Diurnal and seasonal characteristics of aerosol ionic constituents over an urban location in Western India: secondary aerosol formation and meteorological influence

Abstract

Water-soluble ionic constituents (Cl^-, NO₃^-, SO₄^2-, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) of PM_2.5 were measured using Ambient Ion Monitor coupled-to Ion Chromatography (AIM-IC) with a time resolution of one hour at Ahmedabad (an urban location in a semi-arid region of western India) during summer and winter to study the diurnal and seasonal variations. Maximum abundances of most of the species were observed during afternoon hours in summer, whereas minimum concentrations were found during winter at the same local time. During summer, the concentration levels of most of the constituents in a diurnal cycle are largely influenced by variations in the ambient relative humidity. During winter, the emission strength and boundary layer dynamics impact the diurnal cycle. Secondary inorganic species like NH₄⁺, NO₃^- and SO₄^2- exhibit significant differences in their relative abundances during winter and summer. Four factors have been derived by Positive Matrix Factorization (PMF) analysis of the water-soluble ionic constituents. These are secondary, sea-salt, mineral dust sources, while the fourth factor is associated with K⁺, which is likely to be due to emissions related to biomass burning.