Trends and Variability of PM2.5 at Different Time Scales over Delhi: Long-term Analysis 2007-2021

The present study investigated the long-term inter-annual, seasonal, and monthly trend analysis and variability of PM2.5 on different times scales over the national capital, Delhi, India, using high -resolution surface observations from six stations during 2007-2021. The non-parametric Mann -Kendall and Theil-Sen slope estimator were used to study the temporal variations. The long-term PM2.5concentration showed an overall small but statistically significant decreasing trend with an average decrease of -1.35 (95% CI: -2.3, -0.47) mu g m-3 year-1. Seasonal trends revealed a significant decreasing value of -3.05 mu g m-3 year-1(p < 0.1) for summer, an insignificant declining trend of -1.95 mu g m-3 year-1 for monsoon. Similarly no significant trend detected for the post the post monsoon and winter season. Except for December and January, all months displayed a decreasing trend for PM2.5 concentration. These findings indicate that particle pollution over the city is declining at a very slow rate. A rising trend was found for relative humidity and surface pressure, whereas a declining trend for wind speed and PBLH was observed. No trend was observed for temperature and rainfall. The Pearson linear correlation between PM2.5 and meteorological variables was studied using monthly mean data. Rainfall, air temperature, PBLH, and wind speed showed a negative correlation with PM2.5, whereas surface pressure had a positive correlation and relative humidity displayed an inverted U-shape relationship. The average concentration of PM2.5 in the study period of 15 years remained 125 +/- 86 mu g m-3 (ranging between 20 to 985 mu g m-3) and during winter, summer, monsoon, and post-monsoon seasons it was 174 +/- 75, 101 +/- 48, 66 +/- 50, and 192 +/- 93 mu g m-3 respectively. Minimum of the monthly averaged PM2.5 concentration was observed in August, while maximum is November. Satellite data of fire events showed that the crop residue burning over the Punjab region had a significant contribution to the peak PM2.5levels in Delhi during the crop burning period. Government agencies need more strict action plans, especially during winter, to comply with air quality standards.