Impacts of subway development on air pollution and vegetation in Tabriz and Shiraz, Iran

Introduction: Presently, air pollution is viewed as a critical environmental challenge that has deleterious effects on human health and ecosystems. The subway system is extensively developed in numerous countries with the objective of minimizing traffic congestion and pollutant emissions. The aim of the present study is to explore the impact of metro activities on air pollution and, subsequently, urban vegetation inside the two metropolises of Tabriz and Shiraz in comparison to prior years. Materials and methods: To assess air quality before and after the establishment of the metro, we collected average data for Particulate Matters less than 2.5 µm (PM2.5), Particulate Matters less than 10 µm (PM10), SO2, NO2, O3, and CO, as well as the Air Quality Index (AQI), retrieved from monitoring stations in Tabriz and Shiraz between 2014 and 2019. We used the average of the Normalized Difference Vegetation Index (NDVI) calculated by the Landsat 8 satellite in the second phase in order to numerically determine the status of urban vegetation across two timeframes. Results: Preliminary evidence revealed that the average concentration of pollutants in Tabriz, excluding NO2, fell after the launch of the metro system in 2016. Simultaneously, several pollutants, including O3, NO2, and PM2.5, and consequently the AQI, increased following the subway's establishment in Shiraz in 2017. Moreover, it was observed that decreasing emissions in Tabriz amplified vegetation, whereas reducing air quality in Shiraz lowered the NDVI values. Conclusion: Although it appears that the metro's operation improved environmental conditions in Tabriz, a similar outcome was not evident in Shiraz. Therefore, it is suggested that future studies consider meteorological variables whenever addressing the metro's efficiency.