Spatial and Temporal Variability of the PM 2 . 5 / PM 10 Ratio in Wuhan , Central China

1 School of Resource and Environmental Sciences, Wuhan University, Wuhan 430079, China 2 Key Laboratory of Geographic Information System, Ministry of Education, Wuhan University, Wuhan 430079, China 3 School of Architecture and Urban Planning, Huazhong University of Science and Technology, Wuhan 430074, China 4 State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, Wuhan 430079, China


INTRODUCTION
Particulate matter (PM) has been a growing public concern due to its severe health impacts and significant visibility reduction (Dockery and Pope, 1994;Dominici et al., 2014;Zhang et al., 2015;Fang et al., 2016).PM is usually divided and nominated by its aerodynamic diameter, and the most widely monitored particles are PM 10 and PM 2.5 , which refer to particles with aerodynamic diameter less than or equal to 10 or 2.5 micrometers, respectively (Nel, 2005).PM 2.5 is also called fine particle while particle with a diameter between 2.5 and 10 micrometers is coarse particle (PM 2.5-10 ).Coarse particles (PM 2.5-10 ) are mainly produced from natural processing, such as re-suspension of local soil, dust storm, as well as from anthropogenic sources like road dust and various industrial processes (Querol et al., 2004).Fine particles (PM 2.5 ) comprise largely primary and secondary anthropogenic combustion products originated from traffic and energy production (Li et al., 2004).Compared with PM 10 , PM 2.5 gets more attention due to its smaller size, longer atmospheric lifetime and severer health risks (Dominici et al., 2014).
Since fine and coarse particles come from diverse sources and perform different physic-chemical properties, the PM 2.5 /PM 10 ratio can provide crucial information relating to the particle origin, its formation process and its effects on human health (Speranza et al., 2014;Blanco-Becerra et al., 2015).Higher ratios of PM 2.5 /PM 10 attribute particle pollution to anthropogenic sources and smaller ratios indicate considerable involvement of coarse particles, which might be related to natural sources, e.g.dust storm (Sugimoto et al., 2016).Actually, the PM 2.5 /PM 10 ratio is often used to characterize the underlying atmospheric processes within the local environment (Yu and Wang, 2010;Chu et al., 2015).Sugimoto et al. (2016) has estimated the fraction of mineral dust in particulate matter in East Asia using the PM 2.5 /PM 10 ratio.PM 2.5 was not systematically observed and reported in United States until 1998, and it has just been introduced into the air quality monitoring system in 2013 in China (Yu and Wang, 2010).Identifying and quantifying the PM 2.5 /PM 10 ratios can provide useful information for retrospective prediction of PM 2.5 concentration without direct PM 2.5 measurements, which has been successfully applied in California and Taipei (Yu and Wang, 2010;Blanchard et al., 2011;Chu et al., 2015).However, all of these applications of the PM 2.5 /PM 10 ratio need evidence of its spatio-temporal variability.
PM concentration is influenced by various factors such as land use, population density, meteorology condition (Xu et al., 2016b), and there is obvious spatio-temporal heterogeneity of PM concentrations at intra-city or regional scales.Therefore, it is evident that the fraction of fine particles (PM 2.5 ) in PM 10 also varies at time and space (Parkhurst et al., 1999;Zhou et al., 2016).PM 10 can be mainly composed of fine particles (PM 2.5 ), for instance, PM 2.5 contributes 67% of PM 10 in three east-central U.S. states and the average PM 2.5 /PM 10 ratio is 0.60 at 20 European areas (Parkhurst et al., 1999;Eeftens et al., 2012).However, the ratio is relatively lower in desert area because of the significant contribution of coarse particles, for example, the average PM 2.5 /PM 10 ratio is 0.33 in Saudi Arabia (Khodeir et al., 2012).Previous studies also showed that the PM 2.5 /PM 10 ratios were generally higher in the eastern (0.7) than in the central or western (0.5)United States (USEPA, 2004).The PM 2.5 /PM 10 ratio shows apparent seasonal and diurnal variability as well.Speranza et al. (2016) found that the ratio of PM 2.5 to PM 10 , rearranged from published data according to seasons of sampling, was more than 0.6 in colder seasons (autumn-winter) with a lower ratio (< 0.5) in warmer seasons (spring-summer).While in Africa, the PM 2.5 /PM 10 ratio is higher during the wet season than dry season due to dust re-suspension, a major contribution to PM 10 (Akinlade et al., 2015).As for daily variation, it was found that the minimum values of PM 2.5 /PM 10 ratio in one day occurred during traffic hours in consequence of re-suspended coarse road dust (Querol et al., 2001;Evagelopoulos et al., 2006).
Compared with numerous studies demonstrating the variability of PM concentrations (Eeftens et al., 2012;Hu et al., 2014;Akinlade et al., 2015;Ghim et al., 2015;Huang et al., 2015;Zhang and Cao, 2015;Zhou et al., 2015;Xu et al., 2016a), few studies have reported on the spatio-temporal variability of the PM 2.5 /PM 10 ratio (Blanco-Becerra et al., 2015;Zhang and Cao, 2015;Munir, 2016;Zhou et al., 2016).The goal of this study is to investigate the spatiotemporal variability of the PM 2.5 /PM 10 ratio at a city scale systematically and comprehensively.Taking Wuhan, central China, as an example, both long-term, short-term temporal variation and spatial distribution, spatial disparity of the PM 2.5 /PM 10 ratio will be analyzed based on hourly measurements of PM 2.5 and PM 10 concentrations at nine monitoring sites in urban area (Urban-sites) and one remote Background-site in 2013-2015.To our knowledge, this is the first study aiming at the spatio-temporal variability of the PM 2.5 /PM 10 ratio specially.

