Comparison of sampling artifacts and ion balances for a Berner low-pressure impactor and a virtual impactor

Between April 1996 and June 1997, parallel fine particle aerosol samples were collected with a virtual impactor (VI) and a Berner low-pressure impactor (BLPI) at both an urban and a rural site in the Helsinki area. The average size distributions for particulate sulphate, ammonium, oxalate, succinate, MSA, Na+, K+ and Mg2+ were similar at the two sites, while average concentrations were slightly higher at the urban site. Average BLPI/VI ratios of Na+, K+ and succinate were close to one indicating that no major sampling artifacts for these ions were present. The corresponding ratios were larger than one for Ca2+ and Mg2+, suggesting that some particle bounce-off occurred in the BLPIs. The ratios smaller than one for pyruvate, malonate, oxalate, ammonium, glutarate, MSA, and sulphate suggest that some fraction of these ions was formed from gaseous precursors on the VI Teflon filters. Particulate nitrate evaporated from the VI Teflon filters and from the BLPI, the average evaporation percentages varying between about 50%–65%. Chloride seemed to evaporate from the VI Teflon filters, the evaporation being more pronounced at the urban site where the concentration of gaseous SO2 was higher. The fine particle cation/anion ratios were lower for the VI at both sites, and higher at the rural site for both sampler types. The contribution of organic anions to total anions was higher during the summer than during the winter for fine particles, even though these contributions were similar for ultrafine particles.