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

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 mu m) and fine (< 2.0 mu m) 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 (IQ for major inorganic anions (MSA(-), NO(2)(-), NO(3)(-), Cl(-), Br(-), SO(4)(2-), oxalate) and cations (Na(+), K(+), NH(4+), Mg(2+), Ca(2+)). 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 SO(4)(2-) and NH(4+) accounting for about 90% of fine ionic mass. In the coarse fraction the 2 2-main ionic components were Ca(2+) and NO(3)(-) followed by SO(4)(-). Significant day and night difference in the mass concentrations was observed only for fine NO(3)(-). The molar ratios of fine NH(4+) to SO(4)(2-) indicated their complete neutralization to (NH(4))(2)SO(4). 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.