Emission ratio and isotopic signatures of molecular hydrogen emissions from tropical biomass burning
In this study, we identify a biomass-burning signal in molecular\nhydrogen (H-2) over the Amazonian tropical rainforest. To quantify this\nsignal, we measure the mixing ratios of H-2 and several other species as\nwell as the H-2 isotopic composition in air samples that were collected\nin the BARCA (Balanco Atmosferico Regional de Carbono na Amazonia)\naircraft campaign during the dry season. We derive a relative H-2\nemission ratio with respect to carbon monoxide (CO) of 0.31 +/- 0.04 ppb\nppb(-1) and an isotopic source signature of -280 +/- 41 parts per\nthousand in the air masses influenced by tropical biomass burning. In\norder to retrieve a clear source signal that is not influenced by the\nsoil uptake of H-2, we exclude samples from the atmospheric boundary\nlayer. This procedure is supported by data from a global chemistry\ntransport model. The Delta H-2/Delta CO emission ratio is significantly\nlower than some earlier estimates for the tropical rainforest. In\naddition, our results confirm the lower values of the previously\nconflicting estimates of the H-2 isotopic source signature from biomass\nburning. These values for the emission ratio and isotopic source\nsignatures of H-2 from tropical biomass burning can be used in future\nbottom-up and top-down approaches aiming to constrain the strength of\nthe biomass-burning source for H-2. Hitherto, these two quantities\nrelied only on combustion experiments or on statistical relations, since\nno direct signal had been obtained from in-situ observations.