Field comparison of measurement methods for sulphur dioxide and aerosol sulphate

Sampling methods for sulphur dioxide and particulate sulphate were compared at the Virolahti background station in southeastern Finland in 1993–1994. The methods compared included a monitor, two-stage and three-stage filter packs, an absorption solution and a passive sampler. No significant difference in the sulphur dioxide and sulphate results between the two filter pack methods was found (n = 28). The sulphur dioxide results from the monitor were 6% lower than the results from the two-stage sampler (n = 164). The average of sulphur dioxide results for six months from the daily absorption solution was only 4% higher than the results from the two-stage sampler, despite the significant absolute and relative bias between the two methods. The sulphate results from the prefilter of the low-volume absorption solution were considerably lower than the results of the two-stage sampler. The results from the two-week and the one-month passive samplers were in good agreement with the other methods, although their results were somewhat higher. The relative standard deviation for four parallel monthly passive samplers varied between 2% and 15%, with a median of 6%. The comparison showed that the passive sampler can be used to replace the absorption solution method, but at clean background stations the sampling time should not be shorter than one month. For measuring daily sulphur dioxide concentrations the sensitive monitor and the two- or three-stage filter pack methods are the viable alternatives.