Emission ratio and isotopic signatures of molecular hydrogen emissions from tropical biomass burning

In this study, we identify a biomass-burning signal in molecular hydrogen (H-2) over the Amazonian tropical rainforest. To quantify this signal, we measure the mixing ratios of H-2 and several other species as well as the H-2 isotopic composition in air samples that were collected in the BARCA (Balanco Atmosferico Regional de Carbono na Amazonia) aircraft campaign during the dry season. We derive a relative H-2 emission ratio with respect to carbon monoxide (CO) of 0.31 +/- 0.04 ppb ppb(-1) and an isotopic source signature of -280 +/- 41 parts per thousand in the air masses influenced by tropical biomass burning. In order to retrieve a clear source signal that is not influenced by the soil uptake of H-2, we exclude samples from the atmospheric boundary layer. This procedure is supported by data from a global chemistry transport model. The Delta H-2/Delta CO emission ratio is significantly lower than some earlier estimates for the tropical rainforest. In addition, our results confirm the lower values of the previously conflicting estimates of the H-2 isotopic source signature from biomass burning. These values for the emission ratio and isotopic source signatures of H-2 from tropical biomass burning can be used in future bottom-up and top-down approaches aiming to constrain the strength of the biomass-burning source for H-2. Hitherto, these two quantities relied only on combustion experiments or on statistical relations, since no direct signal had been obtained from in-situ observations.