Temporal and Spatial Variations of Nitrogen Dioxide Concentrations in Kolkata, India

Abstract. Air pollution is a primary environmental concern in urban areas. This study examines the temporal and spatial variations in nitrogen dioxide (NO₂) concentrations in Kolkata from 2019 to 2023 using Sentinel-5P satellite data. The application of statistical techniques, including Global Moran's I and Fast Fourier Transform (FFT), highlights changes in NO₂ spatial distribution and identifies dominant periodicities. Yearly analysis reveals notable fluctuations in NO₂ levels, with a significant decline of approximately 9.1% between 2019 and 2020, attributed to reduced vehicular and industrial activities during the COVID-19 lockdown. However, by 2023, NO₂ concentrations had returned to pre-pandemic levels attributed to the resumption of economic activities. Spatial analysis reveals higher NO₂ concentrations in central built-up areas, including Ballygunge, Bhowanipore, and Park Street. At the same time, peripheral regions such as Metiabruz and Behala show lower levels, likely due to vegetated areas. Global Correspondence values indicate significant shifts in NO₂ distribution patterns over the study period. The pattern shifted during the COVID-19 pandemic but stabilized by 2023, aligning with pre-pandemic emission levels, but the monthly pattern was preserved. The FFT analysis reveals a dominant annual cycle with a frequency of 0.0833 cycles per month (12-month period) and an amplitude of 0.3520, along with a significant overall average component. Seasonal variations show higher concentrations in winter due to increased emissions and reduced levels in summer due to photolysis and monsoon rains. These findings underscore the importance of effective pollution management and continuous air quality monitoring to improve air quality in Kolkata.