Satellite-Based Clustering of Pre-Monsoon Wildfires and Variability of Tropospheric NO₂ and CO in Nepal

Dipson Bhandari 

Nepal's pre-monsoon season transforms the country's southern lowlands into one of South Asia's most active wildfire landscapes. However, spatially explicit assessments of wildfire dynamics and trace gas variability at high temporal resolution remain limited. This study uses a satellite-based methodology to quantify wildfire activity and its atmospheric impacts across Nepal during 2021–2024. Visible Infrared Imaging Radiometer Suite (VIIRS) active fire observations were processed into a daily gridded dataset to derive fire count and fire radiative power (FRP) metrics. Wildfire hotspots and their spatial dynamics were identified using percentile-based thresholding combined with connected-component labeling to delineate contiguous clusters of extreme fire activity. Tropospheric nitrogen dioxide (NO₂) and total carbon monoxide (CO) were analyzed using Sentinel-5P TROPOMI products over these clusters across distinct physiographic zones from lowlands to Himalaya. Results reveal pronounced interannual variability in wildfire extent, with the largest hotspot cluster observed in 2022 (~6,718 km²) across western and mid-western Nepal. Fire activity exhibits a strong seasonal peak in April, accounting for 62% of annual detections. Wildfire episodes drive substantial trace gas enhancements, with NO₂ showing extreme intra-monthly variability including episodic increases exceeding 1,200% relative to baseline conditions. Lagged correlation analysis indicates peak associations between fire activity and trace gas concentrations at 1 day for NO₂ and 2 days for CO. Persistent hotspot regions are identified in the Terai and Siwalik zones, particularly in Bardiya, Banke, Dang, Surkhet, Parsa, and Bara districts. Despite limited local fire activity, enhanced seasonal and episodic trace gas patterns were also observed in the Himalayan region.