Revival of Compound Heat–PM_2.5 Extremes in Pre-Monsoon India: Meteorological Shifts and Fueled Secondary Aerosol Production (2017–2024)

The dual crises of climate change and air pollution are creating a dangerous convergence. Climate change is already fueling more frequent, intense, and longer-lasting heat waves worldwide. When these extreme heat events combine with polluted air, they produce a synergistic effect, posing health threats far greater than either would alone. India has emerged as a critical hotspot for these compound risks, with Delhi alone experiencing 388 heat–PM_2.5 co-extreme (PM_2.5 > 75 μg m^–3 and daily maximum 2-m air temperature (T_max) > 35°C) days during 2017–2024. However, such co-extremes have received limited attention. By integrating surface and satellite observations with GEOS-Chem modeling, we find a marked revival of pre-monsoon heat–PM_2.5 co-extremes over India after 2020. The regional mean reaches 17.5 days in April 2022, accounting for over 70% of recent PM_2.5 extremes. Satellite observations reveal substantial increases in major PM_2.5 precursors during this time, and GEOS-Chem sensitivity experiments show that changing meteorological conditions act as primary amplifiers. The meteorological shift features a strengthened anticyclonic circulation over northern India, inducing extreme heat and southeasterly anomalies that facilitate greater pollutant transport. As a result, secondary organic aerosol formation increases markedly and organic carbon yields rise by ~2.3 times, thus dominating these co-extremes. These insights provide an important early warning about how circulation shifts could drive future increases in co-extremes in India.