Changing tropospheric NO₂ dynamics across Indian air pollution hotspots

Area of Science:

• Atmospheric Chemistry and Air Quality Monitoring
• Environmental Science and Policy

Background:

• The National Clean Air Programme (NCAP) targets particulate matter, yet Nitrogen dioxide (NO2) also poses significant health risks and contributes to secondary aerosol formation.
• Understanding NO2 trends is crucial for effective air pollution management in India's non-attainment cities.

Purpose of the Study:

• To analyze the spatiotemporal trends of tropospheric NO2 vertical column density (VCD) from 2005-2023 across 131 polluted Indian cities.
• To identify factors influencing NO2 trends and evaluate the impact of recent air quality policies.

Main Methods:

• Utilized Ozone Monitoring Instrument (OMI) data for tropospheric VCD NO2 analysis.
• Employed statistical methods including linear regression, Seasonal-Trend decomposition using LOESS (STL), and the Mann-Kendall test.
• Applied random forest methods to determine feature importance influencing NO2 trends.

Main Results:

• A statistically significant increasing trend in tropospheric VCD NO2 was observed across most zones from 2005-2023.
• A notable declining trend in NO2 was observed from 2017-2023, particularly in the Indo-Gangetic Plain (IGP), linked to policy interventions.
• NO2 levels in 2023 approached those seen during the COVID-19 lockdown, indicating policy effectiveness.

Conclusions:

• While overall NO2 levels increased until 2017, subsequent declines suggest successful implementation of vehicular emission standards, EV promotion, and NCAP.
• Factors like month, meteorology, and forest fires significantly influence NO2 trends, varying by geographic zone.
• Continued monitoring and policy refinement are essential for sustained air quality improvement in India.