Spatio-temporal patterns of tropospheric NO2 over India during 2005-2019

Air pollution is the leading environmental health risk factor globally. Nitrogen dioxide (NO2) is a criteria pollutant associated with adverse health effects. In India, the focus has been mostly on ambient PM2.5; hence understanding the space-time variability of NO2 and the potential role of different NO2 sources are required. This work aimed to address these two gaps by first examining a long-term spatial heterogeneity of NO2 and linking it to the major contributing sources (e.g., power plants and industries) over the Indian region, and secondly by examining the variability in land use patterns and its changes over the years 2005-2019. We analyzed tropo-spheric NO2 vertical columnar densities (VCDs) from the Ozone Monitoring Instrument (OMI) onboard the Aura satellite at a resolution of 0.1 degrees x 0.1 degrees for the period 2005 to 2019. We calculated linear trends in tropospheric NO2 VCDs and performed a significance test to ensure real changes. We also analyzed Global Human Settlement Layer (GHSL) data to identify the land use as 'high-density urban,' 'low-density urban,' 'rural,' and 'no settle-ment.' The spatial analysis found higher tropospheric NO2 VCDs (>5 x 10(15) molecules/cm(2)) in eastern (coal belt with many power plants) and northern India, along with a few locations in central India. The change in tropospheric NO2 VCDs during the last 15 years showed an increasing trend (>2 x 1013 molecules/cm(2)) in most parts of India. Whereas during the last decade (2010-2019), the annual average trend over the country had reduced by up to similar to 76% relative to 2005-2009. Tropospheric NO2 VCDs increased by 12.5%-29.6% from 2005 to 2019 across all levels of land settlement in India. Our study suggested considering NO2 as an important pollutant in addition to PM2.5 and developing a NO2 exposure model for further health impact assessment studies.