Evaluation of improved inlets and annular denuder systems to measure inorganic air pollutants

Abstract

Modifications were made to various components of an annular denuder system for the collection of ambient SO 2, HNO 3, HNO 2, NH 3, SO 4 2−, and NO 3 −. The annular denuder was modified to incorporate a laminar restoration section as part of the annular denuder itself in order to reduce construction costs and to minimize leaks in the system during field operation. Teflon-coated glass impactors and cyclone inlets for the annular denuders were developed and evaluated for their inertness to acidic gases collected in the annular denuders. Paired samples were collected over a period of several months on the roof of the EPA laboratory in Research Triangle Park, NC, in order to compare the new inlets. For 17 sampling runs when two different types of inlets were compared, average differences for the measurement of SO 2, HNO 3 and HNO 2 were 3.4, 6.4 and 8.3%, respectively. For five sampling runs using pairs of identical impactor inlets, duplication for SO 2 and HNO 3 improved slightly. Annular denuder systems using cyclone and impactor preseparators yielded no significant differences in their abilities to collect HNO 3 and SO 2. The results are supported by the laboratory data of Appel et al. (1987, EPA/APCA Symp., Research Triangle Park), which indicate that no HNO 3 is retained in the inlets tested. Coarse-particle bounce through the system was shown to be a potential problem, although this had no apparent effect on acidic gas collection.