Spatio-Temporal Relationship Between MODIS Fire Activity, Aerosol Loading and PM2.5 During the Paddy Residue Burning Season in North-Western India, 2021

Abstract

The burning of paddy residue is one of the largest seasonal sources of aerosol loading and particulate pollution in north-western India, concentrated in the weeks after the kharif rice harvest. This study focuses on the spatio-temporal link between active fire occurrence, Aerosol Optical Depth (AOD) and ground-level PM_2.5 across Punjab, Haryana, and the adjoining Rajasthan districts of Hanumangarh and Sri Ganganagar during 2021. Fire activity was tracked using Terra-MODIS cumulative fire frequency (MOD14A1), with Aqua-MODIS included for sensor comparison, columnar aerosol loading came from MODIS MAIAC AOD (MCD19A2), and ground-level pollution came from CPCB station PM_2.5 records. Data were analysed at monthly, seasonal, district and weekly resolution, with the September–December window examined in most detail. The year had two clear periods of burning. The first was a smaller wheat-residue burning phase during April–May, followed by a much stronger paddy-straw burning phase in October–November. During the later period, Terra cumulative fire frequency, AOD and PM_2.5 all reached their highest levels. At the weekly scale, fire activity reached its peak in the first week of November, with a cumulative fire frequency of 1,773 and a mean AOD of 1.071. PM_2.5 reached its highest value one week later, at 173.5 µg/m³ in the second week of November. This one-week difference suggests that aerosols remained in the atmosphere after fire activity had started to decline, with slower dispersion near the ground. The areas with high AOD were also not always located directly over the main fire hotspots, suggesting that smoke was transported away from the fields where burning occurred rather than remaining only around the source areas.Pearson correlation supported these patterns, cumulative fire frequency and AOD were strongly associated (r = 0.724, p = 0.002, R² = 0.523), as were cumulative fire frequency and PM_2.5, though more moderately (r = 0.560, p = 0.024, R² = 0.314). AOD and PM_2.5 tracked each other most closely (r = 0.900, p < 0.001, R² = 0.811). Together, the three datasets, satellite fire detection, satellite aerosol retrieval, and ground-based particulate monitoring, give a consistent, weekly-resolved picture of how the 2021 paddy-burning season built up, peaked, and dispersed across the region.