Identifying ‘persistent temperature inversion’ events in aSubalpine Basin using Radon-222

Abstract. One year of meteorological and atmospheric radon observations in a topographically-complex Subalpine Basin are used to identify ‘persistent temperature inversion’ (PTI) events. PTI events play a key role in public health due to the accumulation of urban pollutants that they cause. Two identification techniques are compared: a new method, based on single-height radon measurements from a single centrally-located station, and an existing approach based on observations from eight weather stations around the Subalpine Basin. After describing the radon-based method (RBM), its efficacy is compared with that of the existing pseudo-vertical temperature gradient method (TGM). The RBM identified 6 PTI events over the year (4 in winter, 2 in autumn), a subset of the 17 events identified by the TGM. The RBM is demonstrated to be more consistent in its identification of PTI events, and more selective of persistent strongly stable conditions. Furthermore, its performance is seasonally independent. The comparatively poor performance of the TGM was attributed to seasonal inconsistencies in the validity of the method’s key assumptions (influenced by mesoscale processes, such as local drainage flows, nocturnal jets, and intermittent turbulence influence), and a lack of snow cover in the basin for the 2016–2017 winter period. Corresponding meteorological quantities for RBM PTI events (constituting 27 % of the autumn–winter cold season ), were well characterised. PTI wind speeds in the basin were consistently low over the whole diurnal cycle (typically 0.2–0.6 m s −1 ). The comparative efficacy of the RBM for PTI air quality assessment is demonstrated using hourly PM 10 observations throughout the year. Peak hourly mean PM 10 concentrations for winter (autumn) PTI events were underestimated by 13 µg m −3 (11 µg m −3 ) by the TGM compared with the RBM. Only the RBM indicated that nocturnal hourly mean PM 10 values in winter PTI events can exceed 100 µg m −3 , the upper threshold of low-level short-term PM 10 exposure according to World Health Organisation guidelines. The efficacy, simplicity and cost effectiveness of the RBM for identifying PTI events has the potential to make it a powerful tool for urban air quality management in complex terrain regions; for which it adds an additional dimension to contemporary atmospheric stability classification tools. Furthermore, the long-term consistency of the radon source function will enable the RBM to be used in the same way in future studies, enabling the relative magnitude of PTI events to be gauged, which is expected to assist with the assessment of public health risks.