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We present a differential optical absorption spectroscopy (DOAS) instrument, called "miniDOAS", optimised for optical open-path field-measurements of ambient ammonia (NH₃) alongside nitrogen oxide (NO) and sulfur dioxide (SO₂). The instrument is a further development of the miniDOAS presented by Volten et al. (2012). 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. Multiple linear regression including terms of an autoregressive–moving-average model is used to determine concentrations. For NH₃ the random uncertainty is about 1.4 % of the concentration, and not better than 0.2 µg m⁻³. Potential biases for the slope of the calibration are given by the precision of the differential absorption cross sections (±3 %) and for the offset by the precision of the estimation of concentration offsets (<i>c</i>_ref) introduced by the reference spectrum <i>I</i>_ref. Comparisons of miniDOAS measurements to those by NH₃ acid trap devices showed good agreement. The miniDOAS can be flexibly used for a wide range of field trials, such as micrometeorological NH₃ flux measurements with approaches based upon horizontal or vertical concentration differences. Results from such applications covering concentration dynamics of less than one up to several hundreds of µg m⁻³ are presented.