Mass and ionic composition of atmospheric fine particles over Belgium and their relation with gaseous air pollutants

Abstract

Mass, major ionic components (MICs) of PM_2.5, and related gaseous pollutants (SO₂, NO_x, NH₃, HNO₂, and HNO₃) were monitored over six locations of different anthropogenic influence (industrial, urban, suburban, and rural) in Belgium. SO₄²⁻, NO₃⁻, NH₄⁺, and Na⁺ were the primary ions of PM_2.5 with averages diurnal concentrations ranging from 0.4–4.5, 0.3–7.6, 0.9–4.9, and 0.4–1.2 µg m⁻³, respectively. MICs formed 39% of PM_2.5 on an average, but it could reach up to 80–98%. The SO₂, NO, NO₂, HNO₂, and HNO₃ levels showed high seasonal and site-specific fluctuations. The NH₃ levels were similar over all the sites (2–6 µg m⁻³), indicating its relation to the evenly distributed animal husbandry activities. The sulfur and nitrogen oxidation ratios for PM_2.5 point towards a low-to-moderate formation of secondary sulfate and nitrate aerosols over five cities/towns, but their fairly intensive formation over the rural Wingene. Cluster analysis revealed the association of three groups of compounds in PM_2.5: (i) NH₄NO₃, KNO₃; (ii) Na₂SO₄; and (iii) MgCl₂, CaCl₂, MgF₂, CaF₂, corresponding to anthropogenic, sea-salt, and mixed (sea-salt + anthropogenic) aerosols, respectively. The neutralization and cation-to-anion ratios indicate that MICs of PM_2.5 appeared mostly as (NH₄)₂SO₄ and NH₄NO₃ salts. Sea-salt input was maximal during winter reaching up to 12% of PM_2.5. The overall average Cl-loss for sea-salt particles of PM_2.5 at the six sites varied between 69 and 96% with an average of 87%. Principal component analysis revealed vehicular emission, coal/wood burning and animal farming as the dominating sources for the ionic components of PM_2.5.