Seasonal Variations and Composition of Water-Soluble Ions in PM_2.5 at an Urban Site in Kenitra City, Morocco

A comprehensive study was carried out in the urban area of Kenitra City, Morocco, covering the period from 2020 to 2021. During this study, 60 effective PM_2.5 samples were collected for a duration of 24 hours using a dichotomous sampler and Nuclepore track-etched polycarbonate filters with a diameter of 37 mm. Ion chromatography was employed to determine the content of water-soluble ions, including SO₄²⁻, NO₃⁻, Cl⁻, F⁻, NH₄⁺, Ca²⁺, Na⁺, and K⁺.

The results revealed that the average mass concentration (± standard deviation) of the six ions in PM2.5 was 3.24±1.32 µg.m^-3, constituting approximately 18.7% of the total mass concentration. Among the ions, the concentrations followed the order of Na⁺ > NO₃⁻ > SO₄²⁻ > K⁺ > F⁻ > NH₄⁺ > Cl⁻. The mass concentration and most of the chemical components of PM_2.5 exhibited significant variations throughout the seasons, with higher levels observed during the summer and lower levels during the winter.

The major components of water-soluble ions in PM_2.5 were found to be secondary inorganic species (NO₃^-, SO₄^2- and NH₄⁺) and sea salt species (Na⁺ and Cl^-), contributing an average of 78% to the total PM_2.5 ions. The concentrations of these three ions varied across the four seasons, with the highest levels observed in summer, followed by winter, autumn, and spring. Notably, NH₄⁺ showed a significant correlation with SO₄²⁻, with a maximum correlation coefficient of 0.85 during summer and a minimum coefficient of 0.39 during autumn. The ratio of [NO₃⁻]/[SO₄²⁻] was found to be lower than unity, indicating that the main sources of sulfur and nitrogen in the Kenitra atmosphere were prioritized from stationary sources (typically
associated with power plants, industrial and commercial activities, and other large-scale facilities).