Evaluation of a diode laser based photoacoustic instrument combined with preconcentration sampling for measuring surface-atmosphere exchange of ammonia with the aerodynamic gradient method

We present here a novel instrument for measuring surface-atmosphere exchange fluxes of ammonia. The instrument is the upgraded version of a recently developed near-infrared diode laser based photoacoustic ammonia concentration monitoring instrument, i.e. the original instrument is supplemented with two additional sampling lines, an appropriate gas handling system and an advanced software controlling gradient measurements. As a result of these developments, ammonia concentration can be measured simultaneously at three different heights above the ground and ammonia fluxes can be calculated from these data using the aerodynamic gradient method. The instrument operates fully automatically, requires minimal maintenance and has a temperature controlled, waterproof housing which makes it suitable for measurements even under harsh field conditions. Preliminary tests on stability and accuracy were carried out during two two-week field measurement campaigns, with the three sampling inlets being placed at the same height together with the inlet of a reference instrument. The readings of the three channels agreed well (with correlation coefficients above 0.96). Comparison to reference instruments showed good stability of the photoacoustic instrument, there was no measurable zero-drift or change in sensitivity during the tests. Flux measurements were carried out during a three-week field campaign in southern Scotland over fertilized grassland with reference to a wet-chemical AMANDA instrument in gradient configuration. Ammonia fluxes calculated from the data of the two instruments agreed well. Fluxes up to 2500 ng m^-2 s^-1 were observed after fertilization. The minimum detectable ammonia flux was calculated on the basis of “virtual ammonia fluxes”, from measurements carried out with all inlets at the same height and was found to be 60 ng m^-2 s^-1 which ensures reliable measurements above intensively managed grasslands or agricultural fields