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A compact and simple instrument for the sensitive detection of NO₂ in the atmosphere has been developed. NO₂ is sampled in a stripping coil by a selective chemical reaction, converted into a highly absorbing dye, which is detected by long path absorption in a liquid core waveguide. Several interferences were quantified in the laboratory, which so far can all be neglected. The significant interferences against HONO and O₃ were suppressed by using an upstream HONO/O₃ -scrubber. The instrument has a detection limit of 2 pptv, an accuracy of 10 % and a precision of 0.5 % for 3 min time resolution. Thus, the new NO2-LOPAP technique is more sensitive than known commercial NO₂ instruments. The new analyzer is much simpler to apply than other highly sensitive and selective NO₂ methods, e.g. LIF, DOAS, pulsed CRDS or REMPI methods. The new instrument allows an absolute calibration that can be easily performed with liquid nitrite standards, which is a significant advantage over other NO₂ measurement techniques, for which NO₂ calibration gas mixtures are typically necessary. The new instrument has been validated against the chemiluminescence technique during an urban field campaign and against the FTIR technique in a smog chamber under complex photosmog conditions. The data sets exhibit high correlation and excellent agreement.

The instrument was used at an urban field site and direct NO₂ emissions, from engine combustion, were determined. A NO2/NOₓ ratio of (12 ± 1) % was determined, a ratio much higher than 10 years ago. This is explained by high NO₂ emissions from modern diesel vehicles and an increasing diesel vehicle fraction. The ratio is lower than that determined by using a chemiluminescence instrument, which is explained by interferences. The new NO2-LOPAP was also used in laboratory studies on the photolysis of nitroaromatic species. In contrast to a recent study on this topic, in which a luminol NO₂ instrument was used, a significant formation of NO₂ was observed besides formation of nitrous acid (HONO). Both applications demonstrate that the use of selective NO₂ instruments is of paramount importance for the quantification and interpretation of NO₂ sources in the atmosphere.