Water vapor (H₂O_v) is an essential component of the Earth's atmosphere, playing critical roles in climate regulation, weather patterns, and the water cycle. Its sources primarily come from natural processes such as ocean evaporation and terrestrial evapotranspiration. However, during the fossil fuels (e.g., coal, petroleum, natural gas) combustion process, in addition to emitting substantial amounts of CO₂, they also generate significant amounts of water vapor as a byproduct (combustion-derived water vapor sources: CDWV).

In densely populated regions, CDWV can account for more than 10% of water vapor in the planetary boundary layer. Elevated atmospheric water vapor levels in megacities trigger a cascade of adverse environmental impacts: exacerbated air pollution, diminished solar radiation, more frequent extreme weather events and an amplified greenhouse effect. Accurately quantifying CDWV's contribution to total atmospheric moisture is therefore critical to evaluating anthropogenic impacts on the hydrological cycle—and the distinct isotopic signatures of CDWV versus natural water vapor offer a solution to this challenge.

A research team from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences used online observation techniques to characterize the δ¹⁸O_v, δ²H_v and d-excess_v isotopic compositions of water vapor produced by the combustion of coal, natural gas and liquefied gas, as well as water vapor emitted from vehicle exhaust.

The team found that δ¹⁸O_v in fossil fuel combustion-derived water vapor inherits the isotopic signature of atmospheric oxygen (23.9‰), a result of atmospheric O₂'s direct participation in the combustion process. This gives CDWV distinctly positive δ¹⁸O_v values and markedly negative d-excess_v characteristics. Variations in δ¹⁸O_v across different fossil fuels stem from differences in their chemical compositions, as well as the partitioning ratio of ¹⁸O atoms between CO₂ and H₂O during combustion with atmospheric oxygen.

Generally, the observed isotopic variation ranges of CDWV predominantly fall within the theoretical range. The CDWV can be distinguished from natural water vapor in the δ²H-δ¹⁸O space. Leveraging these unique traits—most notably CDWV's significantly positive δ¹⁸O_v and strongly negative d-excess_v— the researchers integrated precisely measured CDWV isotopic compositions with local energy consumption structures.

This method enables the precise isotopic characterization of locally emitted fossil fuel-derived water vapor, creating a critical tool for tracing urban atmospheric moisture sources, quantifying anthropogenic water vapor's contribution to air pollution and assessing its impacts on regional hydrological cycles.

The study was recently published in Environmental Science & Technology. This work was funded by the National Natural Science Foundation of China, the National Key Research and Development Program, among other sources.

Schematic diagram showing the composition of water vapor d-excess_v from anthropogenic sources and natural sources. (Image by IEE)