Abstract. Recent advances in laser spectrometry offer new opportunities to investigate the soil–atmosphere exchange of nitrous oxide. During two field campaigns conducted at a grassland site and a willow field, we tested the performance of a quantum cascade laser (QCL) connected to a newly developed automated chamber system against a conventional gas chromatography (GC) approach using the same chambers plus an automated gas sampling unit with septum capped vials and subsequent laboratory GC analysis. Through its high precision and time resolution, data of the QCL system were used for quantifying the commonly observed nonlinearity in concentration changes during chamber deployment, making the calculation of exchange fluxes more accurate by the application of exponential models. As expected, the curvature values in the concentration increase was higher during long (60 min) chamber closure times and under high-flux conditions (FN2O > 150 µg N m−2 h−1) than those values that were found when chambers were closed for only 10 min and/or when fluxes were in a typical range of 2 to 50 µg N m−2 h−1. Extremely low standard errors of fluxes, i.e., from ∼ 0.2 to 1.7 % of the flux value, were observed regardless of linear or exponential flux calculation when using QCL data. Thus, we recommend reducing chamber closure times to a maximum of 10 min when a fast-response analyzer is available and this type of chamber system is used to keep soil disturbance low and conditions around the chamber plot as natural as possible. Further, applying linear regression to a 3 min data window with rejecting the first 2 min after closure and a sampling time of every 5 s proved to be sufficient for robust flux determination while ensuring that standard errors of N2O fluxes were still on a relatively low level. Despite low signal-to-noise ratios, GC was still found to be a useful method to determine the mean the soil–atmosphere exchange of N2O on longer timescales during specific campaigns. Intriguingly, the consistency between GC and QCL-based campaign averages was better under low than under high N2O efflux conditions, although single flux values were highly scattered during the low efflux campaign. Furthermore, the QCL technology provides a useful tool to accurately investigate the highly debated topic of diurnal courses of N2O fluxes and its controlling factors. Our new chamber design protects the measurement spot from unintended shading and minimizes disturbance of throughfall, thereby complying with high quality requirements of long-term observation studies and research infrastructures.
Abstract. Interactions of reactive nitrogen (Nr) compounds between the atmosphere and the earth's surface play a key role in atmospheric chemistry and in understanding nutrient cycling of terrestrial ecosystems. While continuous observations of inert greenhouse gases through micrometeorological flux measurements have become a common procedure, information about temporal dynamics and longer-term budgets of Nr compounds is still extremely limited. Within the framework of the research projects NITROSPHERE and FORESTFLUX, field campaigns were carried out to investigate the biosphere–atmosphere exchange of selected Nr compounds over different land surfaces. The aim of the campaigns was to test and establish novel measurement techniques in eddy-covariance setups for continuous determination of surface fluxes of ammonia (NH3) and total reactive nitrogen (ΣNr) using two different analytical devices. While high-frequency measurements of NH3 were conducted with a quantum cascade laser (QCL) absorption spectrometer, a custom-built converter called Total Reactive Atmospheric Nitrogen Converter (TRANC) connected and operated upstream of a chemiluminescence detector (CLD) was used for the measurement of ΣNr. As high-resolution data of Nr surface–atmosphere exchange are still scarce but highly desired for testing and validating local inferential and larger-scale models, we provide access to campaign data including concentrations, fluxes, and ancillary measurements of meteorological parameters. Campaigns (n=4) were carried out in natural (forest) and semi-natural (peatland) ecosystem types. The published datasets stress the importance of recent advancements in laser spectrometry and help improve our understanding of the temporal variability of surface–atmosphere exchange in different ecosystems, thereby providing validation opportunities for inferential models simulating the exchange of reactive nitrogen. The dataset has been placed in the Zenodo repository (https://doi.org/10.5281/zenodo.4513854; Brümmer et al., 2022) and contains individual data files for each campaign.
