The Impact of Atmospheric Aerosols on Heat Stress over Indian Subcontinent

The recent trend in global mean temperature is expected to increase the frequency, duration, and intensity of heat waves and heat stress all around the world with a severe impact over the Indian subcontinent. Despite the significant scientific interest in heat stress and aerosols, much less attention has been paid to the effects of aerosols on heat stress considering heat-humid indices. Using the RegCM4 regional climate model coupled with aerosol chemistry, we investigate the aerosol changes in two heat-humid indices, wet-bulb temperature (WBT) and heat index (HI) over the Indian subcontinent from 2015 to 2019. The aerosols induce a reduction in maximum surface temperature, with the highest impact over western India and the IGP. On the contrary to the decrease in temperature, a net increase in WBT is seen due to the increase in near-surface RH. It is very significant as even a moderate increase in hot and humid weather might lead to a large hike in heat stress-related human illness and mortality. The individual effect of an aerosol-induced increase (decrease) in RH (T_max) was found to have positive (negative) effects on WBT and HI. The study also highlights the importance of selecting the appropriate indices when studying heat stress characteristics, as the sensitivity of individual heat stress metrics to RH differs from each other. The present study results that aerosol-induced changes in RH play a decisive role in the assessment of heat stress, which answers the “missing link” between the prevalence of high heat stress despite high aerosol loading (surface cooling) over the Indian region.