Study Area
Wuhan, located in central China, is the capital city of Hubei province where the Yangtze River and Han River join together dividing Wuhan into three parts, Wuchang, Hankou, and Hanyang (Fig. 1).Wuhan is undergoing rapid industrialization and urbanization over the past decades (Jiao et al., 2017).In 2014, more than 10 million people lived in this city with a total GDP exceeding one trillion RMB (approximately 154 billion US dollars) (Xu et al., 2016b).With the vast economic development, the air quality in Wuhan has been under high pressure.Like other megacities in China, serious particle pollution, especially PM 2.5 pollution, was identified in Wuhan in recent years (Guo et al., 2015).

PM Monitoring and Measurements
Currently, there are ten national-controlling ambient air quality monitoring sites in Wuhan city, of which nine sites are located in the urban area (Urban-sites) with one Background-site located about 50 km away from the urban area (Fig. 1).The detailed information on monitoring sites is shown in Table 1.PM 2.5 has not been monitored routinely until 2013 in Wuhan.The real-time mass concentrations of PM 2.5 and PM 10 are measured using the micro oscillating balance method and/or the β absorption method using commercial instruments (Zhang and Cao, 2015).The measurements are available at the website of Wuhan Environmental Protection Bureau (http://www.whepb.gov.cn/).In this study, hourly measurements of PM 2.5 and PM 10 at ten national-controlling monitoring sites from January 18, 2013 to December 31, 2015 were collected.

Data Summarizing and Analysis
Only data available for both PM 2.5 and PM 10 concentrations at the same hour has been included resulting in more than 170,000 records in total.The average ratios of PM 2.5 to PM 10 among diverse temporal scales at each site are summarized step by step (Xu et al., 2016a).At each site, daily averages of the PM 2.5 /PM 10 ratios are summarized from hourly ratios.As a result, daily average ratios are used to calculate monthly averages.These are then used to calculate seasonal and annual average ratios at each site.The season division in this study is as follows: spring (March, April, May), summer (June, July, August), autumn (September, October, November) and winter (December, January, February).In order to compare the variability of PM 2.5 /PM 10 ratios with PM concentrations, the average concentrations of PM 2.5 and PM 10 at diverse temporal scales are summarized as well.The spatio-temporal variability of PM 2.5 /PM 10 ratios in Wuhan is analyzed as the flowchart shown in Fig. 2. Since the pollution level in urban and suburban area is apparently different (Xu et al., 2016a), temporal variability of the PM 2.5 /PM 10 ratio at Urban-sites (nine) and Background-site (one) will be compared.Although temporal and spatial variability of the PM 2.5 /PM 10 ratio will be analyzed separately, spatio-temporal variability affects each other, for example, the distinguishing of Urban-sites and Background-site in temporal variation analysis also reveals the spatial disparity of the ratio.

