Evaluation of a new inference method for estimating ammonia volatilisation from multiple agronomic plots
Abstract. Tropospheric ammonia (NH3) is a threat to the environment and human health and is mainly emitted by agriculture. Ammonia volatilisation following application of nitrogen in the field accounts for more than 40 % of the total ammonia emissions in France. This hence represents a major loss of nitrogen use efficiency which needs to be reduced by appropriate agricultural practices. In this study we evaluate a novel method to infer ammonia volatilisation from small agronomic plots made of multiple treatments with repetition. The method is based on the combination of a set of ammonia diffusion sensors exposed for durations of 3 hours to 1 week, and a short-range atmospheric dispersion model, used to retrieve the emissions from each plot. The method is evaluated by mimicking ammonia emissions from an ensemble of 9 plots with a resistance-analogue-compensation-point surface exchange scheme over a yearly meteorological database separated into 28-days periods. A multi-factorial simulation scheme is used to test the effects of sensor number and heights, plot dimensions, source strengths and background concentrations, on the quality of the inference method. We further demonstrate by theoretical considerations in the case of an isolated plot that inferring emissions with diffusion sensors integrating over daily periods will always lead to underestimations due to correlations between emissions and atmospheric transfer. We evaluated these underestimations as −8 % ± 6 % of the emissions for a typical western European climate. For multiple plots, we find that this method would lead to median underestimations of −16 % with an interquartile [−8 % −22 %] for two treatments differing by a factor of up to 20 and a control treatment with no emissions. We further evaluate the methodology for varying background concentrations and ammonia emission patterns and demonstrate the low sensitivity of the method to these factors. The method was also tested in a real case and proved to provide sound evaluations of ammonia losses from surface applied and incorporated slurry. We hence showed that this novel method should be robust and suitable for estimating ammonia emissions from agronomic plots. Further work should anyway be produced for validating this method in real conditions.