A denuder technique for the measurement of nitrous acid in urban atmospheres

A new denuder set-up for the measurement of nitrous acid in polluted atmospheres is described here. The set-up is composed of one tetrachloromercurate-coated denuder for the removal of SO₂ and two downstream sodium carbonate-coated denuders for the determination of nitrous acid by the differential technique Febo et al., 19, 1517–1530 1990]. The removal of SO₂ is necessary in order to avoid the formation of artifact nitrite on the sulfite layer which results from the interaction between atmospheric SO₂ and the Na₂CO₃ coating. Because of this mechanism, the measurement of HONO by means of the previously used NaClNa₂CO₃Na₂CO₃ denuder set-up is heavily biased in all cases when SO₂ and NO₂ are present at high concentration levels (e.g. urban environments).