Modification of a commerical NOx detector for high sensitivity

We describe modifications of a commercial chemiluminescent NOx detector to provide sufficiently high sensitivity to measure oxides of nitrogen in clean continental air. The changes include a larger reaction chamber, a faster vacuum pump, and a prereactor for background measurement. The detection limit, defined as a signal-to-noise ratio of 1:1 at the 2σ level, is about 0.10 ppb, with a response time of about 3 s. This corresponds to a ten-fold improvement over the original detection limit.