2. NO₂ Measurement Techniques: Pitfalls and New Developments

Reliable measurements of atmospheric trace gases are necessary for both, a better understanding of the chemical processes occurring in the atmosphere, and for the validation of model predictions. Nitrogen dioxide (NO₂) is a toxic gas and is thus a regulated air pollutant. Besides, it is of major importance for the oxidation capacity of the atmosphere and plays a pivotal role in the formation of ozone and acid precipitation. Detection of NO₂ is a difficult task since many of the different commercial techniques used are affected by interferences. The chemiluminescence instruments that are used for indirect NO₂ detection in monitoring networks and smog chambers use either molybdenum or photolytic converters and are affected by either positive (NO_y) or negative interferences (radical formation in the photolytic converter). Erroneous conclusions on NO₂ can be drawn if these interferences are not taken into consideration. In the present study, NO₂ measurements in the urban atmosphere, in a road traffic tunnel and in a smog-chamber using different commercial techniques, i.e. chemiluminescence instruments with molybdenum or photolytic converters, a luminol based instrument and a new NO₂-LOPAP, were compared with spectroscopic techniques, i.e. DOAS and FTIR. Interferences of the different instruments observed during atmospheric measurements were partly characterised in more detail in the smog chamber experiments. Whereas all the commercial instruments showed strong interferences, excellent agreement was obtained between a new NO₂-LOPAP instrument and the FTIR technique for the measurements performed in the smog chamber.