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 (PM_2.5) 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 PM_2.5 (− 4.64 ± 2.68 µg/m³/year) and PM₁₀ (− 8.64 ± 2.68 µg/m³/year) are significantly higher than that in other seasons. PM_2.5 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 PM_2.5 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 PM_2.5 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.