Diffusion denuders are commonly used for collection of water soluble atmospheric gases. Denuder plates made of glass with silica coating were however difficult to handle in field measurements, and also had wettability problems. In this paper, we describe a new design of parallel plate denuder consisting of perfluoralkoxy (PFA) plates and polyester sheets. The parallel plate denuder was coupled on-line to ion chromatography for analysis of atmospheric HNO3 and SO2. With time resolution of 20 min, the detection limits for both gases were 6 ppt(v). Memory effect was found to be significant for HNO3 especially at high humidity with the long sample inlet tube. Particle losses due to diffusion and electrostatic effects in the denuder were investigated and compared with theory. Particle loss in the sample inlet tube was found to play an important role by reducing the number of charged particles entering the denuder. The instrument was applied in two European Union field measurement campaigns, and the intercomparison results are presented in this paper. (C) 2001 Elsevier Science Ltd. All rights reserved.