• radon;
  • baseline;
  • MLO;
  • troposphere;
  • Asian_outflow;
  • Pacific

[1] We use 7 years of hourly radon observations at Mauna Loa Observatory (MLO), together with 10-day back trajectories, to identify baseline air masses at the station. The amplitude of the annual MLO radon cycle, based on monthly means, was 98 mBq m–3 (39 –137 mBq m–3), with maximum values in February (90th percentile 330 mBq m–3) and minimum values in August (10th percentile 8.1 mBq m–3). The composite diurnal radon cycle (amplitude 49 mBq m–3) is discussed with reference to the influences of local flow features affecting the site, and a 3-hour diurnal sampling window (0730–1030 HST) is proposed for observing the least terrestrially influenced tropospheric air masses. A set of 763 baseline events is selected, using the proposed sampling window together with trajectory information, and presented along with measured radon concentrations as a supplement. This data set represents a resource for the selection of baseline events at MLO for use with a range of trace species. A reduced set of 196 “deep baseline” events occurring in the July–September window is also presented and discussed. The distribution (10th/50th/90th percentile) of radon in deep-baseline events (8.7/29.2/66.1 mBq m–3) was considerably lower than that for the overall set of 763 baseline events (12.3/40.8/104.1 mBq m–3). Results from a simple budget calculation, using sonde-derived mixing depths and literature-based estimates of oceanic radon flux and radon concentrations in the marine boundary layer, indicate that the main source of residual radon in the lower troposphere under baseline conditions at MLO is downward mixing from aged terrestrial air masses in the upper troposphere.