Field Intercomparison Exercise on Nitric Acid and Nitrate Measurement: Critical Evaluation of the Results

Summary

A five days intercomparison of measurement techniques for nitric acid and particulate nitrate has been held in the Area della Ricerca di Roma during September 1988. The study design included 4-hours and 12 hours sampling periods, and, for two days, additional 24-hours samplings, so as to evaluate the self-consistence of each method. Also, two samples for each group were taken from a nitric acid permeation source, in order to check the accuracy of the methods m laboratory conditions. The evaluation of the results is not performed through a simple regression of the data, that is by assessing agree ment or disagreement between pairs of methods, but according to the following steps: 1) the drawbacks of the different techniques are critically evaluated; 2) the self-consistence of each method is checked, also in the light of the presence of potential interfering species; 3) a study of the sum and ratio between reported nitric acid and reported nitrate concentrations is carried out; this represents an interesting tool for the data comprehension and interpretation. The application of these criteria to the data set gathered during the intercomparison shows that the diffusion techniques yield the most reliable results, while the 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 nitrates.