Spatio-temporal characterization of tropospheric ozone and its precursor pollutants NO₂ and HCHO over South Asia

In recent decades, South Asia has experienced declining air quality, with much of the attention being focused on extremely high levels of particulate matter. Here, we analyze tropospheric ozone (O₃), formaldehyde (HCHO), and nitrogen dioxide (NO₂) to assess other measures of air quality across South Asia from 2008 to 2018. The IASI-Forli retrieved tropospheric ozone data was validated with ozonesonde, reanalysis (ERA5), satellite (TES), and model simulation products (GEOS-Chem and TOMCAT/SLIMCAT). Space-based observations of these three trace gases were used to conduct a spatio temporal analysis over South Asia using trend analysis (Theil-Sen and linear regression), change-point detection (Pettitt's test), and hotspot identification (Getis-Ord G_i*). We used the formaldehyde-nitrogen dioxide ratio (FNR) to identify NO_x limited, VOC limited, and transitional regimes in South Asia. Counter to previous studies, a statistically significant decrease of HCHO (−0.0041 DU yr⁻¹) and O₃ (−0.064 DU yr⁻¹) was detected for South Asia; however, NO₂ is increasing the 0.001 DU yr⁻¹ over South Asia during 2008–18. The Indo-Gangetic Plains emerged as being critically affected by the three trace gases. Certain parts of southern and south-eastern India are gradually emerging as NO₂ and HCHO hotpots. No significant O₃ hotspots were discernible, though coldspots existed along the Himalaya belt of India, Nepal, and Bhutan and mountainous tracts of Pakistan. FNR indicates the reduction of NO_x in NO_x-limited regime of the Indo-Gangetic Plains reduced the formation of tropospheric O₃ over South Asia.