Spatiotemporal Patterns and Cause Analysis of PM2.5 Concentrations in Beijing, China

According to the monthly comprehensive air index ranking in China in 2016, Beijing ranked in the bottom tenth three times, indicating that the air pollution situation is very serious compared to other cities in China. In this study, we chose 23 urban environmental assessment points, which covered all districts and counties in Beijing. We used ArcGIS software to analyze atmospheric concentrations of particulatematter with a diameter< 2.5 μm(PM 2.5 ) for eachmonth of 2016 in each district/county of Beijing.Our results showed that PM 2.5 concentrations inwinter and springwere generally higher than those in summer and autumn. The higher monthly average PM 2.5 concentrations were primarily in the southwest and southeast areas. The higher annual average values were distributed in Fangshan, Daxing, and Tongzhou, which were closely related to the high terrain in the northwest and the low-lying terrain in the southeast, the “Beijing Bay” terrain, and local climatic conditions. The temporal and spatial distributions of PM 2.5 constitute a warning signal for human life and production during different seasons and regions.


Introduction
In recent years, the frequent occurrence of haze events has gradually influenced human health and living.The composition of haze is very complex, including hundreds of atmospheric particles [1].Aerosol particles 10 m in diameter are the most harmful particles for human health: floating dust, aerosols, and inhalable particles can directly enter the human body and adhere to the upper and lower respiratory tract and lungs, causing rhinitis, bronchitis, and other diseases.These particles will induce lung cancer if they exist in this environment for a long time.Particles > 10 m do not easily enter the respiratory tract.Most 5-10 m particles are deposited in the upper respiratory tract, whereas most 2.5-5 m particles are deposited in the bronchioles and alveoli.However, about 75% of particles < 2.5 m (PM 2.5 ) in diameter are deposited in alveoli.Because the alveolar area has a large surface area and the alveolar wall has an abundant capillary network, the soluble fraction (including some heavy metals) is easily absorbed into the blood and has effects in the human body, while the insoluble fraction, as a foreign body, is phagocytized by macrophages, causing an inflammatory reaction.In addition, PM 2.5 also damage the cardiovascular and nervous systems.Although PM 2.5 particles are small, a large number of toxic and harmful substances readily attach to them.Particles attached to toxic substances have the characteristics of long duration and longdistance transportation in the atmosphere, causing a greater impact on human health and the atmosphere.
In February 2012, the State Council of China revised and issued an Ambient Air Quality Standard (GB3095-2012), in which PM 2.5 was added as a new monitoring indicator.PM 2.5 is equivalent to 1/20 the diameter of a human hair, and particles of this size directly enter the lungs.Although PM 2.5 content is very small compared to other components of the Earth's atmosphere, they have important implications for air quality and visibility.In 2013, PM 2.5 was identified as a primary carcinogen by the International Cancer Research Institute.The composition of PM 2.5 is very complex and mainly depends on the source, which can be natural (soil dust, volcanic eruption, and forest fires) or anthropogenic (burning of fossil fuels, emission of motor vehicles, and 2 Advances in Meteorology burning and dust of straw waste), which are the most harmful to human health [2].Secondhand smoke is a major source of PM 2.5 , followed by cooking fumes.The frequent hazy weather in Beijing is caused by several factors, including industrial development, coal-based energy consumption, and increases in motor vehicle ownership and urban construction projects.
The objective of the present study was to investigate the temporal and spatial patterns of PM 2.5 in Beijing in 2016 and to analyze the causes of their formation.The findings provided by this study will support environmental policymakers who should make informed decisions pertaining to urban planning to mitigate atmospheric pollution events in Beijing.

2.
1. Data Sources.In this study, we selected 23 urban environmental assessment points, covering all districts and countiesin Beijing, and collected data on the average PM

Temporal Patterns of PM 2.5 in Beijing in 2016
3.1.1.Current Situation.The Beijing-Tianjin-Hebei region and surrounding areas (including Shanxi, Shandong, Inner Mongolia, and Henan) suffer from severe pollution, which in 2016 in Beijing persisted for more than 40 days.For nearly half of the year, Beijing experiences a minimum of light pollution, while March and October-December are the major pollution seasons.Heavy pollution has occurred many times in Beijing in December, and 35.5% of days were considered a heavy pollution, which was significantly higher than that in other months.The annual average PM 2.5 concentration in Beijing in 2016 was 73 g/cm 3 , which was reduced by 9.9% from the year earlier, and the annual average concentrations in 2016 of three other major atmospheric pollutants, PM 10 , sulfur dioxide, and nitrogen dioxide, were 92, 10, and 48 g/cm 3 , respectively, which were reduced by 9.8%, 28.6%, and 4.0%, respectively, compared with that in the year earlier.It is undeniable that Beijing still experiences severe air quality issues, as shown by the hazy weather conditions highlighted by the monitoring data.

