Abstract
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 ΔH2 / ΔCO 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 in-situ observations.
Highlights
Molecular hydrogen (H2) is the second most prominent reduced trace gas in the atmosphere with an average abundance of about 530 parts per billion (Ehhalt, 1999; Novelli et al, 1999)
During the BARCA-Large Scale Biosphere–Atmosphere (LBA) project air samples were collected over the Amazonian tropical rainforest during the dry season www.atmos-chem-phys.net/13/9401/2013/
Mixing in the lower boundary layer leads to lower H2 mixing ratios in those samples due to soil uptake
Summary
Molecular hydrogen (H2) is the second most prominent reduced trace gas in the atmosphere with an average abundance of about 530 parts per billion (ppb) (Ehhalt, 1999; Novelli et al, 1999). The model study by Pieterse et al (2011), for example, shows a large plume of H2 over the Amazonian rainforest and other southern tropical areas in the austral spring and summer (cf their Fig. 6) This illustrates that the local H2 emissions from biomass burning in the tropical rainforest can be a major seasonal contributor to the total source and might locally affect atmospheric levels of trace gas concentrations. We characterise the isotopic signature of the biomass-burning source These two estimates support both bottom-up and top-down approaches to quantify the contribution of tropical biomass burning to the global and local H2 budget. 2. The H2 / CO emission ratio is derived and discussed, and the isotopic composition of H2 from biomass burning is studied in Sect.
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