activities and meteorological factors(Li et al., 2017). For example, at the beginning of autumn, PM2.5 and PM10 concentrations in the Bohai-rim urban agglomeration gradually rise, resulting in the onset of high pollution areas in the Shandong, Shanxi, Henan, and Hebei provinces that continue until winter, while in spring PM2.5 and PM10 concentrations begin to decrease.
PM2.5 and PM10 pollution were the most serious in winter, and the distribution range was also the widest. The PM2.5 concentrations mostly ranged from 70 to 130 µg/m3, while the PM10 concentrations mostly ranged from 150 to 200 µg/m3. Two primary reasons underlie this phenomenon. First, exhaust gas emissions are large during heating periods for cities in northern China. Second, green plant coverage is low during winter and the dilution of pollutants is reduced, leading to higher PM2.5 and PM10 concentrations than in other seasons.
The PM2.5 and PM10 concentrations decreased in autumn relative to winter, with PM2.5 concentrations decreasing to 35–60 µg/m3 in the area of 70–130 µg/m3 and spreading to the north. Relative to winter levels, the PM2.5 and PM10 concentrations in South China were significantly decreased, which may be related to typhoons and wetter climates.
Summer seasons exhibited the lowest PM2.5 and PM10 concentrations and the best air quality. The PM2.5 and PM10 concentrations in summer were significantly lower than in autumn and winter. Specifically, PM2.5 concentrations ranged between 15 and 30 µg/m3 in most areas, and PM10 concentrations ranged between 20 and 55 µg/m3, with higher values appearing in northern areas. The high level of air quality in the summer arises primarily because ocean winds prevail in China from June to August. Ocean winds blow hot and humid air over the ocean to the land which can then greatly reduce PM2.5 and PM10 concentrations. However, PM10 concentrations in Xinjiang, in addition to PM2.5 concentrations on the eastern coast exhibited high values that were primarily related to the topography of China.
Areas in the northern region still use heating in the spring, and thus, the high-value areas of PM2.5 and PM10 concentrations in the northern region are more significant than in the south, although areas with low concentrations in the north are lesser than those of the summer. PM2.5 concentrations increased to 50–90 µg/m3, and PM10 concentrations increased to 90–150 µg/m3.
Monthly spatial characteristics
Areas with high monthly average concentrations of PM2.5 and PM10 appeared around Henan and northwestern Xinjiang from January 2016 to 2018 (Figs. 12 and 13). In most areas of Henan, the monthly average concentration of PM2.5 reached 100 µg/m3 and the monthly average concentration of PM10 reached 140 µg/m3, and these improved in February. The monthly average PM2.5 concentrations in high-value areas centered around Henan generally decreased by 20–30 µg/m3, while the monthly average PM10 concentration decreased by 30–40 µg/m3. Except for two high-value areas, the monthly average PM10 concentrations in most areas of the country were below 70 µg/m3, while the monthly average PM2.5 concentrations were below 50 µg/m3. Beginning in March, the high-value area centered around Henan gradually weakened, and a development trend appeared to the north. The air quality in the south improved due to the onset of the rainy season. The average monthly PM2.5 concentrations in the Fujian and Zhejiang provinces were below 35 µg/m3.
Nationwide air quality improved in April and May, which was directly related to the end of heating use in northern cities. The regions with higher values included Shandong, Shanxi, Henan, and Hebei. These provinces are concentrated in areas of China with heavy industry and have relatively dense populations. The air quality in the southern provinces has improved significantly, considering the Yangtze River as the dividing line for the country. The monthly average PM2.5 concentrations were lower than 35 µg/m3 and the monthly average PM10 concentrations were lower than 60 µg/m3. Similar observations were made from June to August, with high PM2.5 values in Xinjiang decreasing. In these three months, high-value areas appeared in the Bohai-rim urban agglomeration, the Jianghuai Plain, and the Fenwei Plain. The average PM2.5 concentrations in the south and west of the Yangtze River were lower than 30 µg/m3. After clearing by the marine monsoon climate from June to August, the monthly average PM10 concentrations in most parts of China were lower than 70 µg/m3.
A PM2.5 high-value area in Xinjiang began to appear in September, and the monthly average PM2.5 concentration in the south of the Yangtze River also increased. The monthly average PM10 concentrations also generally increased by 10–20 µg/m3 nationwide. PM2.5 and PM10 high-value areas centered around Hebei began to appear in September, while PM2.5 and PM10 concentrations increased throughout the country. High-value areas centered around the Shandong, Shanxi, Henan, and Hebei provinces gradually became obvious from November to December. By December, the average concentration of PM2.5 in most areas south of the Yangtze River exceeded 60 µg/m3.