Simultaneous determination of nitrogen dioxide and peroxyacetyl nitrate in ambient atmosphere by carbon-coated annular diffusion denuder

A diffusion denuder for simultaneous collection of ambient nitrogen dioxide and peroxyacetyl nitrate (PAN), based on quantitative collection on an alkaline active carbon surface, has been developed. Nitrogen dioxide yields nitrite anion whereas PAN yields nitrate which are extracted after sampling with water and analysed by ion chromatography. Laboratory and field tests concerning sorption efficiency, effect of humidity and capacity were carried out. The pre-exponential and exponent for the first term of the Gormley-Kennedy equation applied to annular geometry were estimated. It was found that for PAN the agreement between the experimental and calculated values is quite good and compatible with a "perfect sink" behaviour of carbon, whereas, in the case of NO2, the experimental collection efficiency is lower than theory but still suitable for ambient sampling. Nitrogen oxide and ozone do not interfere in the determination. SO2, HONO and HNO3 which could interfere can be removed by using two diffusion denuders placed in line before the carbon denuder: one coated with tetrachloromercurate (TCM), for the removal of SO2 and HNO3, and the second, coated with carbonate, for the removal of HONO. The detection limit for 24 h sampling is better than 0.05 and 0.06 ppb for NO2 and PAN, respectively. The response for NO2 was compared during a field campaign in Milan with the corresponding integrated response of a chemiluminescence analyser, while data for PAN were compared with the integrated response of a GC-ECD technique. The results obtained are reported and discussed.