Description: Purge and trap system (P&T) with gas chromatograph to analysed methane and nitrous oxide. The in-house designed purge and trap system (P&T) determines methane (CH4) and nitrous oxide (N2O) concentrations in seawater samples by dynamic headspace method. After desorption of volatile compounds with an inert ultrahigh purity carrier gas (Helium 99.999 % and additional preparation by purifier – VICI – Valco Instruments Co. Inc.), the gases were analyzed by a gas chromatograph (GC) (Agilent 7890B), equipped with a flame ionization detector (FID) for CH4 measurements and an electron capture detector (ECD) for N2O measurements. The analytical system consists of four main components: Firstly, the purge chamber (200 x 24 mm), with integrated frit (porosity 2, Erich Eydam KG, Kiel, Germany) to purge the seawater with helium to displaced dissolved gases. Secondly, the trap (stainless steel, 700 mm x 1/8”, U-shaped), filled with HayeSep D (60/80 mesh, CS Chromatographie Service GmbH, Langerwehe, Germany) to enrich the relevant gas compounds. Thirdly, a connected calibration sampling loop for incorporate gas standards. Finally, the GC, which is connected via 10-port-2-pos valve, is used to separate CH4 and N2O with a special column circuit and a Deans-Switch unit, which regulates the carrier gas flow to the FID or ECD. The GC is run isothermally at 45 °C with a helium flow of 3 mL min-1 respectively 6 ml min-1 in column 1 and column 2 (Length 10 m or 30 m, HP-PLOT/Q+PT, Diam. 530 µm, film 40 µm). Column one allows the pre-separation and separation of undesirable compounds and through column two a further separation between CH4 and N2O is realized. In the FID-mode (Deans-Switch regulates the carrier gas flow to the FID) CH4 is detected during the first 5.0 minutes. After 5.8 minutes the GC system switches in the ECD-mode via Deans-Switch and N2O is detected (6.3 min total runtime). The FID works at 250 °C and the ECD at 345 °C. The FID uses Helium and ECD uses 10 % carbon dioxide in nitrogen as make up gas (in each case 25 ml/min). To ensure the measurement's accuracy, calibration standards are measured daily before and after the actual water samples. The calibration range is chosen, so that the samples are within the limits of the calibration methane (CH4) and nitrous oxide (N2O). The standards follow the same time sequence and pathway as the samples. For CH4 and N2O a standard deviation lowers than 1 % is desirable. After successful calibration, the water samples can be measured. The seawater (poisoned with mercury (II) chloride) is stored at 4 °C in 200 ml glass bottles without a headspace. A 100 ml glass syringe (volume calibrated, Fortuna®, Optina®, Poulten & Graf GmbH, Germany) pulls the seawater sample from the glass bottle without creating air bubbles. Around 10 mL of the sample are pushed out of the syringe over valve three into the purge vessel (filled to 1/5). The exact volume of the sample to be measured is calculated with the help of a caliper. The loaded carrier gas passes through a Nafion drying tube (type: MD-050-24S-2092917-06, Ansyco, Karlsruhe, Germany), removing water residues. Then a helium stream (around 100 mL min) transports the water free gases to the cooling trap, where they are capture and concentrated. After 10 minutes of purge time, the trap is placed in a hot water bath (95 °C) and via 10-port-2-pos valve the gas components travel to the GC. At this point in time, the column system is connected to the FID. Methane has a shorter retention time (5.0 min) than N2O, so it is possible to switch after 5.8 min to the ECD-mode to detect N2O (with a 6.3 min retention time). At the end of the measurement day, standards will be remeasured to determine the drift behaviour of the system. In addition, a blank (purged seawater) can be measured, whereby experience shows that the concentration is zero.

Description: Upwelling systems are significant sources of atmospheric nitrous oxide (N₂O). The Benguela Upwelling System is one of the most productive regions worldwide and a temporally variable source of N₂O. Strong O₂ depletions above the shelf are favoring periodically OMZ formations. We aimed to assess underlying N₂O production and consumption processes on different temporal and spatial scales during austral winter in the Benguela Upwelling System, when O₂-deficiency in the water column is relatively low. The fieldwork took place during the cruise M157 (August 4th – September 16th 2019) onboard the R/V METEOR. This expedition included four close-coastal regions around Walvis Bay at 23°S, which presented the lowest O₂ concentrations near the seafloor and thus may provide hotspots of N₂O production. Seawater was collected in 10 L free-flow bottles by using a rosette system equipped with conductivity-temperature-depth (CTD) sensors (SBE 911plus, Seabird-electronics, USA). Incubation experiments were performed using stable isotope ¹⁵N-tracers. Seawater samples for ¹⁵N-tracer incubations and natural abundance N₂O analysis were collected from 10 L free-flow bottles and filled bubble-free into 125 mL serum bottles. The samples for natural abundance N₂O analysis were immediately fixed with saturated HgCl₂ and stored in the dark. To perform the incubation, we added ¹⁵N-labeled NO₂-, NO₃⁻ and NH₄⁺ to estimate the in-situ N₂O production rates and associated reactions. To determine a single rate, the bottles were sacrificed after tracer addition, and within the time interval of 12 h, 24 h and 48 h by adding HgCl₂. Rates were calculated based on a linear regression over time. Total N₂O and natural abundance isotopologues of N₂O were analyzed by using an isotope ratio mass spectrometer (IRMS, Delta V Plus, Thermo Scientific). NO₂- production was additionally analyzed by transforming ¹⁵NO₂- to ¹⁵N₂O following the azide method after McIlvin & Altabet (2005) and the nitrogen isotope ratio of N₂O was measured by an IRMS. N₂ production was determined via an IRMS (Flash-EA-ConfloIV-DELTA V Advanced, Thermo Scientific) by injecting headspace from exetainers. The N₂O yield per nitrite produced and the N₂O yield during denitrification was calculated. Samples for natural abundance N₂O was sampled and measured in triplicates and is shown as an average with standard deviation (SD). In order to estimate the contribution of different N₂O producing pathways by major biological processes and the extent of N₂O reduction to N₂, the dual-isotope mapping approach was applied to natural abundance isotopologues of N₂O, which uses the relative position of background-subtracted N₂O samples in a δ¹⁵Nˢᴾ-N₂O vs. δ¹⁸O-N₂O diagram (Yu et al., 2020; Lewicka-Szczebak et al., 2020).