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Abstract:
In this study, we identify a biomass-burning signal
in molecular hydrogen (H2) over the Amazonian tropical
rainforest. To quantify this signal, we measure the mixing ratios
of H2 and several other species as well as the H2 isotopic
composition in air samples that were collected in the BARCA
(Balanço Atmosférico Regional de Carbono na Amazônia)
aircraft campaign during the dry season. We derive a relative
H2 emission ratio with respect to carbon monoxide (CO)
of 0.31±0.04 ppb ppb−1 and an isotopic source signature
of −280 ± 41‰ 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 H2, we exclude
samples from the atmospheric boundary layer. This procedure
is supported by data from a global chemistry transport
model. The 1H2 /1CO 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 H2 isotopic source
signature from biomass burning. These values for the emission
ratio and isotopic source signatures of H2 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 H2. Hitherto, these two quantities
relied only on combustion experiments or on statistical relations, since no direct signal had been obtained from insitu
observations.
