عنوان مقاله [English]
Air pollution has become a major environmental problem in most parts of the world, especially in the metropolises of developing countries, along with population growth, agricultural development, urban development, industrial development and increasing motor transportation (Joulaei et al., 2017).
Among air pollutants, PM2.5 is one of the most dangerous pollutants. Numerous studies have shown that PM2.5 can damage human lung tissue and increase the risk of chronic respiratory, cardiovascular and cancer diseases (Wang and etal, 2019; Vinikoor-Imler and etal, 2011).
Statistical and spatial methods have been widely used to identify Spatial-temporal patterns of various air pollutants (Alijani, 2015)
The purpose of this study was to use temporal and spatial analysis methods and Geographic Information System (GIS) affecting the emission of PM2.5 in Mashhad in a period of 5 years.
Materials and Methods
Mashhad City in Northeastern of Iran, the second largest metropolis in Iran, has 23 air quality monitoring stations. In this study, was used of Geographic Information System (GIS), geo-statistical functions, spatial analysis, geographical processes and sub-tools of each to identified the spatial and temporal patterns of the main air pollutant in Mashhad.
Daily data of particulate matter smaller than 2.5 microns (PM2.5) were collected from air quality stations in the 5-year period (2014-2019). Initially, the data was verified. Then the PM2.5 concentrations were calculated for each station daily and monthly scale. Maps were used by Geographic Information System (GIS). In the next step, local spatial correlation analysis or Local Moran Index (LMI) and hot spots analysis was performed by Getis –Ord-Gi statistic for this pollutant.
Daily and monthly average concentration of pollutants in the late spring and early summer, showed sometimes winds with dust, so that the PM2.5 concentration is increased. Seasonal maps were prepared using the Algebra Map function.
In the spring and summer, the Western and North-Western regions of Mashhad have the lowest density of particulars maters (19.3 μg / m3). The comparison of the average concentration of PM2.5 in different seasons of the year shows that autumn has the highest concentration.
In the winter, the intensity of PM2.5 concentration has decreased. However, in the winter, with an average concentration of 30 μg / m3 and it is in the second rank of air pollution after autumn and the lowest concentration is located in the North-West and West of Mashhad as in previous seasons.
According to the local spatial autocorrelation analysis, the concentration of particulate matter in the Eastern areas of the city is significantly higher than its neighboring areas, which is marked on the map with high cluster (HH) and low clusters (LL) are spread in the North-West.
The annual map of PM2.5 concentration was drawn by combination of seasonal maps in a 5-year period.
This map provides a more complete understanding of how PM2.5 is distributed in the air of Mashhad. According to this map, the eastern and southeastern regions of Mashhad with an average concentration of 36.8 μg / m3 have the highest concentration than the Western and North-Western.
The study of effective factors in the emission of PM pollutants in the Mashhad city showed that the total length of the urban transportation network in hot spots is 283.287 meters and the average speed of vehicles in this section is 36 kilometers per hour. While, the total length of the network in the cold spot is 424342 meters and the average speed of cars is 46 kilometers per hour.
Emissions in cold spots are also lower than in high-traffic areas due to longer communication lines, faster vehicle speeds and light traffic.
Geographical point of view show that, Mashhad plain is located between two the Binalood mountains in the south and the Hezar Masjed mountains in the north, which causes the formation of a special pattern and canalization of wind from the South-East to the North-West. High density of suburban population, more agricultural lands, rural roads, sand mines, brick kilns and cement factories in the East and South-East of the city due to high concentration of PM2.5 and the formation of hot spots in this area. However, the location of the cold spot in the North-Western regions of Mashhad was due to lower population density, much more green space per capita, higher altitude, greater distance from industries, adjective to Torqabeh and Shandiz areas.
Spatial autocorrelation analysis showed that the most Particulate Matter (PM2.5) or hot spots in East and Southeast Mashhad, and 22.3 percent of the total area of the city. On a seasonal scale, hotspots are the largest in spring with 25% of the total area of the city, then winter with 24%, summer with 21.8% and finally autumn with 17.2%. Against, areas with low concentrations of PM2.5 (cold spots) have been formed in the North-West of Mashhad. On an annual scale, 25.5% of the city is in the cold spots and 22.3% is in the hot spots. The population is 21% and the area of industrial - service uses is 30% more in the hot spot than the cold spot, which directly increases the emission of air pollutants in hot spots. While, in the cold spots the parks area is 70%, residential land use is 67%, the total area of green spaces is 62% is more than hot spots. On the other hand, other factors such highways and main roads, the length of the transportation network, increase the speed of vehicles in these areas.