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 cycle. 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 Tmax > 35 °C) days during 2017–2024. However, such compound 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 strengthened anticyclonic circulation over northern India, inducing extreme heat and southeasterly anomalies that facilitate greater transport of pollutants. As a result, secondary organic aerosol formation increases markedly and organic carbon yields rising 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 compound extremes in India.