Towards robust on-site ammonia emission measuring techniques based on inverse dispersion modeling

Ammonia is recognized as one of the major atmospheric pollutants affecting air and ecosystem quality. The application of N fertilizers is a major source of NH3 emissions. It is necessary to develop simple, accurate and low cost measurement techniques to obtain representative data for a wide range of regions and agricultural practices. This information would improve national inventories and support decision-making processes regarding strategies for NH3 emission abatement. Measurement techniques can be complex, adjusted to specific conditions, labor-intensive and costly. This work analyses different methods aimed at reaching a balance between the accuracy and precision required for measurements and their complexity. Three techniques were tested under: semiopen passive chambers (SOCs) and an inverse dispersion model (IDM) combined with two different NH3 air concentration measuring techniques: ALPHA® passive samplers and acid bubblers. Different setups were evaluated testing different heights over the emitting surface. These techniques were assessed using micrometeorological mass balance integrated horizontal flux (IHF) with passive flux samplers as a reference method. The SOC results showed a close linear relationship with the IHF results (R2=0.784, p<0.01), although emissions after 16 days were 10.6% higher. The bLS IDM with acid bubblers showed promising results, although they were labor-demanding and required a power supply. The IDM with ALPHA® samplers placed at a 1.25-m height was demonstrated to embrace precision and close agreement with the IHF observations in our experimental conditions (R2=0.768, p<0.01; no difference from line 1:1; mean bias: 0.041 kg N ha−1h−1 and rRMSE: 46.1%). However, these data require special attention for periods of drastically changing weather, when the NH3 fluxes determined with air concentration samplers moved away from the overall emission pattern. Longer sampling intervals and the assumption of neutral atmospheric conditions in IDM may decrease costs, simplify the procedure and provide a cost-efficient alternative to the IHF method.