Abstract. In order to get a comprehensive picture of the distribution of nitrous oxide (N 2 O) in the North Atlantic Ocean, measurements of dissolved nitrous oxide were made during three cruises in the tropical, subtropical and cold-

[1] We use transit time distributions calculated from tracer data together with in situ measurements of N 2 O to estimate the concentration of biologically produced N 2 O ([N 2 O] xs ) and N 2 O production rates in the central North Atlantic Ocean. Our approach to estimation of N 2 O production rates integrates the effects of potentially varying production and decomposition mechanisms along the transport path of a water mass. We find that previously used approaches overestimate the oceanic equilibrium N 2 O concentrations by 8-13% and thus underestimate the strength of N 2 O sources in large parts of the water column. Thus the quantitative characteristics of the [N 2 O] xs /AOU relationship used as an indicator of nitrification are distorted. We developed a new parameterization of N 2 O production during nitrification depending linearly on AOU and exponentially on temperature and depth, which can be applied to calculate N 2 O production due to nitrification in the entire ocean including oxygen minimum zones.

[1] Differences in isotopic composition between the various sources of H 2 are large, but only a few measurements have been carried out to constrain them. Two conflicting values have been published for H 2 from biomass burning. Both rely on the assumption that the isotopic composition of H 2 should scale with the isotopic composition of the precipitation at the location where the biomass grew. Here we test this hypothesis using 18 wood samples collected from various locations around the globe. The sample locations cover a range of dD content of H 2 in precipitation, from below −120‰ in Siberia and Canada to −15‰ in Zimbabwe. The results confirm the predicted dependence of the H 2 isotopic composition on the precipitation in the sampling region. The water content itself is found to at most slightly affect the results. Furthermore, dD of H 2 depends strongly on combustion efficiency. Thus, the isotopic composition of H 2 from biomass burning shows a strong variability around the globe and between different stages of a fire. It is suggested that, rather than a global bulk number, global models that attempt to reproduce the spatial and temporal distribution of dD in H 2 should incorporate explicitly the variability of dD(H 2 ) from biomass burning on dD in precipitation.

Nitrous oxide (N2O) was measured during the first German SOLAS (Surface Ocean – Lower Atmosphere Study) cruise in the tropical North Atlantic Ocean on board R/V Meteor during October/November 2002. About 900 atmospheric and dissolved N2O measurements were performed with a semi‐continuous GC‐ECD system equipped with a seawater‐gas equilibrator. Surface waters along the main transect at 10°N showed no distinct longitudinal gradient. Instead, N2O saturations were highly variable ranging from 97% to 118% (in the Guinea Dome Area, 11°N, 24°W). When approaching the continental shelf of West Africa, N2O surface saturations went up to 113%. N2O saturations in the region of the equatorial upwelling (at 0–1.5°N, 23.5–26°W) were correlated with decreasing sea surface temperatures and showed saturations up to 109%. The overall mean N2O saturation was 104 ± 4% indicating that the tropical North Atlantic Ocean is a net source of atmospheric N2O.

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