Atmospheric methane (CH₄) has been growing steadily since 2007 for reasons that are not well understood. Here we determine sources of methane using a technique informed by atmospheric measurements of CH₄ and its isotopologue ¹³CH₄. Measurements of ¹³CH₄ provide for better separation of microbial, fossil, and fire sources of methane than CH₄ measurements alone. Compared to previous assessments such as the Global Carbon Project, we find a larger microbial contribution to the post-2007 increase.

Stable water isotopes in marine boundary layer vapour are strongly influenced by the strength of air–sea fluxes. Here, we investigate a distinct vapour isotope signal observed in the warm sector of Southern Ocean cyclones. Single-process air parcel models are used together with high-resolution isotope-enabled simulations with the weather prediction model COSMOiso to improve our understanding of the importance of air–sea fluxes for the moisture cycling in the context of extratropical cyclones.

The Earth's water cycle contains the common H₂O molecule but also the less abundant, heavier HDO. We use their different physical properties to study tropical ice clouds in model simulations of the West African monsoon. Isotope signals reveal different processes through which ice clouds form and decay in deep-convective and widespread cirrus. Previously observed variations in upper-tropospheric vapour isotopes are explained by microphysical processes in convective updraughts and downdraughts.

We use high-resolution numerical isotope modelling and Lagrangian backward trajectories to identify moisture transport pathways and governing physical and dynamical processes that affect the free-tropospheric humidity and isotopic variability over the eastern subtropical North Atlantic. Furthermore, we conduct a thorough isotope modelling validation with aircraft and remote-sensing observations of water vapour isotopes.

For the first time, we included stable isotopes in the Lagrangian particle dispersion model FLEXPART to investigate firewood home heating aerosol. This is an innovative source apportionment methodology since comparison of stable isotope ratio model predictions with observations delivers quantitative understanding of atmospheric processes. The main outcome of this study is that the home heating aerosol in residential areas was not of remote origin.