Determination of Nitric Acid in Ambient Air by Gas Chromatography/Photoionization Detection after Collection in a Denuder

Abstract

A technique, adaptable to automation, has been applied to monitoring ambient concentrations of HNO₃. HNO₃ is preconcentrated and separated from particulate nitrate by sampling air through a diffusion tube (denuder) coated with Al₂(SO₄)₃. The HNO₃ collected is released as NO _x by thermal desorption. Then NO _x is converted into NO and analysed selectively by gas chromatography/photoionization detection. The minimum detectable concentration (defined as three times the standard deviation of the denuder blank) was 0.014 μg/m³ for a 30-L sample. Results for ambient air sampling with a minimum time resolution of 30 min are presented. No decrease in collecting efficiency was observed for denuders passing 30 sampling runs in ambient air for a minimum sampling duration of 30 min. The precision of the determination of HNO₃ concentrations in ambient air, expressed as the relative standard deviation, is in the range of 6-15%.