Increasing temperatures in the Arctic over recent decades have been interpreted as 'Arctic amplification' of the warming due to increasing concentrations of greenhouse gases. Understanding the mechanisms for such amplification has become a major research priority. Often the approach is 'event-based', focusing on the structure and frequency of 'cold-air outbreaks' (CAOs) and 'warm-air intrusions' (WAIs) which potentially exchange heat between the Arctic and the mid-latitudes. This paper describes results from a carefully planned observational campaign using the HALO high-altitude research aircraft, in which flight paths were planned in conjunction with weather forecast information so that certain air masses were sampled more than once, giving information on transformation of the air mass properties. Complementary model studies are used to allow interpretation of the observations and assessment of the model capability. This paper, beyond its specific conclusions, is, through its comprehensive combination of observations and modelling, likely to set the standard for future research on this important topic.

This study is of particular relevance not only to the geoscientific community but also to the broader public and media, as it touches on wildfire resilience, atmospheric health, and climate feedbacks—all central to contemporary environmental discourse. It presents a timely and highly relevant analysis of the stratospheric effects of the record-breaking 2023 Canadian wildfire season—an event of global environmental and public interest. Using a combination of satellite, airborne, and ground-based observations, alongside chemistry-transport model simulations, the study uncovers a surprising and counterintuitive result: despite the extreme scale and intensity of the fires, the vertical reach of smoke into the stratosphere was relatively shallow.