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: The high surface productivity triggered by nutrient-rich Benguela upwelled waters results in significant enrichment of organic carbon in the sub-surface waters due to enhanced mineralization in the water column and benthic fluxes. Hence, microbial O2-consuming processes are promoted, driving oxygen depletion that favors trace gases i.e. methane (CH4) and nitrous oxide (N2O) production at relatively shallow depths. During upwelling, gas-rich subsurface waters are also transported towards the surface waters, enhancing trace gas sea-air fluxes. We investigate the variability of these fluxes on seasonal and shorter timescales to understand the intensity of the Benguela upwelling system in gas emissions. The data might serve as a base for projections under a changing climate. The fieldwork took place during the cruise M157 (August 4th – September 16th, 2019) onboard the R/V METEOR, which encompassed close-coastal and open ocean regions between Mindelo (Cape Verde) and Walvis Bay. The main transect lines around 18, 23 and 25°S represents the Angola-Benguela frontal zone, Walvis Bay and Lüderitz upwelling cells respectively, which are suggested to represent some regional hotspots of trace gas emissions to the atmosphere, in particular in the vicinity of the upwelling cells. The partial pressures of CH4, N2O, and CO2 as well as oxygen saturation in surface water were determined using IOW's self-built Mobile Equilibrator Sensor System (MESS). The system was described in details elsewhere (Sabbaghzadeh et al., 2021) but in brief, it consists of a custom-built equilibrator (combined shower-head/bubble type) with a water flow rate of about 5 l min-1 and an airflow rate of ~ 4 l min-1, which is linked to two off-axis integrated cavity output laser spectrometers (oa-ICOS, Los Gatos Instruments) for the detection of CH4 / CO2 and N2O / CO. Seawater was supplied by a pump installed at a water depth of about 6 m in the moon pool on board of RV METEOR. oa-ICOS sensors combine a highly specific infrared band laser with a set of reflective mirrors and achieve an effective absorption path length of several kilometers. This enables the detection of the trace gases with high accuracy. Three standard gases, provided by the central calibration lab of the European Integrated Carbon Observation System Research Infrastructure (ICOS RI) were used to calibrate the sensors almost daily throughout the entire expedition. To estimate sea-air gas fluxes, the atmospheric concentration of trace gases was also measured at several positions during the cruise using a tube with the inlet positioned to minimize ship contamination. All other ancillary parameters out of the MESS system were synchronized with D-ship data with a simultaneous data reduction to one-minute intervals.