Observed improvement in air quality in Delhi during 2011–2021: Impact of mitigation measures

Abstract. Assessing long-term air quality trends helps evaluate the effectiveness of adopted air pollution control policies. A decade of SAFAR observations revealed that the trend of particulate matter (PM_2.5 and PM₁₀) in Delhi shows a reduction of 2.98 ± 0.53 µg/m³/y (4.91 ± 1.01 µg/m³/y) or overall 29 % (23.7 %) reduction between 2011 and 2021 while vehicles almost doubled but with the implementation of cleaner technologies and stricter industrial regulation. Seasonal negative trends of pre-monsoon (March-April-May; -3.43 ± 1.02 µg/m³/y) and post-monsoon (October–November; -4.51 ± 1.59 µg/m³/y) are relatively higher. The role of trends in dust storms, fire counts and annual rainy days are also discussed. The contribution of meteorology to the trend is estimated using WRF-Chem simulation of PM_2.5 for October when maximum stubble burning occurs and gets transported to Delhi. The model is run with the meteorological initial conditions of 2018, 2015, and 2011 while keeping the emissions of 2018 with identical model configuration and found that meteorology contributed 9.8 % in October, while the observed decline in PM_2.5 is 35 % (best fit) and 25 % (value). The study identifies the governmental control measures at various levels and green initiatives as the significant contributors to air quality improvement during 2011–2021.