Quantification of Water-Soluble Inorganic Ions of PM10 Particles in Selected Areas of Kolkata Metropolitan City, India

Abstract

Several anthropogenic (like industries, vehicles, coal-based thermal power plants, etc.) and natural sources (like lightning, degradation of organic matter, etc.) emit a copious amount of primary gaseous pollutants like Sulphur dioxide (SO₂), Nitrogen dioxide (NO₂) and Ammonia (NH₃), which further contribute in the formation of particulate matters where they are present in the form of water-soluble inorganic ions (WSII) like SO₄²⁻, NO₃⁻, NH₄⁺, respectively. To assess the conversion of primary gaseous pollutants into dissolved ions in PM₁₀, real-time sampling (24 h average) of PM₁₀ and primary gaseous pollutants (SO₂, NO₂, NH₃) from September 2021 to March 2022 over 13 highly polluted places in Kolkata were performed. Then PM₁₀ was quantified using the gravimetric method, while NO₂ and NH₃ were estimated through chemiluminescence and SO₂ was estimated through the fluorescent spectrometric method. Analysis of WSII species in PM₁₀ was also performed through the spectrophotometric method and then to understand the conversion of the primary gaseous pollutants into WSIIs in PM₁₀, sulphur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) and ammonia conversion ratio (NHR) was calculated. The results showed that NO₂ is the most abundant primary gaseous pollutant in the ambient air of Kolkata [highest in Dumdum (73.6 μgm⁻³)] and NO₃^– is the most abundant WSII in PM₁₀ of Kolkata [highest in Bansdroni (3.74 μgm⁻³)] and the SOR and NOR values were significantly higher in Bansdroni and NHR was significantly higher in Santoshpur due to presence of optimum meteorological conditions. Lightning is one of the major natural sources of NO_x. So, due to lightning, atmospheric NOx level increases which then gets associated with particulate matter, increasing the concentration of NO₃⁻ ion in particulate matter due to gas-particle partitioning. This is indicated by strong linear correlation coefficients (R² = 0.746) between a number of flashes on the day of sampling and NO₃⁻ concentration in PM₁₀ aerosol.