3.1 Dust accumulation characteristics and spatial distribution of concentration in Pingshuo mining area
The satellite inversion data of PM2.5 and PM10 concentration in winter in January were selected to obtain the distribution of dust particles in the mining area and Shuozhou City, as shown in Fig. 4, and the three-dimensional distribution characteristics of dust in Pingshuo mining area as shown in Fig. 5. In the figure, the horizontal distance represents the east-west direction, and the vertical distance represents the north-south direction. Shuocheng District is located in the south of Pingshuo Mining area.
It can be seen from Fig. 4(a) that there is a discontinuity between the dust concentration generated in the mining area and the concentration in the urban area, and the dust particles mainly diffuse to the southeast, but not to the urban area. Moreover, it can be seen from Fig. 4 (b) that PM10 is mainly concentrated in the northeast of the mining area in winter, but there is a discontinuity between the concentration and Pingshuo mining area. At the same time, it can be seen from Fig. 5 that dust in open-pit mining areas has obvious concentration characteristics in winter, but the diffusion range is limited to the vicinity of the open-pit mine, the dust concentration around the mining area is low, and the dust distribution in winter presents the characteristics of "strong concentration and weak dispersion". In addition, the open-pit mine may also have a dust suction effect in winter, resulting in a decrease in the dust concentration around the mine and an increase in the concentration above the mine.
In order to describe the spatial distribution characteristics of dust in Pingshuo mining area more clearly and accurately, the inversion data of PM2.5 concentration were selected, and the spatial distribution of PM2.5 concentration in Pingshuo mining area and urban area was obtained. The geographical ranges of Antaibao, Anjialing and Dong open pit mines were identified. According to the climatological definition (in the northern hemisphere, March to May is spring, June to August is summer, September to November is autumn, and December to February is winter), the PM2.5 concentration distribution maps of Shuozhou and Pingshuo mining areas were divided into four seasons, as shown in Fig. 6.
Compared with other seasons, PM2.5 is concentrated in the mining area in winter, with the highest concentration reaching 44µg/m3. This is because the temperature of the atmosphere near the ground in the northern winter is lower than that of the upper atmosphere, forming a stable inversion structure, and the air can not convection up and down, and it is difficult to disperse after the dust accumulation, and it is easy to occur serious dust pollution events. In spring, dust concentration in the mining area is still concentrated, and the dust concentration near the mining area is higher, but the concentration degree is lower than that in winter, and the maximum concentration in the mining area is 39µg/m3. In summer and autumn, the concentration of PM2.5 in the mining area decreased significantly compared with that in winter and spring. In addition, the geographical range of the mining area and part of the urban area is identified. Dust generated in the mining area spreads along the downwind direction, and the PM2.5 diffusion range is about 3Km in winter. Dust generated in the mining area cannot spread to the urban area, and there is discontinuity between the mining area and the urban area. In spring, PM2.5 only shows high and low distribution near the mining area, which is greatly affected by customs, and the PM2.5 concentration near the urban area is low, and the dust in the mining area does not directly affect the urban area. With higher temperature and increased precipitation in summer and autumn, PM2.5 concentration decreases overall, and there is no significant difference between the PM2.5 concentration in mining area and urban area.
Similarly, the distribution diagram of PM10 concentration is shown in Fig. 7, and its distribution rule is roughly consistent with that of PM2.5 concentration. However, the concentration of PM10 in spring is higher than that in winter, with a maximum value of 174.4µg/m3. This is due to the high concentration of PM10 particles affected by strong sandstorms in spring due to windy and sandy weather. When the concentration decreased in summer and autumn, the diffusion range of PM10 was about 3.5Km, and the spatial diffusion pattern showed a seasonal distribution. The research results can reflect that the dust produced in Pingshuo mining area has little or no influence on the urban area, and the contribution of dust in mining area to urban environmental pollution is small.
In summary, according to the geographical location identification and spatial distribution analysis results of Pingshuo mining area and urban area, the following conclusions can be drawn: The spatial distribution of PM2.5 in the whole Shuozhou City and Pingshuo Mining area is low in summer and autumn, and high in winter and spring. In the spatial distribution map, there is an obvious spatial discontinuity at the junction between Pingshuo Mining area and urban area, which is low at the junction and high in urban area and mining area. Moreover, dust diffusion degree varies with time. It shows that there is no direct correlation between Pingshuo mining area and the local increase of dust concentration in the urban area. The local increase of dust in the mining area does not affect the urban area, and the diffusion of dust particles to the surrounding area is small.
3.2 Characteristics of dust migration in Pingshuo mining area
The inversion data of PM2.5 concentration for 9 consecutive months were selected, and the selected research area was divided into four areas outside the mining area, the mining area, the urban boundary area of the mining area and the urban area according to their relative geographical location. The dust diffusion and distribution rules of different months were sorted out according to the four seasons of winter, spring, summer and autumn, as shown in Fig. 8.
