Nordic intercalibration of airborne equipment for measurement of sulphate and sulphur dioxide

Intercalibration experiments with airborne equipment belonging to Nordic laboratories were carried out and the filter samples analyzed by the various methods in use at the laboratories. The results were subjected to statistical analyses of correlation and regression. Intercalibration on the ground comprised Danish, Norwegian, Swedish, and Finnish equipment and an airborne intercalibration was later carried out with Danish and Norwegian equipment installed in two aircraft. Although the sulphate results exhibit good mutual correlation, the results from the samples taken on the ground differ by up to 50%, the main reason being an overestimation by one of the methods used (X-ray fluorescence). Further, the differences between the results from the samples taken in the air are of the same magnitude, except in a single instance where the results differed by a factor of 3; the reason for this is not clear. The sulphur dioxide results exhibit rather poor mutual correlation; the results from the samples taken on the ground differ at most 20%, but in some cases, however, this is the net result of significant loss of reaterial in the apparatus and an overstimation of sulphur dioxide by the analytical methods. The samples taken in the air had too low a content of sulphur dioxide for meaningful comparisons.