A miniDOAS instrument optimised for ammonia field-measurements

We present a DOAS instrument, called "miniDOAS", optimised for optical open-path field-measurements of ambient ammonia (NH.sub.3) alongside nitrogen oxide (NO) and sulphur dioxide (SO.sub.2). The instrument is a further development of the miniDOAS presented by Volten et al. (2012). Here, we use a temperature-controlled spectrometer, a deuterium light source and a modified optical arrangement. The system was set up in a robust, field-deployable, temperature-regulated housing. For the evaluation of light spectra we use a new high-pass filter routine based upon robust baseline extraction with local regression. In order to fit differential absorption cross-sections to the measurements, multiple linear regression is performed including terms of an autoregressive-moving-average model. For NH.sub.3 the instrument's precision is 0.8 to 1.4 %. Accuracy is larger than precision and derives from the precision, uncertainty in DOAS absorption cross-sections (± 3 %) and an uncertain estimation of concentration offsets (c.sub.0) present through the definition of reference spectrum I.sub.0 . Accuracy will at minimum be approximately 0.3 μg m.sup.−3 . The limit of detection against I.sub.0 is around 0.2 μg m.sup.−3 . Comparisons of miniDOAS measurements to those by NH.sub.3 acid trap devices showed good agreement. The miniDOAS can be flexibly used for a wide range of field trials, such as micrometeorological NH.sub.3 flux measurements with approaches based upon horizontal or vertical concentration gradients. Results from such applications, covering concentration dynamics of sub-ppb to ppm mixing ratios, are presented.