Day-night variability of EC, OC, WSOC and inorganic ions in urban environment of Indo-Gangetic Plain: implications to secondary aerosol formation

Abstract

This manuscript reports on the day- and night-time variability of EC, OC, WSOC and inorganic ions in ambient aerosols (PM_2.5 and PM₁₀) sampled from an urban site (Kanpur) in the Indo-Gangetic Plain. The chemical data also provide evidence for the secondary aerosol formation and variability in the composition of particulate matter. The aerosol mass is dominated by fine-mode particles and PM_2.5/PM₁₀ mass ratio exhibit significant temporal variability (range: 0.46 to 0.86). The chemical composition suggests that total carbonaceous aerosols (TCA = 1.6 × OC + EC) and water-soluble inorganic species (WSIS) account for nearly 50 and 20% of the PM_2.5 mass, respectively. The mass concentrations of PM_2.5, EC and OC show about 30% increase during night-time. A significant linear relation between EC-OC (R² = 0.66) and OC-K⁺ (R² = 0.59) and their characteristic ratios suggest biomass burning emission as a dominant source. The average WSOC/OC ratio is relatively high in the day-time samples (0.66 ± 0.11) compared to that in the night-time (0.47 ± 0.07); suggesting increased contribution of secondary organic aerosols. The mass fraction of particulate NO₃^- increases by a factor of five during night-time due to relatively stable NH₄NO₃ and/or its secondary formation from the hydrolysis of N₂O₅. Although the concentration of SO₄^2- is noticeably higher during day-time (~20%), the day-night variability of particulate-NH₄⁺ is insignificant. The concentrations of OC, EC and inorganic species (K⁺, NH₄⁺, NO₃^- and SO₄^2-) show 2 to 4 fold increase during the haze events.