Radon (222Rn) as a tracer of cave air exchange

Abstract Radon ( 222 Rn) was employed as a tracer of seasonal, diurnal and hourly-scale cave air flow. Niedźwiedzia Górna Cave in the Kraków-Częstochowa Upland, having a well-studied thermal regime representative of similar underground spaces in the temperate climate, was chosen for the research. Relationships between changes in 222 Rn activity concentration in cave air and changes in cave microclimate parameters (air temperature) and meteorological parameters (temperature, pressure and wind speed) were compared. Data registered at one-hour intervals throughout a period of 8.5 months (from mid-December 2019 to the end of August 2020) were used. On a seasonal scale, the process of cave air exchange with the atmosphere takes place at the upper level of the cave in the first half of the year (from December to June) and at the bottom level - in the first quarter of the year (January-March). The air exchange is inhibited as the atmospheric air temperature equals the average temperature of the air at the lower level of the cave (+ 8.4 °C) in April and May. At the upper level, it is the atmospheric air temperature higher than the temperature inside the cave (+ 8.5 °C) which stops air exchange in July and August. On a diurnal scale, 222 Rn is an effective tracer of air exchange lasting about two weeks in winter (January-February) and as a range from 6 to 9 days in transitional periods in April and May. In an hourly cycle, effective air exchange takes place at daytime and ends at night in warmer parts of the cave and starts at night and continues until noon or the afternoon in colder parts.