Nordic intercomparison for measurement of major atmospheric nitrogen species

Abstract

A comparative study of measurements of atmospheric gaseous nitric acid and ammonia and particulate nitrate and ammonium concentrations was conducted using various types of filter packs and denuder systems. Some of the filter packs used are recommended by the European Monitoring and Evaluation Programme (EMEP) and are widely used in the Nordic Countries and elsewhere in Europe. In addition, nitrogen dioxide was measured using iodide-impregnated sintered glass filters and the differential optical absorption spectrometer method. Particulate nitrate and ammonium concentrations were measured with two different size segregating samplers: a Berner low-pressure impactor and a virtual impactor. The weather conditions were most of the time cool and humid. The agreement between different measurements was good for most species, but poor for gaseous ammonia. The virtual impactor collected on average 17% more nitrate than the denuder systems, 24% more than filter packs and 37% more than the Berner low-pressure impactor. The nitrate discrepancy is believed to be due to inefficient coarse particle transport into the denuder systems, fine particle nitrate evaporation in the filter packs and Berner low-pressure impactor and, to a lesser extent, possible collection of nitric acid by the virtual impactor. A substantial fraction of PM 2.5 may be lost if the nitrate evaporation cannot be accounted for. The denuder systems did not collect particles larger than about 5.6 μm equivalent aerodynamic diameter efficiently. Interferences were observed on the nitric acid measurements determined by a NaOH coated denuder. NaCl coated denuders seemed to operate well in this study since interferences were not observed.