Monthly Mean PM 2.5 Concentration Distribution in
Beijing. Figure 1 shows the monthly average PM 2.5 concentrations in Beijing in 2016.It is clear that the average PM 2.5 concentration was highest in December (mean = ∼133 g/cm 3 , SD = 21 g/cm 3 versus ∼100 g/cm 3 , SD = 13 g/cm 3 for November).The average PM 2.5 concentration in March 2016 was ∼92.8 g/cm 3 (SD = 8 g/cm 3 ).Significant increases in PM 2.5 concentrations were detected from August to December.PM 2.5 -induced pollution in March and October-December was more serious than in other months, in which the average concentrations ranged from 45 to 70 g/cm 3 .Fluctuations in summer and autumn were small.The lowest concentration, ∼43.5 g/cm 3 , occurred in February.Figure 1 shows that PM 2.5 concentration declined in March, increased in August, and reached its maximum in December.The mean PM 2.5 concentrations in summer and autumn were lower than those in winter and spring, indicating better air quality during summer and autumn in Beijing.

Monthly Mean PM 2.5 Concentrations in Beijing:
District and County Data. Figure 2 shows the monthly average PM 2.5 concentration distributions in the 16 districts and counties of Beijing.We found that the PM 2.5 concentration of each county was lowest in February and around August.Statistical analysis showed that the concentrations in February and August were significantly lower than those in the other months ( < 0.05).Monthly average concentrations from April to August in each county were all low, but the differences among these months were significant for each county ( < 0.05).The PM 2.5 concentrations in districts and counties rose rapidly beginning in September and attained their highest levels in December.Statistical analysis showed significant differences in monthly average concentrations among all months ( < 0.05), except between May and September ( > 0.05).Overall, the concentrations at the district and county levels were consistent with the regionally averaged concentrations in Beijing.Moreover, the monthly changes in regional PM2.5 concentrations were significantly correlated among the 16 regions in Beijing ( < 0.05).
Figure 2 shows that the highest monthly average PM 2.5 concentration occurred in Fangshan district (161 g/cm 3 ) and  the lowest concentration was in Miyun district (37 g/cm 3 ).The yearly average PM 2.5 concentrations in Daxing and Fangshan districts were the highest among all districts, (80.6 ± 28.8 and 80.5 ± 34.0 g/cm 3 , respectively), ranging from 49 to 146 g/cm 3 and 36 to 161 g/cm 3 , respectively.The mean concentrations in Yanqing and Miyun districts were the lowest among all regions (57.8 ± 17.1 and 59.7 ± 20.0 g/cm 3 , respectively) ranging from 30 to 95 g/cm 3 and 35 to 88 g/cm 3 , respectively.The highest monthly concentrations in Yanqing and Miyun districts were <100 g/cm 3 .Overall, our results show higher PM 2.5 concentrations in southern compared with the northern regions of Beijing.

