Climatic isotope signals in tree rings masked by air pollution: A case study conducted along the Mont Blanc Tunnel access road (Western Alps, Italy)

► Influence of air pollution on the climatic signal recorded in the δ13C chronologies. ► Air pollution loads strongly influence the photosynthetic process. ► Significant long-term changes in δ18O, δ15N and total N concentration. ► Trees respond to pollution events but less to the lack of pollution for short times.