The water vapour isotopic composition (1H216O, H218O and 1H2H16O) of the Atlantic marine boundary layer has been measured from 5 research vessels between 2012 and 2015. Using laser spectroscopy analysers, measurements have been carried out continuously on samples collected 10–20 meter above sea level. All the datasets have been carefully calibrated against the international VSMOW-SLAP scale following the same protocol to build a homogeneous dataset covering the Atlantic Ocean between 4°S to 63°N. In addition, standard meteorological variables have been measured continuously, including sea surface temperatures using calibrated Thermo-Salinograph for most cruises. All calibrated observations are provided with 15-minute resolution. We also provide 6-hourly data to allow easier comparisons with simulations from the isotope-enabled Global Circulation Models. In addition, backwards trajectories from the HYSPLIT model are supplied every 6-hours for the position of our measurements.

Abstract. Nitrous oxide (N 2 O) is an important and strong greenhouse gas in the atmosphere. It is produced by microbes during nitrification and denitrification in terrestrial and aquatic ecosystems. The main sinks for N 2 O are turnover by denitrification and photolysis and photo-oxidation in the stratosphere. In the linear N=N=O molecule 15 N substitution is possible in two distinct positions: central and terminal. The respective molecules, 14 N 15 N 16 O and 15 N 14 N 16 O, are called isotopomers. It has been demonstrated that N 2 O produced by nitrifying or denitrifying microbes exhibits a different relative abundance of the isotopomers. Therefore, measurements of the site preference (difference in the abundance of the two isotopomers) in N 2 O can be used to determine the source of N 2 O, i.e., nitrification or denitrification. Recent instrument development allows for continuous positiondependent δ 15 N measurements at N 2 O concentrations relevant for studies of atmospheric chemistry. We present results from continuous incubation experiments with denitrifying bacteria, Pseudomonas fluorescens (producing and reducing N 2 O) and Pseudomonas chlororaphis (only producing N 2 O). The continuous measurements of N 2 O isotopomers reveals the transient isotope exchange among KNO 3 , N 2 O, and N 2 . We find bulk isotopic fractionation of −5.01 ‰ ± 1.20 for P. chlororaphis, in line with previous results for production from denitrification. For P. fluorescens, the bulk isotopic fractionation during production of N 2 O is −52.21 ‰ ± 9.28 and 8.77 ‰ ± 4.49 during N 2 O reduction.The site preference (SP) isotopic fractionation for P. chlororaphis is −3.42 ‰ ± 1.69. For P. fluorescens, the calculations result in SP isotopic fractionation values of 5.73 ‰ ± 5.26 during production of N 2 O and 2.41 ‰ ± 3.04 during reduction of N 2 O. In summary, we implemented continuous measurements of N 2 O isotopomers during incubation of denitrifying bacteria and believe that similar experiments will lead to a better understanding of denitrifying bacteria and N 2 O turnover in soils and sediments and ultimately hands-on knowledge on the biotic mechanisms behind greenhouse gas exchange of the globe.

Nitrous oxide (N₂O) is an important and strong greenhouse gas in the atmosphere and part of a feed-back loop with climate. N₂O is produced by microbes during nitrification and denitrification in terrestrial and aquatic ecosystems. The main sinks for N₂O are turnover by denitrification and photolysis and photo-oxidation in the stratosphere. The position of the isotope ¹⁵N in the linear N&amp;thinsp;=&amp;thinsp;N&amp;thinsp;=&amp;thinsp;O molecule can be distinguished between the central or terminal position (isotopomers of N₂O). It has been demonstrated that nitrifying and denitrifying microbes have a different relative preference for the terminal and central position. Therefore, measurements of the site preference in N₂O can be used to determine the source of N₂O i.e. nitrification or denitrification. Recent instrument development allows for continuous (on the order of days) position dependent <i>δ</i>¹⁵N measurements at N₂O concentrations relevant for studies of atmospheric chemistry. We present results from continuous incubation experiments with denitrifying bacteria, <i>Pseudomonas fluorescens</i> (producing and reducing N₂O) and <i>P. chlororaphis</i> (only producing N₂O). The continuous position dependent measurements reveal the transient pattern (KNO₃ to N₂O and N₂, respectively), which can be compared to previous reported site preference (SP) values. We find bulk isotope effects of &amp;minus;5.5&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;0.9 for <i>P. chlororaphis</i>. For <i>P. fluorescens</i>, the bulk isotope effect during production of N₂O is &amp;minus;50.4&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;9.3 and 8.5&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;3.7 during N₂O reduction. The values for <i>P. fluorescens</i> are in line with earlier findings, whereas the values for <i>P. chlororaphis</i> are larger than previously published <i>δ</i>¹⁵N_<i>bulk</i> measurements from production. The calculations of the SP isotope effect from the measurements of <i>P. chlororaphis</i> result in values of &amp;minus;6.6&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;1.8. For <i>P. fluorescens</i>, the calculations results in SP values of &amp;minus;5.7&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;5.6 during production of N₂O and 2.3&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;3.2 during reduction of N₂O. In summary, we implemented continuous measurements of N₂O isotopomers during incubation of denitrifying bacteria and believe that similar experiments will lead to a better understanding of denitrifying bacteria and N₂O turnover in soils and sediments and ultimately hands-on knowledge on the biotic mechanisms behind greenhouse gas exchange of the Globe.

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