Trends and Variability of PM_(2.5) 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 PM_(2.5) 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 PM_(2.5) concentration showed an overall small but statistically significant decreasing trend with an average decrease of -1.35 (95% CI: -2.3, -0.47) μg m^(-3) year^(-1). Seasonal trends revealed a significant decreasing value of -3.05 μg m^(-3) year^(-1) (p ＜ 0.1) for summer, an insignificant declining trend of -1.95 μg 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 PM_(2.5) 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 PM_(2.5) and meteorological variables was studied using monthly mean data. Rainfall, air temperature, PBLH, and wind speed showed a negative correlation with PM_(2.5), whereas surface pressure had a positive correlation and relative humidity displayed an inverted U-shape relationship. The average concentration of PM_(2.5) in the study period of 15 years remained 125 ± 86 μg m^(-3) (ranging between 20 to 985 μg m^(-3)) and during winter, summer, monsoon, and post-monsoon seasons it was 174 ± 75, 101 ± 48, 66 ± 50, and 192 ± 93 μg m^(-3) respectively. Minimum of the monthly averaged PM_(2.5) 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 PM_(2.5) levels in Delhi during the crop burning period. Government agencies need more strict action plans, especially during winter, to comply with air quality standards.