Institute of Chemistry, University of Campinas (UNICAMP), 13083-970, Campinas, São Paulo, Brazil.
Université de Pau et des Pays de L'Adour, E2S UPPA, CNRS, IPREM, Institut des Sciences Analytiques et de Physico-chimie pour L'Environnement et Les Matériaux, Pau, France.
Chemosphere. 2023 Oct;339:139779. doi: 10.1016/j.chemosphere.2023.139779. Epub 2023 Aug 9.
Mercury (Hg) emissions from forest fires, especially tropical forests such as the Amazonian forest, were shown to contribute significantly to the atmospheric mercury budget, but new methods are still necessary to improve the traceability and to reduce the great uncertainties related to this emission source. Recent studies have shown that the combustion process can result in Hg stable isotope fractionation that allows tracking coal combustion Hg emissions, as influenced by different factors such as combustion temperature. The main goal of the present study was, therefore, to investigate for the first time the potential of Hg stable isotopes to trace forest fire Hg emissions and pathways. More specifically, small-scale and a large scale prescribed forest fire experiments were conducted in the Brazilian Amazonian forest to study the impact of fire severity on Hg isotopic composition of litter, soil, and ash samples and associated Hg isotope fractionation pathways. In the small-scale experiment, no difference was found in the mercury isotopic composition of the samples collected before and after burning. In contrast, the larger-scale experiment resulted in significant mass dependent fractionation (MDF δHg) in soils and ash suggesting that higher combustion temperature influence Hg isotopic fractionation with the emission of lighter Hg isotopes to the atmosphere and enrichment with heavier Hg in ashes. As for coal combustion, mass independent fractionation was not observed. To our knowledge, these results are the first to highlight the potential of forest fires to cause Hg isotopic fractionation, depending on the fire severity. The results also allowed to establish an isotopic fingerprint for tropical forest fire Hg emissions that corresponds to a mixture of litter and soil Hg isotopic composition (resulting atmospheric δHg, ΔHg and ΔHg were -1.79 ± 0.24‰, -0.05 ± 0.04‰ and -0.45 ± 0.12‰, respectively).
汞（Hg）排放来自森林火灾，特别是亚马逊森林等热带森林，对大气汞预算有重要贡献，但仍需要新的方法来提高可追溯性，并减少与这种排放源相关的巨大不确定性。最近的研究表明，燃烧过程会导致汞稳定同位素分馏，从而可以跟踪煤燃烧汞排放，受燃烧温度等不同因素的影响。因此，本研究的主要目的是首次调查汞稳定同位素追踪森林火灾汞排放和途径的潜力。更具体地说，在巴西亚马逊森林中进行了小规模和大规模的计划森林火灾实验，以研究火灾严重程度对凋落物、土壤和灰分样品中 Hg 同位素组成的影响，以及相关的 Hg 同位素分馏途径。在小规模实验中，燃烧前后采集的样品中汞同位素组成没有差异。相比之下，较大规模的实验导致土壤和灰分中存在显著的质量依赖分馏（MDF δHg），表明较高的燃烧温度会影响 Hg 同位素分馏，较轻的 Hg 同位素排放到大气中，而较重的 Hg 同位素在灰分中富集。与煤燃烧不同，没有观察到质量独立分馏。据我们所知，这些结果首次强调了森林火灾根据火灾严重程度导致 Hg 同位素分馏的潜力。结果还为热带森林火灾 Hg 排放建立了一个同位素指纹，对应于凋落物和土壤 Hg 同位素组成的混合物（导致大气 δHg、ΔHg 和 ΔHg 分别为-1.79±0.24‰、-0.05±0.04‰和-0.45±0.12‰）。
