Characterization and diurnal variation of size-resolved inorganic water-soluble ions at a rural background site

Abstract

Water-soluble inorganic ions in aerosol samples have been studied. The sample collection took place during summer in 2003 at a European background site which is operating within the framework of the European Monitoring and Evaluation Program. Gent type PM10 stacked filter unit (SFU) samplers were operated in parallel on a day and night basis to collect particles in separate coarse (2.0–10 μm) and fine (<2.0 μm) size fractions. Particulate masses were measured gravimetrically; the filters from one of the SFU samplers were analyzed by particle-induced X-ray emission spectrometry (PIXE) and instrumental neutron activation analysis (INAA). Filters from the other SFU sampler were analyzed by ion chromatography (IC) for major inorganic anions (MSA⁻, NO₂⁻, NO₃⁻, Cl⁻, Br⁻, SO₄²⁻, oxalate) and cations (Na⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺). The water-soluble inorganic ions measured were responsible for 44% and 16% of the total fine and coarse particulate mass, respectively. In the fine size fraction, the main ionic components were SO₄²⁻ and NH₄⁺ accounting for about 90% of fine ionic mass. In the coarse fraction the main ionic components were Ca²⁺ and NO₃⁻, followed by SO₄²⁻. Significant day and night difference in the mass concentrations was observed only for fine NO₃⁻. The molar ratios of fine NH₄⁺ to SO₄²⁻ indicated their complete neutralization to (NH₄)₂SO₄. According to the cation-to-anion ratios the coarse particles were alkaline, while the fine particles were slightly acidic or neutral. By comparing the corresponding concentrations obtained from PIXE/INAA and IC, we determined the water-extractable part of the individual species. We also investigated the effect of long-range transported air masses on the local air concentrations, and we found that the air quality of this background monitoring station was affected by regional pollution sources.