<span style="font-size:14px"><span style="font-family:Arial,Helvetica,sans-serif">The study reports day and nighttime temporal variation of prominent water-soluble inorganic species (WSIS) of fine and coarse mode particulate matter at a site in central Delhi for 2012 and 2013. The two years’ concentrations of major ionic species (NH₄⁺, K⁺, Ca²⁺ and Mg²⁺ and NO₃^-, SO₄^2-) showed that mean levels of both ?cations and ?anions were higher in nighttime than daytime in both PM_2.5 and PM₁₀. Moreover, the difference of mean levels in daytime and nighttime was noticed higher in PM_2.5 compared to PM₁₀. Among anions, more variation was observed for NO₃^-, i.e., ~49% higher level in nighttime compared to daytime, while~9% higher values in the nighttime for SO₄^2-. Higher variation of NO₃^- level might be due to active participation of NOx in daytime photo-oxidation process. Among cations,Ca²⁺ and Mg²⁺ showed higher concentration in daytime compared to nighttime. Higher day and nighttime variation of Ca²⁺ and Mg²⁺ in coarse particulate matter suggested their dominance in PM₁₀ compared to PM_2.5. In PM₁₀, the highest Ca²⁺ concentration was observed in daytime summer and least in nighttime monsoon, indicating the role of loose soil and meteorological conditions. The higher daytime SO₄^2- concentration in monsoon compared to summer show edits significant formation as secondary aerosols in fine mode. The reactive nitrogen species NO₃^- and NH₄⁺were correlated during the day and nighttime in PM_2.5 and PM₁₀.Moreover, NH₄⁺ was highly correlated to SO₄^2- and NO₃^-, mainly in the form of (NH₄)₂SO₄ and NH₄NO₃.</span></span>