Study of temporal variability and mass closure of PM2.5 and its chemical constituents during weak south-west monsoon

Abstract

Ambient PM2.5 (n = 42) samples have been collected during a weak South-west (SW) monsoon (July‒September, 2015) from central part of the Indo-Gangetic Plain (IGP; at Kanpur). Organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), water-soluble inorganic species (WSIS) and metals have been measured in this study. Air-mass back trajectory (AMBT) analysis reveals atmospheric transport from three major pathways indicated as Type-1 (n = 24; traversing through western region), Type-2 (n = 8; traversing eastern region) and Type-3 (n = 10; terrestrial air-masses). Total carbonaceous aerosols (TCA), mineral dust (MD) and WSIS in Type-1 air-mass contributed 18.7 ± 8%, 40.3 ± 14.5% and 35.2 ± 13.4%, respectively to the PM2.5 mass. TCA, MD and WSIS mass fractions constituted 27.1 ± 13.2%, 29.1 ± 7.9% and 34.9 ± 10.7% in Type-2 air-mass and 25.2 ± 14.1%, 34.6 ± 14% and 33.3 ± 9.8% in Type-3 air-mass, respectively. Mineral dust percentage fraction and composition (based on Fe/Al ratio) suggests significant differences between Type-1 and Type-2 air-masses. Based on statistical two-tailed t-test, the mineral dust composition of Type-1 versus Type-3 and Type-2 versus Type-3 air-masses look near similar. However, average mass ratios of OC/EC (6.2–6.4), WSOC/OC (≈0.50; exception being difference in Type-2 versus Type-3) and K+BB/OC (0.05–0.07; K+BB: biomass burning derived potassium) look near similar in PM2.5 associated with all the three air-masses; indicating carbonaceous aerosols from well-mixed sources with dominant contribution from biomass burning emission. A significant abundance of organic aerosols and mineral dust in presence of high moisture content and their mixing over IGP has implications to perturbation in regional scale radiative forcing.