<p>Ammonia (NH<sub>3</sub>)<sub> </sub>emissions stem mainly from agricultural sources and affect environment, climate and human health, thereby concomitantly reducing fertilizer nitrogen use efficiency. Reliable and representative measurements for typical field conditions as well as for potential mitigation options are needed as a basis for recommendations to policy makers and farmers. However, there is still uncertainty about the reliability of different NH<sub>3</sub> measurement methods for emissions from low intensity sources, such as synthetic fertilizers, and a lack of data on simultaneous comparative evaluations of different methods. The joint research project NH<sub>3</sub>-Min aims at comparing combinations of different NH<sub>3</sub> samplers and sensors (e.&#160;g. acid traps, dynamic chamber, laser-based techniques), flux calculation approaches (e.&#160;g. IHF, ZINST, bLs-Windtrax, eddy covariance), and scales (small scale multi-plots, field scale) to accurately quantify emissions and evaluate mitigation options (e.&#160;g. use of inhibitors, injection, form of nitrogen).</p> <p>This poster focuses primarily on the quantification of NH<sub>3</sub> concentrations and fluxes determined by a quantum cascade laser spectrometer (QCL) within an eddy covariance setup and the comparison to low-cost approaches, such as ALPHA passive diffusion samplers in combination with backwards Lagrangian stochastic (bLs) modelling (Windtrax). Measurements were carried out in Central Germany during the vegetation period in 2021 and 2022 in a winter wheat crop field, which received 3 urea fertilizer applications (to a total of 170 kg N) per year.</p> <p>First results showed that under high ambient NH<sub>3</sub> concentrations, time-integrated QCL values compared fairly well with those from ALPHA samplers. Under a low concentration regime, however, a significant underestimation of ALPHA values was observed, thereby providing a basis for an estimation of the method-specific detection limit. High-frequency losses using a co-spectral method in the process of eddy flux calculation were estimated to be in the range of 25 to 30%. We found clear diurnal flux courses and emission peaks after each urea application. The net loss of NH<sub>3</sub> summed up to 3.6 kg N ha<sup>-1</sup> over the whole measurement period (March&#160;&#8211; July). In further steps, we will evaluate the performance of the Windtrax model for estimating NH<sub>3</sub> losses from field-scale fertilizer applications and investigate the sensitivity of differences in input concentrations on modelled NH<sub>3</sub> emissions. Our study is a step towards better comparability and integration of different NH<sub>3</sub> measurement techniques and is expected to provide useful tools for robust estimations of NH<sub>3</sub> emission factors for synthetic fertilizer applications.</p> <p>The project is supported by funds of the German Government&#8216;s Special Purpose Fund held at Landwirtschaftliche Rentenbank.</p>
<p>Drained agriculturally used peatlands are hotspots of greenhouse gas emissions. Their exchange of reactive nitrogen (N<sub>r</sub>) with the atmosphere, however, has been less investigated, mainly due to challenges in accurate and feasible flux measurements. In this study, we present data from a two-year field campaign on intensively managed grassland on bog peat soil. We used a modified custom-built converter called TRANC with fast time response connected to a dual-channel chemiluminescence detector (CLD) for eddy flux measurements of total reactive nitrogen (&#8721;N<sub>r</sub>) and total odd nitrogen (NO<sub>y</sub>). The difference of the two channels was taken for an estimation of reduced nitrogen (NH<sub>x</sub>). We found good agreement between time-integrated N<sub>r</sub> concentrations from TRANC and a DELTA denuder sampler system. Over the two-year observation period, roughly two thirds of all recorded half-hourly &#8721;N<sub>r</sub> and NH<sub>x</sub> fluxes were positive, i.e. indicating ecosystem N<sub>r</sub> loss to the atmosphere. Emission peaks occurred after fertilization events and mainly during the warmer months. Monthly median &#8721;N<sub>r</sub> fluxes were ranging between -8 to 57 ng N m<sup>-2</sup> s<sup>-1</sup>. We further found an enhancement of emissions under dry conditions and clear diurnal patterns in all N<sub>r</sub> fluxes with peaks occurring around noon and close-to-neutral exchange during nighttime. The net loss of &#8721;N<sub>r</sub> to the atmosphere was calculated to reach 9.3 kg N ha<sup>-1</sup> in 2020 and 6.7 kg N ha<sup>-1</sup> in 2021. Our setup allowed for an estimation of NH<sub>x</sub> emission factors, at least for the organic inputs. Taking one week after each fertilization into account by summing up all recorded and gap-filled fluxes, emission factors for NH<sub>x</sub> were in the range of 1.2 to 2.5% of added fertilizer nitrogen. Our study demonstrates the applicability of the modified TRANC converter for eddy flux measurements and provides useful data for understanding N cycling in agroecosystems to derive sustainable management options for farmers and conservationists.</p>
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