Seasonal variations in size distribution, water-soluble ions, and carbon content of size-segregated aerosols over New Delhi

Abstract

Size distribution, water-soluble inorganic ions (WSII), and organic carbon (OC) and elemental carbon (EC) in size-segregated aerosols were investigated during a year-long sampling in 2010 over New Delhi. Among different size fractions of PM₁₀, PM_0.95 was the dominant fraction (45%) followed by PM_3–7.2 (20%), PM_7.2–10 (15%), PM_0.95–1.5 (10%), and PM_1.5–3 (10%). All size fractions exceeded the ambient air quality standards of India for PM_2.5. Annual average mass size distributions of ions were specific to size and ion(s); Ca²⁺, Mg²⁺, K⁺, NO₃⁻, and Cl⁻ followed bimodal distribution while SO₄²⁻ and NH₄⁺ ions showed one mode in PM_0.95. The concentrations of secondary WSII (NO₃⁻, SO₄²⁻, and NH₄⁺) increased in winters due to closed and moist atmosphere whereas open atmospheric conditions in summers lead to dispersal of pollutants. NH₄⁺and Ca²⁺were dominant neutralization ions but in different size fractions. The summer-time dust transport from upwind region by S SW winds resulted in significantly high concentrations of PM_0.95 and PM_3–7.2 and PM_7.2–10. This indicted influence of dust generation in Thar Desert and its transport is size selective in nature in downwind direction. The mixing of different sources (geogenic, coal combustions, biomass burning, plastic burning, incinerators, and vehicular emissions sources) for soluble ions in different size fractions was noticed in principle component analysis. Total carbon (TC = EC + OC) constituted 8–31% of the total PM_0.95 mass, and OC dominated over EC. Among EC, char (EC1) dominated over soot (EC2 + EC3). High SOC contribution (82%) to OC and OC/EC ratio of 2.7 suggested possible role of mineral dust and high photochemical activity in SOC production. Mass concentrations of aerosols and WSII and their contributions to each size fraction of PM₁₀ are governed by nature of sources, emission strength of source(s), and seasonality in meteorological parameters.