Tropospheric concentrations of nocturnal NO2 measured at our rural site in New Zealand (45¿S, 170¿E) are presented. We also describe the long path spectroscopic absorption technique used. The system features a beam splitter just before the exit slit of a scanning monochromator, and the broad spectral band signal seen at a second detector is used to remove the effect of amplitude modulations caused by atmospheric flicker. A signal ratioing technique produces nearly 2 orders of magnitude improvement in this site. With a one hour observation period detection thresholds of 20 ppt have been achieved from absorptions over a 9.2 km path. The results display a wide range of mixing ratios from the detection threshold, up to in excess of 1 ppb on isolated days. A strong absorption feature at 442.6 nm is identified as a water vapour absorption, and the absorption cross section is found to be (3.1¿0.3)¿10-26 cm2 molecule-1 at 0.5 resolution.