Unidentified interference in DOAS measurements of ozone

A differential optical absorption spectroscopy (DOAS) system was designed and operated in Christchurch, New Zealand, during the winter of 1997. We report the detection of an unidentified absorber in the 279 to 289 nm spectral region, which is used by many commercial DOAS systems for the measurement of atmospheric ozone on short absorption paths. Tests were performed to ensure that this absorber is not an artifact of the analysis procedure. The presence of the unidentified absorber can have serious consequences on ozone concentrations obtained with the DOAS technique when primary pollutant concentrations are high, while it is hardly noticeable under clean conditions. It is argued that either the absorber may be specific to New Zealand emissions, possibly due to the widespread use of wood fires for domestic heating, or that its influence is generally overlooked because interest in ambient ozone usually concentrates on summer smog conditions, where primary pollutant concentrations are generally lower than during winter.