Long-Term Variation of the Ratio
The long-term variation of the PM 2.5 /PM 10 ratio will be analyzed from seasonal and monthly perspectives.The daily average PM 2.5 /PM 10 ratios and PM concentrations summarized from nine Urban-sites during the whole study period are shown in Fig. 3.
PM concentrations fluctuate prominently day by day, with daily average PM 10 concentrations varying from 15 to 415 µg m -3 and PM 2.5 varying from 7 to 356 µg m -3 .During the whole three years, 40% days exceed the 24-hour Level-II limitation (75 µg m -3 ) for PM 2.5 concentration, and 27% days violate the 24-hour Level-II limitation (150 µg m -3 ) for PM 10 concentration, demonstrating the severity of particle pollution in Wuhan (China Ambient Air Quality Standards (GB 3095-2012)).The Level-II limitations are applicable to residential areas, commercial-transport-residential mixed areas, cultural areas, industrial areas and rural areas (CAAQS, GB 3095-2012).Due to the noticeable fluctuation of PM concentrations, the daily average PM 2.5 /PM 10 ratios vary significantly as well.The daily mean ratios change in 0.2-0.9 with an average ratio of 0.62 for the whole three years in Wuhan urban area, indicating the dominant fraction of fine particles in PM 10 .
There is a periodical variation for both PM concentrations and PM 2.5 /PM 10 ratios in Wuhan urban area.A high level of seasonality can be observed that PM pollution in winter is the most serious and it drops to a comparatively low level in summer.These fluctuations have been broadly  reported in China (Zhang and Cao, 2015;Zhou et al., 2015).The daily average PM 2.5 /PM 10 ratios display a similar varying pattern to PM concentrations in Wuhan urban areas.Overall, the ratio declines from spring to summer, reaching the lowest, and then it increases to the highest in winter.The seasonal averages of PM concentrations and PM 2.5 /PM 10 ratios at nine Urban-sites and one Background-site in 2013-2015 are presented in Table 2.
In terms of seasonality of the PM 2.5 /PM 10 ratios, both Urban-sites and Background-site experience the highest ratios in winter, but the ratio drops down to the lowest (0.55) in summer at Urban-sites while Background-site experiences the lowest (0.63) in spring (Table 2).The highest value of PM 2.5 /PM 10 ratio in winter was also observed in other  (Gu et al., 2010;Huang et al., 2015;Zhou et al., 2015).Higher PM 2.5 /PM 10 ratios relating to colder seasons (autumn and winter) were found as well in a meta-analysis (Speranza et al., 2016).The increasing of fuel consumption for domestic and industrial heating in winter leads to an increase of fine particle emissions (Liu et al., 2016).The secondary aerosol, one of the major source of fine particles, is also accelerated due to the lower mixing height in winter (Huang et al., 2014).On the other hand, the stable atmospheric conditions in winter are favorable for the dry deposition of coarse particles, but also favor the accumulation of fine particles in the air, resulting the domination of fine particles in PM 10 in winter (Huang et al., 2015).At nine Urban-sites, PM 2.5 concentration decreases from the highest to the lowest by 64% but PM 10 concentration declines by 49%, indicating a stronger seasonal disparity of PM 2.5 than PM 10 .
For more detailed information about the long-term variation, monthly variation of the PM 2.5 /PM 10 ratios and PM concentrations are presented in Fig. 4. The error bars indicate one standard deviation of the ratios at Urban-sites and Background-site.Fig. 4 shows that monthly variation of the PM 2.5 /PM 10 ratios generally share the same variation trend with PM concentrations, especially the ratios at nine Urban-sites.PM 2.5 and PM 10 concentrations decrease from January to July and then increase to December in addition to some local undulations.However, there is obvious difference for the variation of PM 2.5 /PM 10 ratio between Urban-sites and Background-site.The PM 2.5 /PM 10 ratios at Urban-sites drop to the lowest in July (0.51) while the ratio at Background-site experiences the minimum in April (0.59), although both of them reach to the maximum in February.Another disparity between Urban-sites and Backgroundsite is the variation amplitude of the ratio.The ratio varies more greatly at Urban-sites but there is only imperceptible changes for the ratio at Background-site from March to December, which indicates more complex and changing PM sources in urban area.