Spatial Patterns of PM 2.5 in Beijing in 2016
3.2.1.Current Situation.The topography of Beijing is described as "Beijing Bay," being surrounded by mountains on three sides: the west, north, and northeast sides [3].The southeast is a large plain gradually leaning to "Bohai Bay." Beijing has a typically temperate and semihumid continental monsoon climate: it is hot and rainy in summer and cold and dry in winter.There were spatial concentration differences in PM 2.5 in Beijing according to the data of monitoring stations located in each district and county.The highest PM 2.5 concentrations occurred southeast and southwest of Beijing, mainly in Daxing, Tongzhou, and urban areas in the southeast, whereas lower PM 2.5 concentrations occurred in the northern, northwestern, and northeastern suburbs.The most serious pollution caused by haze is always in the southeast and urban areas of Beijing, reflecting the more developed economy, and higher population density, in these regions [4].More hazy weather confers more risks to human health.Figure 4 shows the spatial patterns in monthly average PM 2.5 concentrations during 2016.In January, mean PM 2.5 concentrations in Changping, Miyun, Yanqing, and Huairou districts were low, whereas those in Daxing, Fangshan, and Pinggu districts were high.In February, the mean concentrations in Miyun, Changping, Yanqing, Huairou, Mentougou, and Haidian districts were low, whereas those in Fangshan and Daxing districts were high.In March, the concentrations in Changping, Miyun, and Huairou were low, and the concentrations in Daxing, Fengtai, Tongzhou, and Shijingshan were high.In April, the concentrations in Huairou, Changping, and Mentougou were low, whereas the concentrations in Daxing, Fangshan, and Shijingshan were high.In May, the average concentrations in Beijing were low compared to those of other months; the concentrations in Tongzhou, Shunyi, and Huairou were low, whereas those in Daxing and Shijingshan were high.Air quality in all districts improved somewhat in June, compared to that in the month earlier.The concentrations in Changping, Huairou, Miyun, Yanqing, and Mentougou were lower than those in other districts, whereas those in Tongzhou and Fangshan were higher.In July, the concentrations in Changping and Mentougou were lower versus other districts, whereas those in Dongcheng, Xicheng, Chaoyang, and Tongzhou were higher.In August, the concentrations in Miyun, Changping, and Yanqing were lower versus other districts, and those in Daxing, Daxing, and East-West districts were higher.The PM 2.5 concentrations in all districts in this month were lower than the averaged values of other months, and the air quality in Beijing in August was generally better than other months.In September, the PM 2.5 concentrations in Miyun, Yanqing, Changping, and Huairou were low versus other districts, whereas the concentrations in Tongzhou and Daxing districts were high.In October, the concentrations in Haidian, Yanqing, Pinggu, and Mentougou were low, while the concentration in Shijingshan was the highest among all districts.In November, the concentrations in Yanqing, Huairou, and Miyun were low, whereas the concentrations in Fangshan, Daxing, and Fengtai were high.In December, the concentrations of PM 2.5 in all districts were higher compared to all other months, and the concentrations in Miyun and Yanqing were lower than the average level in Beijing.Moreover, the concentration in Fangshan was the highest among all districts of Beijing during December.

Seasonal Average PM 2.5 Concentrations by District and
County. Figure 5 shows the spatial distribution of quarterly average PM 2.5 concentrations in Beijing in 2016.Clear seasonal differences were observed in the spatial distributions of PM 2.5 concentrations.In spring, the mean PM 2.5 concentrations were 54.7-89.0g/cm 3 at the district/county level.The highest PM 2.5 concentrations and pollution levels were observed in the southwest and southern regions, followed by the eastern regions, primarily including Fangshan, Daxing, Fengtai, Tongzhou, and Pinggu districts.The mean PM 2.5 concentrations in northern Beijing, Changping, Miyun, and Huairou were lower than those in other regions.In summer, the PM 2.5 concentrations in all districts/counties were 49.3-63.5 g/cm 3 .The concentrations in the south and southwest of Beijing, including Daxing and Fangshan, and the main urban areas, including Shijingshan, Dongcheng, and Xicheng, were high.This finding may be related to the urban heat island effect.In general, the PM 2.5 concentration showed a decreasing trend from the south to north of Beijing; the concentration in the northern region was low, indicating better air quality in this region.In autumn, the mean PM 2.5 concentrations in all counties were 43.3-60.0g/cm 3 .The mean concentrations in the southeast and southern regions, including Tongzhou, Daxing, and Xicheng, were high, whereas those in the west, southwest, and north were low.The mean concentrations in all counties were higher in winter than in other seasons.The average winter PM 2.5 concentrations in all districts were in the range of 74.7-119.7 g/cm 3 , and the average was >100 g/cm 3 in most regions, indicating that air quality was lowest at this time of year.Statistical analysis showed that the winter values were significantly higher than those in other seasons ( < 0.01).In winter, pollution in the southwestern and southeastern regions, including Fangshan, Fengtai, and Tongzhou, was most serious, followed by that in Daxing and Shijingshan.The PM 2.5 concentrations in Yanqing, Miyun, Huairou, and Mentougou were lower compared to those in other regions.Spatial differences in PM 2.5 concentrations at the district/county level were primarily related to the local climate and topographical conditions and to the productivity levels and living habits of Beijing residents during winter [5,6].

