RESEARCH BRIEF

Size-resolved characteristics of inorganic ionic species in atmospheric aerosols at a regional background site on the South African Highveld North-West University's research published in Journal of Atmospheric Chemistry

Andrew D. Venter^I; Pieter G. van Zyl^I; Johan P. Beukes^I; Jan-Stefan Swartz^I; Miroslav Josipovic^I; Ville Vakkari^II; Lauri Laakso^{I, II}; Markku Kulmala^III

^IUnit for Environmental Sciences and Management, North-West University, Potchefstroom, South Africa
^IIFinnish Meteorological Institute, Helsinki, Finland
^IIIDepartment of Physics, University of Helsinki, Finland

Atmospheric aerosols or particulate matter (PM) are important components of the atmosphere with high temporal and spatial variability, which can have significant impacts on air quality and climate change. Detailed physical and chemical characterisation are crucial in establishing the impacts of atmospheric aerosol. Aerosols consist of organic and inorganic species, and the composition and concentration of these species depends on their sources, chemical transformation and sinks. In this study an assessment of major inorganic ions determined in three aerosol particle size ranges, i.e. PM₁, PM_1-2.5 and PM_2.5-10 collected for one year at Welgegund was conducted.

Sulphate (SO₄^2-) and ammonium (NH₄⁺) dominated the PM₁ size fraction, while SO₄^2- and nitrate (NO₃^-) dominated the PM_1-2.5 and PM_2.5-10 size fractions. SO₄^2- had the highest contribution in the two smaller size fractions, while NO₃^- had the highest contribution in the PM_2.5-10 size fraction. SO₄^2- and NO₃^- levels were attributed to the impacts of aged air masses passing over major anthropogenic source regions. Comparison of inorganic ion concentrations to levels thereof within the western Bushveld Igneous Complexes - a source region influencing Welgegund - indicated higher levels of most inorganic species within the source region. However, the comparative ratio of SO₄^2- was significantly lower due to SO₄^2- being formed distant from SO₂ emissions and submicron SO₄^2- having longer atmospheric residencies. Aerosols at Welgegund were determined to be generally acidic, which was mainly attributed to high concentrations of SO₄^2-.

PM₁ and PM_1-2.5 fractions revealed a seasonal pattern, with higher inorganic ion concentrations measured from May to September. Higher PM concentrations were attributed to decreased wet removal, more pronounced inversion layers trapping pollutants, and increases in household combustion and wild fires during winter. Back trajectory analysis also revealed higher concentrations of inorganic ionic species corresponding to air mass movements over anthropogenic source regions.