FIELD INTERCOMPARISON EXERCISE ON NITRIC-ACID AND NITRATE MEASUREMENT ROME, 1988) - A CRITICAL APPROACH TO THE EVALUATION OF THE RESULTS

A 5-day intercomparison of measurement techniques for nitric acid and particulate nitrate was carried out at the Area della Ricerca di Roma (Montelibretti, Rome, Italy) during September 18-24, 1988. Sixteen groups from eleven European countries participated in the experiment, intercomparing the performances of several denuder and filter pack systems. In order to obtain a better characterization of the performance of each technique, in addition to HNO3 and NO3- field sampling, the protocol included HNO3 determinations from a pure source and temporal self-consistency tests. Many ancillary measurements were also undertaken, which proved to be of help in the interpretation of the data. The evaluation of the results is not performed through a simple linear regression of the data, that is by assessing agreement or disagreement between pairs of methods, but by comparing the results obtained in both the additional tests and the field samplings with the predicted deposition pattern. Once the reliability of each single technique has been evaluated in the light of the potential interfering mechanisms, a comparison between the results yielded by groups using the same technique and, finally, different techniques is carried out. The application of these criteria to the data set gathered during the intercomparison shows that the diffusion techniques yield the most reliable results, while teflon-nylon filter packs do not allow a correct discrimination between nitric acid and nitrate, particularly in the presence of high ammonium nitrate concentrations. Filter packs using a cellulose prefilter are only able to measure total nitrate.