Measurements of Combustion-Derived Water Vapor Isotopic Composition from Different Fossil Fuels

Fossil fuel combustion not only emits large amounts of CO₂ but also generates significant water vapor (H₂O_v). Currently, research on the isotopic composition of combustion-derived water vapor (CDWV) predominantly relies on theoretical calculations. This has hindered the comprehensive understanding of the characteristic patterns in isotopic variations and associated isotopic fractionation mechanisms during CDWV formation. In this study, we systematically investigated the variation characteristics in the isotopic composition of water vapor emitted from the combustion of various fuels and initially established the characteristic isotopic signatures of CDWV. The δ¹⁸O-H₂O_v produced by fossil fuel combustion inherits the isotopic signature of atmospheric O₂, resulting in a significantly positive δ¹⁸O_v value (> +10‰) and markedly negative d-excess_v (< -200‰) characteristic. Through this study, we demonstrated the reliability and universality of the theoretical values of CDWV isotopic composition. Through experiments, we observed the dynamic variations in isotopic compositions and water vapor mixing ratios during fossil fuel combustion processes. The distinct isotopic disparity between the CDWV and the natural water vapor enables us to use d-excess_v as a quantitative tracer for estimating the contribution of anthropogenic fossil fuel combustion to atmospheric moisture.