Review of Secondary Aerosol Formation and Its Contribution in Air Pollution Load of Delhi NCR

The elevated levels of fine particulate matter (PM) and its associated health concerns are one of the major cause of apprehension for society as well as for policymakers in the region of Delhi-NCR. Atmospheric secondary aerosols are reported to hold one of the major fractions of PM. Considering its adverse effects and complex process of formation, the present article focuses on a critical review of its categorization based on formation mechanism, contribution to overall ambient aerosol load in the Delhi region, and other responsible factors. The formation of secondary aerosol is primarily governed by the abundance of precursor pollutants with variations in relative humidity, temperature, and solar radiation, which is evident in the seasons of post-monsoon and winter when there are higher emissions of NOx, SOx, NH 3 , VOCs, HCl, etc., which along with conducive meteorology, favor the increased rate of particle formation. The review reveals that secondary inorganic aerosols comprising of sulfate, nitrate, and ammonium contribute 24%, 15%, 21%, and 23% of PM 10 during winter, summer, monsoon, and post-monsoon seasons, respectively, whereas the same is found to be higher in PM 2.5 , with 29%, 22%, 27%, and 28%. Thus, highlighting the need for comprehensive knowledge of secondary aerosol formation and its contributions in metropolitan areas such as Delhi, which will assist policymakers in determining the policies needed to curtail city-specific precursor factors, thereby reducing the ambient secondary aerosol concentration.