Causes Analysis of PM 2.5 Concentrations
3.4.1.Causes of PM 2.5 Temporal Distributions.The average PM 2.5 concentrations in winter and spring were significantly higher than those in summer and autumn in Beijing.The average PM 2.5 concentration was highest in December.The concentrations from April to September showed moderate fluctuations, and the levels in this period tended to be lower versus the rest of the year.The reasons for this were particulate emissions caused by coal consumption in the north in winter and the transportation of particulates by the frequent southwesterly winds, which led to a large number of polluted air masses and warm, humid air flows.Moreover, local emissions further increased pollution in Beijing during the winter [7].In spring, the hazy weather decreased significantly, reducing the likelihood of particle retention and decreasing the concentrations of particles.In addition, wind speed was high in spring (generally 5 m/s), particularly in the northwest and the east, which was reflected in the lower mean PM 2.5 concentrations during spring in Beijing compared with winter.Summer and autumn are not heating seasons and there were adequate sunlight and moderate wind speed (<4 m/s).Internal emissions in Beijing have played a leading role in PM 2.5 concentrations over the years.These emissions include automobile exhaust emissions and dust arising from construction sites, which aggravate air pollution in Beijing [8,9].

Causes of PM 2.5 Concentration Spatial Distribution
Patterns.PM 2.5 concentrations in Beijing were high in the south and low in the north; the concentrations in the southeast and southwest border regions were the highest across the entire city.The eastern region had the second highest concentration, which also matched the overall mean for Beijing.The northern region generally had acceptable PM 2.5 concentrations.According to the Environmental Monitoring Center, Beijing Environmental Protection Bureau, PM 2.5 concentrations in Yanqing, Miyun, and Huairou were the lowest among all districts in Beijing, while those in Daxing, Fangshan, Tongzhou, and Fengtai were the highest.The difference in PM 2.5 concentrations between the north and south was significant, and air quality in the north was better than that in the south due to natural and anthropogenic factors.
Beijing has a semi-basin terrain.The northern, western, and eastern regions of Beijing are surrounded by mountains, and only the eastern and southern regions are fully exposed.The main wind directions in Beijing are easterly, southerly, and northwesterly.Low wind speed mainly occurs in the southern and eastern directions, with high wind speed mainly occurring in easterly and northwesterly directions [10].In general, the winter season has the most serious pollution; it is cold and dry with little rain and is significantly influenced by the dry cold air mass from Siberia, which forms northerly and northwesterly winds.Wind blowing from the north to the south leads to clean air in the northern region of Beijing, in which there are fewer people and less industrial development.Meanwhile, the polluted air in the southern region is due to the direction of the wind.In addition, the southeast region of Beijing is almost situated within the plains and includes populated areas, such as the large industrial areas in the Hebei and Tianjin regions.A large number
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Changping County Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Tongzhou County Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Pinggu County Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Yanqing County

Figure 2 :
Figure 2: Monthly average PM 2.5 concentrations in the 16 districts of Beijing in 2016.

Figure 3 :
Figure 3: The spatial distribution of annual average PM 2.5 concentrations in Beijing in 2016.
Figure 3 shows the spatial distribution of the annual average PM 2.5 concentrations in Beijing in 2016.The annual average PM 2.5 concentrations in Fangshan and Daxing districts were the highest (∼80 g/cm 3 ) among all districts; the concentrations in Yanqing and Miyun districts were <60 g/cm 3 , those in Huairou, Changping, and Mentougou districts were in the range of 60-65 g/cm 3 , those in Haidian, Pinggu, and Shunyi districts were in the range of 65-70 g/cm

Figure 4 :Figure 5 :
Figure 4: Monthly average PM 2.5 concentrations in the 12 months of 2016 in Beijing.
2.5 concentrations in each month of 2016.The data were in the fo0rm of monthly air quality data for Beijing, released by the Beijing 3, and those in other regions were in the range of 70-80 g/cm 3 .A decreasing trend in PM 2.5 concentrations was observed from south to north: there was more serious PM 2.5 pollution in the south and better air quality in northern regions.