Measurement Uncertainty of Sulphur and Nitrogen Containing Inorganic Compounds By 1-Stage and 2-Stage Filter-pack Methods

Abstract

The sampling and analysis properties of 1-stage and 2-stage filter-pack methods were studied in detail in monitoring of sulphur and nitrogen containing inorganic gases and particles (sulphur dioxide, sulphate, sum of nitric acid and nitrate and total ammonium). The limit of detection and the limit of quantitation for 24-h samples were estimated using the results of a short-term field experiment completed with available data from long-term monitoring and internal quality assurance. Furthermore, the combined expanded measurement uncertainty including sampling and analysis (Utot) was estimated for filter-pack methods in order to give a tool for distinguishing long-term trends in air quality from the measurement variability. Utot was found to be very near the analytical uncertainty when measuring higher air concentration levels, being ± 4.0% for sulphur concentrations > 1.0 μg m−3, ± 3.0% for sulphate concentrations > 0.5 μg m−3, ± 3.5% for the sum of nitrate and nitric acid concentrations > 0.3 μg m−3 and ± 4.5% for total ammonium concentrations > 0.8 μg m−3. At the lower air concentration range Utot increases significantly due to the field blank values. The precision of the 24-h filter-pack sample results expressed by means of modified median absolute difference (M.MAD) and coefficient of variance (CoV) gave 8.3% for sulphur dioxide and 5.4% for particulate sulphate. For the sum of gaseous nitric acid and particulate nitrate the CoV was 5.5% and for total ammonium 4.3%. In addition the suitability of the 24-h filter-pack methods in weekly sampling was proved.