Short-Term Variation of the Ratio
Daily variation of the PM 2.5 /PM 10 ratios and PM concentrations at Urban-sites and Background-site in 2013-2015 is presented in Fig. 5.The 19:00 is the turning point of daily variation of the ratios, so the hour begins at 19:00 in Fig. 5.An obvious night-day differences of PM 2.5 /PM 10 ratios can be found in Fig. 5.The PM 2.5 /PM 10 ratios at both Urban-sites and Background-site increase from 19:00 all along to the early morning at 6:00, with the peak value of 0.65 at Urban-sites and 0.72 at Background-site.While in the daytime, the ratios decrease until nightfall (18:00), although there is a modest rise at Urban-sites during post meridiem (12:00-14:00).It is the changes of temperature that should be taken into consideration to understand the evident night-day differences of PM 2.5 /PM 10 ratios.The underground temperature is lower than the air temperature at night, which is called temperature inversion.Temperature inversions lead to stable atmospheric conditions constraining vertical airflow, which is favorable to the dry deposition of  coarse particles and the accumulation of fine particles (Wallace and Kanaroglou, 2009).Therefore, the PM 2.5 /PM 10 ratios increase gradually at night.In the daytime, as a consequence of the re-suspended coarse road dust and dynamic human activities, the PM 2.5 /PM 10 ratios decline gradually.The daily variation of PM 2.5 /PM 10 ratios in four seasons are presented in Fig. 6 to identify the seasonality of the daily variation of the ratios.
The turning point of daily variation trend for PM 2.5 /PM 10 ratios is one or two hours later in summer and autumn, since the temperature inversion occurs later in the two seasons.To keep consistent with Fig. 5, the time in four sub-divisions begins at 19:00 as well.It shows similar daily variation but also disparities of PM 2.5 /PM 10 ratios in four seasons (Fig. 6).Daily variation of PM 2.5 /PM 10 ratios at Urban-sites is more stable and smooth.However, there are local fluctuations in daily variation at Background-site, especially during nighttime in winter (Fig. 6(d)), which might be related to the limited number of Background-site (only one) in Wuhan.PM 2.5 /PM 10 ratios increase dramatically in the nocturnal period except for the ratio at Background-site in winter.The ratios generally share the same decreasing trend in the daytime in four different seasons.The gap of ratios between Urban-sites and Background-site in nighttime is greater than that in the daytime (Figs. 5 and 6).As a consequence of the urban heat islands effect, the ground temperature is higher in urban areas than suburban in nighttime.Therefore, the intensified temperature inversion in suburban areas will lead to more stable atmospheric conditions, which contributes to the rising of PM 2.5 /PM 10 ratios.Although the ratio varies more like a unimodal pattern in Fig. 5 summarized from all four seasons.The daily variation of ratio in four sub-divisions shows a more obvious bimodal pattern, particularly in summer.Merely, the afternoon peak is not as noticeable as the peak in early morning.Actually, the bimodal pattern of daily variation of PM 2.5 /PM 10 ratios also have been reported in Catalonia, Spain (Querol et al., 2001) and Kozani, Greece (Evagelopoulos et al., 2006).

Spatial Distribution of the Ratio
The spatial distribution of three-year average PM 2.5 /PM 10 ratios and PM concentrations in Wuhan urban area are presented in Fig. 7. Apparent spatial disparity of PM pollution level and PM 2.5 /PM 10 ratios in Wuhan urban area can be recognized in Fig. 7.The three-year average PM 2.5 concentrations vary from 68 µg m -3 at GX to 87 µg m -3 at GH, and PM 10 concentrations change in 115-135 µg m -3 .The most serious PM 2.5 and PM 10 pollutions take place at GH site, where it is a traditional industrial base.The GX site experiences both the lowest PM 2.5 and PM 10 concentrations probably due to lower population density (Table 1).The proportion of fine particles in PM 10 also shows great variability varying from 58% to 66%.The maximum ratio (0.66) is found at the most polluted site (GH), demonstrating the high contribution of industrial emissions to fine particles, which has also been proved by another industrial site (WJS) with higher PM 2.5 /PM 10 ratio.ZK and GX sites are located in industrial zones as well, but the ratios at those sites are relatively low.In spite of less population, these two industrial parks are planned for development of high and advanced technologies with lower industrial emission; on the other hand, these two industrial parks are newly built in recent years with much area of bare land resulting in considerable suspension of soil dust (Xu et al., 2016b).As for LY site, located in a scenery zone, PM concentrations and the ratio at this site are still relatively high because it is situated in the downwind of the Qingshan traditional industrial base (GH).

