Surface-to-mountaintop transport characterised by radon observations at the Jungfraujoch

Atmospheric composition measurements at Jungfraujoch are affected intermittently by boundary-layer air which is brought to the station by processes including thermally driven (anabatic) mountain winds Using observations of radon-222, and a new objective analysis method, we quantify the land-surface influence at Jungfraujoch hour by hour and detect the presence of anabatic winds on a daily basis. During 2010-2011, anabatic winds occurred on 40% of days, but only from April to September. Anabatic wind days were associated with warmer air temperatures over a large fraction of Europe and with a shift in air-mass properties, even when comparing days with a similar mean radon concentration. Excluding days with anabatic winds, however, did not lead to a better definition of the unperturbed aerosol background than a definition based on radon alone. This implies that a radon threshold reliably excludes local influences from both anabatic and non-anabatic vertical-transport processes.