The FLUXNET2015 dataset provides ecosystem-scale data on CO 2 , water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.

Abstract. Recent advances in laser spectrometry offer new opportunities to investigate ecosystem–atmosphere exchange of environmentally relevant trace gases. In this study, we demonstrate the applicability of a quantum cascade laser (QCL) absorption spectrometer to continuously measure ammonia concentrations at high time resolution and thus to quantify the net exchange between a seminatural peatland ecosystem and the atmosphere based on the eddy-covariance approach. Changing diurnal patterns of both ammonia concentration and fluxes were found during different periods of the campaign. We observed a clear tipping point in early spring with decreasing ammonia deposition velocities and increasingly bidirectional fluxes that occurred after the switch from dormant vegetation to CO2 uptake but was triggered by a significant weather change. While several biophysical parameters such as temperature, radiation, and surface wetness were identified to partially regulate ammonia exchange at the site, the seasonal concentration pattern was clearly dominated by agricultural practices in the surrounding area. Comparing the results of a compensation point model with our measurement-based flux estimates showed considerable differences in some periods of the campaign due to overestimation of non-stomatal resistances caused by low acid ratios. The total cumulative campaign exchange of ammonia after 9 weeks, however, differed only in a 6 % deviation with 911 and 857 g NH3-N ha−1 deposition being found by measurements and modeling, respectively. Extrapolating our findings to an entire year, ammonia deposition was lower than reported by Hurkuck et al. (2014) for the same site in previous years using denuder systems. This was likely due to a better representation of the emission component in the net signal of eddy-covariance fluxes as well as better adapted site-specific parameters in the model. Our study not only stresses the importance of high-quality measurements for studying and assessing land surface–atmosphere interactions but also demonstrates the potential of QCL spectrometers for continuous observation of reactive nitrogen species as important additional instruments within long-term monitoring research infrastructures such as ICOS or NEON at sites with strong nearby ammonia sources leading to relatively high mean background concentrations and fluxes.

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Abstract. Flux measurements of reactive nitrogen compounds are of increasing importance to assess the impact of unintended emissions on sensitive ecosystems and to evaluate the efficiency of mitigation strategies. Therefore, it is necessary to determine the exchange of reactive nitrogen gases with the highest possible accuracy. This study gives insight into the performance of flux correction methods and their usability for reactive nitrogen gases. The eddy-covariance (EC) technique is today widely used in experimental field studies to measure land surface–atmosphere exchange of a variety of trace gases. In recent years, applying the EC technique to reactive nitrogen compounds has become more important since atmospheric nitrogen deposition influences the productivity and biodiversity of (semi)natural ecosystems and their carbon dioxide (CO2) exchange. Fluxes, which are calculated by EC, have to be corrected for setup-specific effects like attenuation in the high-frequency range. However, common methods for correcting such flux losses are mainly optimized for inert greenhouse gases like CO2 and methane or water vapor. In this study, we applied a selection of correction methods to measurements of total reactive nitrogen (ΣNr) conducted in different ecosystems using the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence detector (CLD). Average flux losses calculated by methods using measured cospectra and ogives were approximately 26 %–38 % for a seminatural peatland and about 16 %–22 % for a mixed forest. The investigation of the different methods showed that damping factors calculated with measured heat and gas flux cospectra using an empirical spectral transfer function were most reliable. Flux losses of ΣNr with this method were on the upper end of the median damping range, i.e., 38 % for the peatland site and 22 % for the forest site. Using modified Kaimal cospectra for damping estimation worked well for the forest site but underestimated damping for the peatland site by about 12 %. Correction factors of methods based on power spectra or on site-specific and instrumental parameters were mostly below 10 %. Power spectra of ΣNr were heavily affected – likely by white noise – and deviated substantially at lower frequencies from the respective temperature (power) spectra. Our study supports the use of an empirical method for estimating flux losses of ΣNr or any reactive nitrogen compound and the use of locally measured cospectra.