Three-Year-Long PM2.5/PM10 Ratio at Nine Sites in the Most Polluted Region in India

PM_2.5/PM₁₀ ratio is essential for understanding particulate pollution’s severity and adverse effects on human beings as it reveals how long the particle will stay in the atmosphere and where it will deposit in the human respiratory tract. The present study focuses on the spatio-temporal variability of the PM_2.5/PM₁₀ ratio from nine sites (six in Delhi, one each in Amritsar, Varanasi and Kolkata) in the Indo-Gangetic Plain (IGP) during the last 3 years (2019–2021) before, during, and after the COVID-19 pandemic-induced lockdown in India. Robust statistics such as median and percentiles have been employed to avoid bias due to non-normal distributions. Considerable spatial and temporal variability was seen throughout the 3 years. Amritsar and one site in Delhi exhibited the least temporal variability in PM_2.5/PM₁₀ (~ 10%) annually. However, the highest average variation over the 3 years was ~ 28%, noticed for one site in Delhi. The PM_2.5/PM₁₀ ratio was high (~ 0.6 ± 0.1) during the post-monsoon (Oct–Dec) and winter (Jan–Feb) seasons. The PM_2.5/PM₁₀ ratio was low (~ 0.4 ± 0.1) in the monsoon season (June–Sep.) and pre-monsoon season (Mar–May). Conditional Bivariate Probability Function (CBPF) and Cluster Analysis using Hysplit data were done to assess the local and long-range source of pollutants arriving at a receptor location. The impact of wind speed and relative humidity on the PM_2.5/PM₁₀ ratio was also analysed. The results of this study would help establish an intricate policy framework for cities in the IGP.