We report on a study using radon to investigate terrestrial influences (e.g., pollution, nutrients and pollen) on boundary layer and free tropospheric air over the Southern Ocean and Antarctica, and the mechanisms by which they arise. Seasonal radon cycles are characterised for 6 stations: Macquarie Island, King Sejong, Neumayer, Dumont d’Urville, Jang Bogo and Dome Concordia. Context is provided for this comparative study by north-south Southern Ocean radon transects by the RV Investigator. Synoptic air mass transport within the marine boundary layer (MBL) dominated radon seasonality at Macquarie Island, in the mid Southern Ocean. A combination of synoptic transport, tropospheric injection, and Antarctic outflow contributed to the seasonality at the sub-Antarctic site King Sejong station. Tropospheric subsidence and injection events delivered terrestrially-influenced air to the Southern Ocean MBL in the vicinity of the “Polar Front”. Antarctic katabatic outflow events were observed to modify trace atmospheric constituent characteristics of the MBL 100-200 km off the coast. Radon seasonality at coastal Antarctic sites was dominated by a combination of local radon sources in summer and subsidence of terrestrially-influenced tropospheric air, whereas those on the Antarctic Plateau were primarily controlled by tropospheric subsidence. Separate characterization of oceanic MBL and katabatic-flow air masses in summer at Dumont d’Urville revealed monthly mean differences of 5 ppbv in ozone and 0.3 ng m-3 in gaseous elemental mercury; primarily due to chemical processes occurring on the Antarctic Plateau. A comparison of observed radon with Antarctic radon simulations by global climate models over recent decades indicated that: (i) some models overestimate synoptic transport to Antarctica in the MBL, (ii) the seasonality of the Antarctic ice sheet needs to be better represented in models, (iii) coastal Antarctic radon sources need to be taken into account, and (iv) the underestimation of radon in subsiding tropospheric air needs to be investigated.