Measuring the biosphere-atmosphere exchange of total reactive nitrogen by eddy covariance using a novel converter

Because nitrogen availability is one of the key limiting factors for the productivity of ecosystems, it also has a strong influence on the greenhouse gas exchange. The joint monitoring of reactive nitrogen exchange is therefore essential for the interpretation of the greenhouse gas budget of ecosystems. However, the exchange of reactive nitrogen with the atmosphere includes various compounds (NO, NO₂, HNO₃, NH₃, NH₃^-, NH₄⁺) and the long-term monitoring of all of them individually would be very expensive and seems unrealistic in practice. An alternative approach is the measurements of the exchange of the sum of all these reactive nitrogen species (total reactive nitrogen: 'TorN'). For this purpose we used a custom built converter system that thermally converts all reduced and oxygenated reactive nitrogen compounds to nitric oxide (NO). The latter is then analyzed by a single NO analyzer with chemiluminescence detection. The converter can be placed on the measurement tower at the air sampling inlet and thus avoids the problematic transport of sticky components like NH₃ and HNO₃ through long sampling tubes. Since commercially available NO detectors allow fast response measurements (1 s or faster) the TorN converter could be used for eddy covariance (EC) flux measurements. In a test field experiment, we operated an EC system for TorN fluxes at the Swiss FLUXNET and NitroEurope site Oensingen for more than two months. The agricultural site provided periods when the TorN exchange was dominated by ammonia emission (after manure application) and other periods dominated by NO₂ deposition. The obtained TorN concentration and flux measurements were compared to the sum of simultaneous measurements of ammonia (by gradient system) and NO₂ (by dynamic chamber system). The results showed a good agreement within the uncertainty of the individual measurements indicating the strong potential of the TorN converter for flux monitoring.