Spatial Disparity of the Ratio
In order to distinguish the PM 2.5 /PM 10 ratios with different distances to the main urban area, those nine Urban-sites are divided into two parts (urban core and urban fringe) depends on whether located inside or outside the 2 nd Ring Road in Wuhan.Obviously, sites 1-5 are located inside the 2 nd Ring Road (urban core) while the other four Urbansites belong to the second part (urban fringe) (Fig. 1), and the  third part is suburban (Background-site).The ratios at three categories of sites in four seasons are presented in Fig. 8.
An obvious tendency can be found in Fig. 8 that the PM 2.5 /PM 10 ratio gradually increase from urban core (0.617, four-season average) to urban fringe (0.630) and further to suburban (0.680).In addition, the magnitude of the spatial disparity varies in different seasons.In winter and spring, the ratios at three categories of sites are similar.The disparity is the most obvious in summer and the ratio at suburban is 16.6%, 23.7% higher than that at urban fringe and urban core, respectively.The ratio at urban fringe is 6.1% higher than that at urban core in summer.But in autumn and winter, the ratio at urban core and urban fringe are almost the same, although the ratio at suburban is significantly higher.
Fine particles as well as coarse particles are emitted and produced due to human activities in urban area, but fine particles can travel further than coarse particles before they are removed from the atmosphere, resulting higher PM 2.5 / PM 10 ratios in suburban (Parkhurst et al., 1999;Zhou et al., 2016) In addition, the lower ratios at Urban-sites are partly attributed to the resuspension of construction dust and road dust because of the massive infrastructure construction (e.g., metro and arterial traffic construction) in Wuhan urban area in recent years (Xu et al., 2016b).Parkhurst et al. (1999) demonstrated that the fractional contribution of PM 2.5 to PM 10 appears greater at remote stations and lower at those stations which are more likely to be influenced by anthropogenic local sources.A previous study in Wuhan also reported higher ratios in suburban site (0.64) than urban site (0.57) in 1995-1996 (Wei et al., 1999).It was also found the highest PM 2.5 /PM 10 ratio at the control site in Ibadan, Nigeria, because of being far from the road and any obvious source of pollutants (Akinlade et al., 2015).

DISCUSSION
This study investigates the spatio-temporal variability of the PM 2.5 /PM 10 ratio from both long-and short-term perspectives at a city scale based on hourly PM observations in 2013-2015.The three-year average PM 2.5 /PM 10 ratio in Wuhan urban area is 0.62.A wide range of ratios of PM 2.5 to PM 10 have been observed across the world.It is reported that 0.6 is regarded as a typical North American PM 2.5 /PM 10 ratio (Dockery and Pope, 1994).In Asia, the ratio is generally less than 0.5, indicating higher coarse particle masses; but urban sites in China have mean PM 2.5 /PM 10 ratio values greater than 0.5 (Hopke et al., 2008).Studies in China recently reported that the mean ratio of 190 cities is 0.56 in 2014-2015 (Zhang and Cao, 2015).Compared with other megacities or urban agglomeration in China (Table 3), the fraction of fine particles in PM 10 in Wuhan urban area is relatively high, indicating the severity of fine particle pollution in Wuhan.
This study demonstrates that the PM 2.5 /PM 10 ratio varies obviously at time and space.The ratio shares approximate seasonality with PM concentrations at Urban-sites, and the highest ratio observed in winter (0.75) is 20% higher than the minimum ratio (0.55) in summer in Wuhan urban area,  Previous study also reported that PM 2.5 comprised about 80% of PM 10 during winter months but with 50% in summer months in Birmingham, UK (Harrison et al., 1997).We found apparent night-day difference of the daily variation of PM 2.5 /PM 10 ratio.The ratio increases in the nocturnal period all the time due to favorable condition for coarse particle sedimentation caused by inversion temperature, and it declines in the daytime with a moderate rise in the afternoon.In terms of the spatial distribution and disparity, the three year average highest ratio (0.66) in urban area is found at the traditional industrial zone (GH site), indicating the considerable contribution of energy combustion and other industrial process to fine particles.The three-year average PM 2.5 /PM 10 ratio at one remote Background-site (0.68) is higher than that at any other Urban-sites (0.58-0.66).We also found an obvious spatial gradient of the ratio, which increases from the urban core to urban fringe, and further to suburban.The variability of PM concentrations has been reported in many previous studies (Eeftens et al., 2012;Huang et al., 2015;Zhou et al., 2015;Xu et al., 2016a), while this study contributes to the spatio-temporal variability of PM 2.5 /PM 10 ratio and its applications.The variation of ratios can reflect the seasonality and type of PM emission sources (Speranza et al., 2016).The considerable variability of the ratio cannot be ignored when we use the ratio in practice.For instance, the seasonal variability of PM 2.5 /PM 10 ratio should be addressed in the retrospective prediction of historical PM 2.5 concentrations without direct measurements.On the other hand, the ratio can be better used if we investigate its variability at first.
There are some limitations in this study as well.At present, only the mass concentrations of PM 10 and PM 2.5 are routinely observed at ambient air quality stations in China (Zhang and Cao, 2015).Although PM compositions characterize particle origins and its formation processes, whose significance has been emphasized all the time (Wen et al., 2016), the diverse chemical compositions of fine and coarse particles were not included in this study constrained by data accessibility.In addition, only the arithmetic mean of PM 2.5 /PM 10 ratio is used in the data analysis, while other robust statistics, e.g., median, various percentiles, and Theil-Sen trend analysis, which are not sensitive to non-normal distributions and to extreme values in both tails of the distributions, can be applied to better understand the variability of the ratios (Munir, 2016).Finally, the PM 2.5 /PM 10 ratio varies at space and time as a consequence of PM fluctuation, but many other factors, such as meteorological condition, affect its value and variability (Blanco-Becerra et al., 2015;Wang et al., 2015).The influencing factors and their relationship with the PM 2.5 / PM 10 ratio can be taken into consideration in further work.