In January, dust accumulation occurred in the mining area, and the PM2.5 concentration decreased first and then increased in the boundary area, but the overall concentration decreased, and the PM2.5 concentration gradually increased in the urban area. The results show that the dust produced in the mining area cannot be diffused to the urban area, and the impact on the urban environment is small, which is consistent with the previous analysis. The PM2.5 concentration in February showed a decreasing trend, but the distribution pattern was basically the same as that in January. According to the obtained results, the corresponding research area can be divided into four stages: Off-mine area, the Gathering area, the Descending area and the Elevation area. In spring (March to May), the concentration of PM2.5 in the mining area is still high in other areas, and the concentration degree and concentration value are gradually reduced, and the distribution law of each area is basically consistent with that of winter. The PM2.5 concentration in summer (June-August) gradually decreases from north to south, which can be divided into four stages: Off-mine impact area, the Stationary area, the Descending area and the stationary area. The PM2.5 concentration in autumn (September) is basically unchanged, roughly around 15ug/m3.
The distribution law of PM10 is basically the same as that of PM2.5 concentration, but in May, the concentration of PM10 suddenly produces agglomeration phenomenon, and the concentration value gradually increases. The reason is that the concentration of PM10 suddenly increases due to the dust generated in spring, resulting in agglomeration phenomenon, and the overall PM10 concentration in spring is greater than that in winter. The PM10 concentration in autumn (September) is basically unchanged at around 41ug/m3.
The results show that the dust diffusion phenomenon of Pingshuo mining area is seasonal distribution, and the accumulation phenomenon is easy to occur near the mining area in spring and winter temperature inversion, and the winter accumulation phenomenon is the most serious. In summer and autumn, the diffusion of dust generated in the mining area is mainly affected by wind speed and temperature factors, and the dust generated in the mining area is difficult to disperse to the urban area. In summer and autumn, the dust outside the mining area will be absorbed by the mining area after it spreads to the mining area, and the dust concentration will gradually decrease. The concentration of PM2.5 and PM10 in each season is basically lower than the national secondary air quality standard (75ug /m3 and 150ug /m3), and only in May, the PM10 concentration is higher than the secondary standard, and the overall pollution degree of the mining area is small. Based on the conclusion of the study, the mining area is not the main source of pollution in urban areas, and in the study of dust pollution control in open pit mines, the seasonal factors of dust diffusion and special factors such as strong dust weather should be fully considered, so as to propose corresponding control measures, which cannot be generalized.
3.3 Analysis of pollution situation in Pingshuo mining area
Statistics on the air quality of PM2.5 and PM10 in Pingshuo Mining area and the proportion of days with different air quality grades were conducted. As shown in Fig. 10, the air quality was the best from June to September, and the percentage of days in the "excellent" level were 100%, 96%, 82% and 100%, respectively. The PM2.5 level decreased from January to May. "Light pollution", "moderate pollution" and "heavy pollution" phenomena are produced, especially in January and March the pollution situation is the most serious, the air quality is poor, the pollution percentage is 19% and 21% respectively, of which March even appeared "serious pollution". The statistical results of PM10 air quality in the mining area are roughly the same as that of PM2.5, and the air quality is above good from June to September, but the frequency of severe pollution increases from January to May, especially in spring, when the frequency of severe pollution is higher. The reason for this phenomenon is that there is less precipitation in spring and the surface is extremely dry and loose. When the wind blows past, it will draw a large amount of dust into the air, causing atmospheric pollution.
To sum up, the months with the best air quality in Pingshuo Mining area are mainly from June to September, the particulate matter concentration in April and May has increased, and the pollution phenomenon is obvious in January and March, and the air quality is poor.
3.4 Correlation analysis of dust concentration between urban area and mining area
In order to verify whether dust generated by production and mining in mining areas has an impact on urban areas, PM2.5 and PM10 concentration distribution rules of three regions in different months were selected respectively, as shown in Fig. 11. From winter to autumn, the concentration of PM2.5 and PM10 showed a decreasing trend. From January to May, the concentration distribution of PM2.5 from the mining area to the urban area showed a trend of decreasing first and then increasing, indicating that the dust generated in the mining area could not spread to the urban area, and there was no obvious correlation with the urban area. From June to September, the concentration distribution of PM2.5 from the mining area to the urban area showed a gradually decreasing trend, but the overall concentration value did not change much, indicating that the concentration in summer and autumn was basically affected by the weather, and the contribution of dust production in the mining area was very low.
Compared with PM2.5, the variation of PM10 concentration in the three regions is not obvious with the month. The PM10 concentration in the three regions shows no correlation from January to March, and the PM10 concentration in the three regions shows a sudden increase in May, and the concentration value ordering in the three regions shows fluctuations, and there is a fluctuation zone. From June to September, the PM10 concentration values in different regions were basically the same and were divided into consistent areas. The results show that mining area is not the root cause of the increase of dust concentration in urban area. The dust concentration at the junction of mining area and Shuocheng District is basically the same in summer and autumn, but in winter and spring, the dust concentration at the junction of mining area and Shuocheng District is lower than that of Shuocheng District and mining area, and the change of dust concentration in mining area has no obvious influence on the urban area.