In highly complex terrain path-averaged (or even volume-averaged measurement by scanning with remote sensing techniques) can be advisable. Compared with single point measuring techniques path-averaging measurement results are a more adequate tool for the comparison with modelling results, especially if the spatial resolution of the models does not match the spatial representativity of the measurements (Schäfer et al., 2005).
An open-path DOAS (Differential Optical Absorption Spectroscopy) is measuring path-averaged NO, NO2, O3, SO2, benzene, toluene, xylene, formaldehyde and ammonium concentrations. The instrument contains an analysis unit (AR 500) and an emitter/receiver (ER 130) which points to three retroreflectors (all from OPSIS GmbH). The positions of the retroreflectors are collected in the software and the step-motor driven emitter/receiver unit is automated and is changing from retroreflector to retroreflector including an optical alignment. The path lengths are 1000 m typically. Each compound requires a different optimum path length depending from its spectroscopic characteristics.
In Schäfer et al., 2008, a typical application of this measuring technique is demonstrated. For the determination of emission rates in heterogeneous areas this technique was successfully introduced for air port air quality studies (Schürmann et al., 2007). Using this measuring approach also the discrimination of polluted and unpolluted (background) air volumes by only one instrument is advisable.