Trends of leading pollutant in a highly polluted global city: processes involved.

The impact of air pollution mitigation policies needs to be studied by evaluating long-term trends of lead pollutant to determine air quality index, the particulate matter (PM). A decade of SAFAR (System of Air quality and weather Forecasting And Research) observations revealed that the trend of particulate matter (PM) with size < 2.5 µm (PM2.5) and size < 10 µm (PM10), respectively, in a highly polluted global city, Delhi, shows a reduction of - 3.12 ± 0.52 µg/m3/year (- 4.68 ± 0.84 µg/m3/year) or overall, 28.8% (25.2%) reduction between 2011 and 2022 due to the implementation of eco-friendly technologies and strict industrial regulation despite doubling of number of vehicles. Seasonal negative trends during post-monsoon of PM2.5 (- 4.64 ± 2.68 µg/m3/year) and PM10 (- 8.64 ± 2.68 µg/m3/year) are significantly higher than that in other seasons. PM2.5 and PM10 show a relatively higher negative trend during winter (- 2.94 ± 1.08 µg/m3/year) and pre-monsoon (- 4.86 ± 2.07 µg/m3/year), respectively. The influence of dust storms, fire counts, and annual rainy days on PM trends is discussed. The contribution of meteorology to the trend is estimated using the WRF-Chem simulation of PM2.5 for October when maximum stubble burning occurs in Haryana and Punjab regions and gets transported to Delhi by upwind flow. The model is run for the post-monsoon month (October) with the meteorological initial conditions of 2018, 2015, and 2011 while keeping the emissions of 2018 with identical model configuration and found that meteorology contributes 9.8%, while the observed decline in PM2.5 is 28.8% during 2011-2022. The study identifies the governmental control measures at various levels and green initiatives as the significant contributors to air quality improvement during 2011-2022.