Accuracy and sensitivity of NH3 measurements using the Dräger Tube Method

Abstract. Regional estimates of ammonia (NH₃) emissions are often missing data from heterogeneous or small fields. Areas with no experienced staff or in-field power supply also prevent the use of accurate and fully established micrometeorological measurement techniques. The Dräger Tube Method (DTM) is a calibrated open-dynamic chamber method, which requires little training to use and is comparatively inexpensive. It uses NH₃ detector tubes (Dräger Tubes), an automatic pump, as well as a chamber system comprised of four stainless steel chambers connected with PTFE tubing. Even though the DTM is often used in countries such as Germany and China, the detection accuracy, precision and sensitivity have not been tested yet. In order to quantify those for the DTM, we simultaneously measured defined NH₃ mixing ratios with the Dräger Tubes, with direct laser absorption spectroscopy (MGA⁷, MIRO Analytical AG, Switzerland) and with cavity ring-down spectroscopy (G2103, Picarro, Inc., USA). Second, we tested the exchange of the tubing material and heating of the tubing under laboratory conditions, as well as PTFE film attachments or wiping of the DTM chamber system with ethanol during outdoor measurements, on performance improvements. Results showed that the Dräger Tubes had a detection limit between 150 and 200 ppb, which is three to four times higher than originally assumed. Dräger Tube concentration measurements also underestimated NH₃ concentrations by 43 up to 100 % for mixing ratios between 50 and 300 ppb, and by 28 up to 46 % for mixing ratios between 500 and 1500 ppb. The PTFE tubing material showed similar performances to the polyester-polyurethane tubing material regarding response time, which was further improved by heating of the tubing to 50 °C. The modifications of the chamber surface and cleaning in the outdoor experiment did not lead to any improvements of NH₃ concentration measurements. The results suggest that the DTM should only be used where alternatives are unfeasible and high NH₃ emissions are to be expected. A further assessment of calibrated DTM with reference methods is required for a comprehensive evaluation and alternative developments for a more appropriate method replacing the DTM in small plot applications is encouraged.