A novel index for assessing the compound drought, heat waves, and air pollution extremes

During extreme droughts and heatwaves, high air pollution levels further intensify the hazardous outcome of compound events. This study proposes a new index to measure the compound PM2.5 drought heat wave (CPDH) hazard. To achieve this, MERRA-2 air pollution data and ERA5 precipitation and temperature data over South Asia were used for a 43-year period from 1980 to 2022. The study employed three CPDH indices to estimate characteristics such as duration, magnitude, and severity. Additionally, the trends of these indices were also studied to understand the spatiotemporal changes in CPDH. The results showed that the annual duration of CPDH ranges from 0 to 9 days, and the magnitude is 0.4 in the western regions, while severity ranges between 0 and 2 in the central and western regions. The trend analysis showed that PM2.5 increased annually by 0.21 to 0.48 μg/m³, heatwave events increased by 0.25 to 0.67 over most of the study area, and drought severity increased with a decrease in SPEI by −0.01 to −0.03 in the west of South Asia over the past 43 years. These changes caused the CPDH magnitude and severity in the western, eastern, and southern regions to more than triple in the last decade compared to the early period. The CPDH duration, magnitude, and severity trends revealed an increase of more than 0.6 days, 0.015, and 0.05 per year in the eastern, southwestern, and western parts of the sub-continent. The study indicates a gradual increase in CPDH events, which can adversely affect public health.