CONCLUSIONS
This study contributes to the spatio-temporal variability of PM 2.5 /PM 10 ratio.Overall, fine particles make up 62% of PM 10 in Wuhan urban area in 2013-2015, revealing the severity of fine particle pollution.The ratio varies obviously at time and space.During one day, the ratio increases in the nocturnal period in all seasons due to favorable condition for coarse particle sedimentation caused by inversion temperature and declines in the daytime with a modest rise in the afternoon.As for the seasonality, the highest ratio is observed in winter (0.75) while it drops to the lowest in summer (0.55) in urban area.There is an apparent spatial gradient of the ratio that gradually increases from urban core (0.617) to urban fringe (0.630) and further to suburban (0.680).The highest ratio is found at industrial zone (GH, 0.66), proving the considerable contribution of energy combustion and other industrial process to fine particles.The implication of this study is that considerable variability of the ratio cannot be ignored when we use the ratio in practice, such as retrospective prediction historical PM 2.5 .In turn, the ratio can be better used to understand atmospheric processes if we investigate its variability at first.

Fig. 1 .
Fig. 1.Study area and the spatial distribution of ten ambient air quality monitoring sites in Wuhan city: (a) location of Wuhan in China; (b) administration boundary of Wuhan city and spatial distribution of ten monitoring sites; (c) detailed distribution of nine monitoring sites in the urban area of Wuhan.

Fig. 2 .
Fig. 2. Workflow of data summarizing and spatio-temporal variability analysis of the PM 2.5 /PM 10 ratio.

Fig. 3 .
Fig. 3. Daily average PM 2.5 /PM 10 ratios and PM concentrations summarized from hourly observations at nine Urban-sites in urban area in 2013-2015 with the solid and dashed horizontal lines representing the 24-hour Level-II limitations for PM 2.5 (75 µg m -3 ) and PM 10 (150 µg m -3 ) concentrations, respectively, according to the China Ambient Air Quality Standards (CAAQS, GB 3095-2012).

Fig. 4 .
Fig. 4. Monthly variation of the PM 2.5 /PM 10 ratios and PM concentrations at Urban-sites and Background-site.

Fig. 5 .
Fig. 5. Daily variation of the PM 2.5 /PM 10 ratios and PM concentrations at Urban-sites and Background-site in 2013-2015.

Fig. 7 .
Fig. 7. Spatial distribution of PM 2.5 /PM 10 ratios and PM concentrations at nine Urban-sites in Wuhan urban area in 2013-2015.

Fig. 8 .
Fig. 8.The average PM 2.5 /PM 10 ratios in urban core, urban fringe, and suburban in four seasons from 2013 to 2015.

Table 1 .
Detailed description of ambient air quality monitoring sites in Wuhan.
rd ring road Industrial, Transportation zone 400 10 Chenhu qihao CH About 50 km away from urban area Natural reserve zone -

Table 3 .
Comparison of the PM 2.5 /PM 10 ratio in other cities or regions across China.