Changing tropospheric NO₂ dynamics across Indian air pollution hotspots

The National Clean Air Programme (NCAP) of India launched in 2019, aims to significantly reduce air pollution in its major hotspots, placing major attention on particulate matter. However, Nitrogen dioxide (NO₂) a criteria pollutant, also needs attention due to its role in secondary aerosol formation and associated adverse health impacts. Considering this, the current study examines the continuously changing tropospheric vertical column density (VCD) NO₂ trends (2005-2023) over 131 polluted cities (non-attainment cities). Based on geographic locations, these cities were categorized into different zones, and their co-located Ozone Monitoring Instrument (OMI) based tropospheric VCD NO₂ trends (yearly, seasonal) were studied. Tropospheric VCD NO₂ over all the zones shows a statistically significant increasing trend (linear regression) with an annual increase of 1 - 4 × 10¹³ molecules cm^-2y^-1. Seasonal-trend decomposition using LOESS (STL) trend component analyzed with linear regression and Mann-Kendall test also showed a similar statistically significant increasing trend in almost all zones across India. However, from 2017 to 2023, tropospheric NO₂ across all zones of India except the Northeast (NEAST) showed a declining trend. The highest decline was over Indo-Gangetic Plain (IGP) of around 7 × 10¹³ molecules cm^-2y^-1 but found statistically significant only with the Mann-Kendall test. Moreover, there has been an average reduction of 1.9 % per year in tropospheric VCD NO₂ over non-attainments cities since 2017 which can be linked to the implementation of upgraded vehicular exhaust standards (Bharat Stage VI), electric vehicle popularisation, NCAP, and strict regulatory implementations. Interestingly, the average VCD NO₂ in 2023 has reached similar levels to the COVID-19 lockdown year. While understanding the factors influencing the trend of tropospheric VCD NO₂, the feature importance of random forest methods shows that factors vary in zones and trends are influenced by month, meteorological factors, and forest fires in different zones across India.