Revisiting the Urban heat and pollution Islands over the Kolkata metropolitan area, India

Abstract

Urban environments alter local climate and air quality through concentrated emissions of heat and pollutants, giving rise to effects such as Urban Heat Island (UHI) and Urban Pollution Island (UPI). These microclimatic phenomena have seen growth due to industrialisation and urbanisation. Their coupled interaction in the context of Indian cities, however, remains poorly understood. In this study, we have investigated the evolution of UHI and UPI over a megacity of India, Kolkata, with a focus on Kolkata Metropolitan Area (KMA) by analysing Remote Sensing (MODIS) and in situ measurements (CPCB) of Land Surface Temperature (LST), UHI, Aerosol Optical Depth (AOD), Direct and Diffuse Solar Radiation, and Particulate Matter (PM_2.5 and PM₁₀) for a collective period of 2001–2024. UHI and UPI were observed over KMA, with both Nighttime LST (NLST) and Daytime LST (DLST) as well as AOD having significantly higher values over the urban region by +1.79 °C, +2.92 °C, and 0.18, respectively, than in rural areas. The results also suggested that the magnitudes of UHI and UPI are increasing over the years for KMA. As a result of increasing UPI, the study observed a simultaneous reduction of direct solar radiation by 22.64 W/m² and an increase of diffuse radiation by 18.82 W/m² over urban areas for the study period. The study suggests that the UPI and UHI may influence each other through direct and diffuse radiative forcing, and that the modulation of urban surface energy balance may occur due to these changes, which may be a link to unprecedented weather events over the region. The results provide a scientific basis for developing policies on regional climate adaptation and air quality management in Indian cities.