Spatial-temporal analysis of day-night time SUHI and its relationship between urban land use, NDVI, and air pollutants in Tehran metropolis

One of the basic factors that should be investigated and monitored in the field of urban heat islands is the exploration and detection of their spatiotemporal changes, which have been well addressed in spatial statistics. The current study aimed to detect the spatiotemporal changes of surface urban heat islands (SUHI) in Tehran metropolis during the daytime/nighttime at monthly and seasonal scales and over the warm and cold periods of the year. The consequences of many elements like as daytime/nighttime land surface temperature (LST) extracted by the MODIS/006/MOD11A1 and the NDVI extracted by MODIS/006/MOD13A2 over a 20-year period (2001–2020) were first investigated. Then, the SUHI index was computed for the study area. The correlations between the heat islands and urban land use (traffic, population density, airport, etc.), air pollutants (CO, NO2, SO2, etc.), and NDVI were investigated in the next stage. Finally, Moran’s algorithm was used to measure the spatial autocorrelation, and Gi statistic was used to analyze the cold and warm spots. The results indicated that the LST trend was constant during the daytime/nighttime, and the NDVI also had a slight rising trend. The results of the SUHI maps indicated that the zones with heat islands during the daytime over the seasons’ warm and cold times are located in the south, southeast, and west of the city. During the nighttime, the central zones of the city as well as some parts in the east and southeast have had higher heat islands. The results of the correlation between the heat islands and land use, vegetation, and air pollutants indicated a direct correlation between the heat islands and the airport and industrial land use over time, while it was inversely correlated with other land uses. During the nighttime, all land uses had a direct correlation with the heat islands. Regarding the air pollutants, PM2.5 and PM10 were most correlated with the heat islands during both daytime/nighttime while other pollutants have been inversely correlated. The heat islands and the NDVI were also inversely correlated during both daytime/nighttime. The OLS (ordinary least-squares) model results also indicated that the R² values during the daytime/nighttime were 0.70 and 0.59, respectively, over the cold period of the year, compared to values of 0.69 and 0.68 over the warm period of the year. The results of global Moran’s I and G*i statistics also indicated that the heat islands of the Tehran metropolis had a spatial structure distributed in a cluster in which the southern, western, southwestern, and northern parts had warm spots during the daytime and cold spots during the nighttime. Moreover, the northern and northeastern parts had cold spots during the daytime, and the central and eastern parts had warm spots during the nighttime.