Emission Inventory and Spatial Distribution of Particulate Matter from Dust Sources in Zhengzhou City, China

Background: With the rapid development of society and economy, Zhengzhou city, a new first-tier city in central China, is facing high levels of particulate matter (PM) pollution. Dust source has been generally recognized as one of the major source of PM emission that cannot be ignored. In this study, we aimed to establish an emission inventory of PM from dust source and characterize its spatial pattern in Zhengzhou, China. Methods: A GISRS based approach was proposed to establish the emission inventory of total suspended particles (TSP), particulate matter 10 (PM 10 ) and particulate matter 2.5 (PM 2.5 ) from dust source. Bare ground, construction sites and paved roads in 2016 was recognized with high resolution remote sensing data.Technical Guidelines for Emissions Inventory of Particulate Matter Emitted from Dust Source recommended by the Ministry of Ecological Environment (MEP) was used to estimate PM emission, and a spatial grid of 3km×3km resolution was established to display the distribution of PM emission from different dust sources. Results: The total emission of TSP, PM 10 and PM 2.5 in Zhengzhou was 237.5 kt·a -1 , 103.7 kt·a -1 and 22.4 kt·a -1 , respectively in 2016. Compared to other cities in China, the PM emission in Zhengzhou was at a relatively high level. Construction dust source was the main emission source in Zhengzhou, the TSP, PM 10 and PM 2.5 emission of which account for 76.42%, 89.68% and 88.39%, respectively of the total emissions, followed by road dust source and soil dust source. Spatially, total TSP, PM 10 , PM 2.5 emission were larger in main urban areas including Jinshui, Zhongyuan, Erqi, Jingkai and other urban districts. Zhongmou, Xingyang and Xinzheng, which bordering with the main urban area, were also the area with high PM emission, while Dengfeng, Gongyi and other remote areas had relatively small emission of PM. Conclusions: The GISRS based approach has good applicability for identifying the emission characteristics in urban area, and it is urgent for Zhengzhou city to take specific measures for the prevention and control of atmospheric pollution under the severe situation of high- level PM emission.

most serious environmental problem. Haze weather frequently occurs in autumn and winter, and the air quality becomes even worse in many developed cities [1][2][3]. Serious air pollution issue not only restricts the sustainable development of regional economy, but also brings great negative influence on human health [4]. As the major components of atmospheric pollutants, particulate matter (PM) has received much attention in the field of atmospheric environment research [5][6][7]. There are various sources of PM emission, of which dust source is the most common source in urban areas. Dust source refers to a certain range of atmospheric particulates that generated from loose surface and entered to the ambient air by natural and human factors, which can be divided into soil dust, construction dust, road dust and the field dust [8]. Studies have shown that the contribution rate of PM 2.5 and PM 10 emission from dust source account for 41.8% and 60.0%, respectively in Nanchong [9], 16.7% and 23.6% in Haikou, respectively [10], 22.1% and 28.7% in Qingdao [11], and 30% and 42% in Tianjin [12], dust source has been generally recognized as one of the major source of PM emission that cannot be ignored.
Previous studies have made great efforts on the estimation of PM emission, nevertheless, most of these studies were carried out in big cities in developing countries such as Beijing, Shanghai in China [13][14], New Delhi in India [15], and focus more on the industrial and vehicle source of PM emissions [16][17][18][19], while few studies have been conducted to investigate the PM emission from dust source in medium-sized cities with rapid growth and severe air pollution problems. Zhengzhou, a fastgrowing megacity city in the central area of China, is facing serious air pollution issue along with the rapid economic development and expansion of urban area. According to the investigation conducted by MEP in 2016, the air quality of Zhengzhou city was the seventh from the bottom among 74 major cities of China, and the number of days exceeding the national air quality standard of PM 10  Xingyang City), and one county (Zhongmou County) in Zhengzhou. Zhengzhou City is characterized by the north temperate continental monsoon climate with an annual mean temperature of 13.9 ℃, the annual average precipitation is 640 mm, which concentrated in June, July and August. The average wind speed is 2.8 ~ 3.2 m/s, the maximum wind speed is 18 ~ 22 m/s, which mostly occurs in April or September. The soil types mainly include Cinnamon soil (62.5% of the total area), fluvo-aquic soil (26.9%), sandy soil (5.1%), stony soil(2.6%), coarse soil (1.9%) and a small amount of red clay (0.8%) and purple soil (0.2%).

