Air pollution trend in Chandigarh city situated in Indo-Gangetic Plains: Understanding seasonality and impact of mitigation strategies.

The long-term trend of air pollutants was studied in Chandigarh, located in Indo-Gangetic Plains of India. The SPM, PM10, NOx, and SO2 depict site-specific variation having different anthropogenic activities. The results indicate that PM10 levels in Chandigarh remain higher than the prescribed annual ambient air quality standards (60 μg m-3) of India. Seasonal Kendall test indicates a declining trend of PM10 for the year 2003 to 2018 at an industrial and commercial site (1996-2016). There is a significant increase in NOx levels at all locations except at commercial sites. The results of the linear regression model and Theil-Sen slope show that SPM is declining at all locations, but results are not statistically significant. Interestingly, PM10 levels at the industrial site display a significant declining trend (1.3% year-1). Similarly, NOx levels are increasing at all sites but having a statistically significant trend at a rural location (8.9% year-1) and residential site (15.2% year-1). Air pollutants show strong seasonal variability having a higher concentration in post-monsoon and winter season, which found to be linked with regional anthropogenic activities such as crop residue burning and use of solid biomass fuel for cooking and other purposes. Lowest PM10 levels were observed during the monsoon and having a decline of 47.4-66.4% as compared to winter levels. Site-specific variations in air pollutants were found to be associated with air quality improvement policies such as shifting of an interstate bus terminal, ban on diesel autos, including strict implementation of air quality norms on industries. As the relative contribution of various pollution sources is still unknown, the seasonal pattern of pollutants will help in knowing the background concentration of pollutants and could help to formulate evidence-based policies to mitigate air pollution under National Clean Air Programme (NCAP).