Long-term trend analysis of criteria pollutants in megacity of Delhi: Failure or success of control policies

The study aimed to analyze the long-term trend of four criteria pollutants (PM 2.5 and NO 2 during 2006–2019; SO 2 and CO during 2000–2019) and the effect of pollution control measures in Delhi. The hourly data of PM 2.5 and NO 2 was taken from six air quality monitoring stations while SO 2 and CO data was retrieved from MERRA reanalysis (Modern-Era Retrospective Analysis for Research and Applications) products. PM 2.5 and NO 2 datasets from six monitoring stations were divided into three categories based on their years of data collection. At all the categories of sites, both the pollutants showed three distinct trends including a decreasing trend which was common from 2015 to 2019. The annual average trends of SO 2 and CO showed two distinct trends including an increasing trend which was common from 2013 to 2019. The study analyzed various control policies, among them Compressed Natural Gases (CNG) implementation in December 2002, is a historic measure which was followed by 13.5% decrease in particulate matter trend and 20% increase in NO 2 trend during 2002–2005 while 7% decrease in CO trend during 2003–2005. The introduction of ultralow sulfur diesel and closure of coal based thermal power plants reduced SO 2 levels significantly during 2011–2012.