Data processing and spatial mapping
Remote sensing data used in this study was acquired from the Chinese Resource Satellite ZY-3. ZY-3 is China's first three linear array stereo mapping satellite, the maximum spatial resolution of which is 2.1 m. Since the launch of the satellite in January of 2012, images from ZY-3 have been widely used in land surveying, territorial mapping, and ground information extraction [22][23][24]. Before identifying the distribution of emission source, ENVI 5.3 was used for the preprocessing of images, including radiometric calibration, geometric correction, image enhancement and splicing and clipping, the 1:50,000-scale topographic map was used for geometric correction, and the error was less than 0.5 pixels after correction.
The spatial distribution of bare ground, construction sites and different types of roads (national road, provincial road, county road and township road) were identified by visual interpretation in ArcGIS 10.2. Every piece of emission source was extracted as a shapefile, the attribute data of each shapefile included name, location, length, area and other association attributes. After the manual extraction, all the shapefiles were overlaid together and a new thematic layer was formed to calculate the emission of TSP, PM 10 and PM 2.5 with method recommended by the MEP, then a grid of 3km × 3 km resolution was established to display the spatial distribution of TSP, PM 10 and PM 2.5 emission from different dust sources.

Methodology 2.3.1 Soil Dust
Soil dust is generated from bare ground such as bare farmland, exposed mountains, tidal flats, dry river valleys, unhardened spaces. The influence factor of emission from soil dust include particle size distribution of surface soil, surface roughness, vegetation coverage, surrounding shielding conditions and meteorological conditions. Bare ground in this study mainly focuses on the farmland in the surrounding urban areas and the open land without hardening or greening.
The formulas for calculating emission of PM from soil dust sources are as follows: Where W Si is the total PM i emission, t/a; E Si is the PM i emission coefficient, t/(m 2 ·a); A s is the area of soil dust source, m 2 ; D i is the dust emission factor of PM i , t/(10 4 m 2 ·a); η is the dust removal efficiency by pollution control technology, %. Since only the dust emission of the exposed ground was considered in this study, the removal efficiency of the exposed ground control efficiency was set to zero.
Where k i is the percentage of PM i in soil dust. The recommended values of TSP, PM 10 and PM 2.5 are 1, 0.30 and 0.05, respectively; I we is the soil wind erosion index, which is 331 for TSP, 99 for PM 10 and 17 for PM 2.5 according to the soil texture of Zhengzhou. f is the ground roughness factor, the value of which is 0.5; L is the unshielded width factor, and the value is 1.0; V is the vegetation coverage factor, and the value is 0.9. C is the climatic factor, u is the annual average wind speed, m/s; PE is the Thomthwaite Precipitation Index (evaporation index), and the PE formula is as follows: Where p is the annual precipitation, mm; T a is the annual average temperature, °C.
The average wind speed of Zhengzhou is 1.984 m/s, the annual precipitation value is 833.1 mm and the annual average temperature is 16.5 °C in 2016.

Construction dust
Construction dust is the dust generated during the construction process, such as urban municipal infrastructure, the construction and demolition of building, the engineering of the equipment installation and building decoration. The influence factors of emission from construction dust source including construction area, construction time and PM emission coefficient. In this study, area of main building construction and decoration construction was considered as the construction area for the calculation of construction dust emission.
The calculation formulas for the emission from construction dust are as follows: Where W Ci is the total PM i emission of the construction dust, t/a; E Ci is the average PM i emission coefficient of the entire construction zone, t/(m 2 ·month); A C is the area of construction zone, m 2 ; T is the period of construction activities, which is generally calculated by the number of construction days/30 (according to the survey, the average construction days in Zhengzhou was 179 d in 2016); η is the removal efficiency by pollution control technology, %.

Paved road dust
Road dust refers to the dust that generated from paved roads and entering the ambient air under dynamic conditions (wind power, motor vehicle rolling, crowd activities, etc.). The paved roads of Zhengzhou can be divided into four categories including national roads, provincial roads, township roads and county roads.
The calculation formulas of PM emission from each category of paved roads are as follows: [Due to technical limitations, these formulas could not be displayed here. Please see the supplementary files section to access these formulas.] Where W Ri is the total emission of PM i from road dust, t/a; E Ri is the average emission coefficient of PM i from different categories of paved roads, g/(km·vehicle); L R is the length of road, km; N R is the average traffic flow on the road during a certain period of time, the vehicle/a; n r is the number of dust-free days; k i is the particle size multiplier of PM i , the recommended value is 3.23 g/ km (TSP), 0.62 g/km (PM 10 ) and 0.15 g/km (PM 2.5 ); sL is road dust load, g/m 2 ; W is the average vehicle weight, t ; η is the removal efficiency by pollution control technology, %, when multiple measures are implemented simultaneously, the maximum removal efficiency should be taken as the final value.
In this study, days with precipitation exceed 0.25 mm/d was taken as the number of dust-free days. In order to obtain the traffic flow data, a 10-day-long monitoring experiment was carried out on each category of paved roads to obtain traffic flow (N R ) and the road dust load of different types of road.
The average vehicle weight and other traffic information were estimated based on the observation data, the emission parameters of different levels of roads in Zhengzhou are shown in Table 1. which were mainly distributed in Zhongmou and Xinzheng. Zhongmou has the largest exposed area, which accouts for 52.50% of the total area of bare ground in Zhengzhou, followed by Xinzheng, Dengfeng and Xinmi. (Fig. 2(a)). Figure 2 Economy Zone also has high PM emission, the main reason is that the soil type is mainly silt soil, which can be easily raised into air by wind and human activities, therefore lead to high emission coefficient. Dengfeng has a complex terrain that decreases from north to south, Xinmi is surrounded with mountains, although the area of bare ground are not small in the two regions, the limitation of topography reduced the effects of wind and human farming activities and lead to low PM emission.
Xingyang, Gongyi and the urban area of Zhengzhou have the minimum PM emission, and it is mainly due to the small area of bare grounds.

Emission Inventory and Spatial Distribution of PM from construction dust
The number of construction sites was 1,793 in Zhengzhou with a total area of 145.25 km 2 in 2016, which was mainly distributed in the urban area ( Fig. 3(a)). The emission of TSP, PM 10 and PM 2.5 from construction dust source was calculated to be 181.5 kt·a -1 , 93.0 kt·a -1 and 19.8 kt·a -1 , respectively.  (Table 2). Among different levels of roads, township road has the largest PM emission, followed by provincial road, county road and national road. This is because the length of township road is far more than other types of roads, and the road dust load of township road is relatively large. Although the dust load of provincial road is small, the heavy traffic volume lead to high PM emissions. It can be seen that the TSP, PM 10 and PM 2.5 emission from road dust is large in Zhengzhou, but compared with other cities of similar size in China, PM emission from road dust in Zhengzhou are still at low level [26][27].
The spatial characteristics of TSP, PM 10 and PM 2.5 emission from road dust source in Zhengzhou was shown in Fig. 4b, c,   The calculation results showed that the emission of TSP, PM 10 and PM 2.5 of Zhengzhou in 2016 were 237.5kt·a -1 , 103.7kt·a -1 and 22.4 kt·a -1 , respectively. By analyzing the source of PM, it can be found that the construction dust was the main dust sources of PM emission in Zhengzhou, TSP, PM 10 and PM 2.5 emission from construction dust accounted for 76.43%, 89.69% and 88.50%, respectively of total dust sources ( Table 3).
The spatial distribution of total PM emission was shown in Fig. 5b, 5c and 5d, it can be seen that the emission of TSP, PM 10 and PM 2.5 were mainly concentrated in the Zhongyuan, Huiji, Jinshui, Zhengdong New District and other main urban areas. Zhongmou, Xingyang and Xinzheng, which bordering with the main urban area, were also the high PM emission area. While the areas that far away from the main urban area had low emission intensity such as Shangjie and Dengfeng. Three aspects of reasons of the emission characteristics was concluded. Firstly, the majority of construction sites are distributed in the main urban area, secondly, the dense road network and heavy traffic flow of the main urban area increases the PM emission, moreover, although soil dust is more prevalent in the suburbs than in the urban area, but due to the small PM emission from soil dust, it doesn't make much difference to the general characteristic of total emissions.
Results from the analysis revealed that the key zones for PM emission control from dust sources are mainly distributed in the main urban area of zhengzhou, which is consist of Zhongyuan, Huiji, Jinshui, Zhengdong New District. Furthermore, areas such as Zhongmou, Airport Economy Zone and Xinzheng are more likely to face increasing serious PM emission problems due to the acceleration of urbanization under policy orientation, so it is necessary to conduct early prevention of PM emission in these potential PM emission areas. Table 3 Emission of TSP, PM 10           territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 5
Spatial distribution of total PM emission from dust sources. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 5
Spatial distribution of total PM emission from dust sources. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Supplementary Files
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