Responses of the atmospheric concentration of radon-222 to the vertical mixing and spatial transportation

Radon-222 (Rn-222) has traditionally been used as an atmospheric tracer for studying air masses and planetary boundary-layer evolution. However, there are various factors that influence its atmospheric concentration. Therefore, we investigated the variability of the atmospheric radon concentration in response to the vertical air mixing and spatial transport in a boreal forest environment in northern Europe. Long-term Rn-222 data collected at the SMEAR II station in southern Finland during 2000-2006 were analysed along with meteorological data, mixing layer height retrievals and air-mass back trajectory information. The daily mean atmospheric radon concentration followed a log-normal distribution within the range <0.1-11 Bq m(-3), with the geometric mean of 2.5 Bq m(-3) and a geometric standard deviation of 1.7 Bq m(3). In spring, summer, autumn and winter, the daily mean concentrations were 1.7, 2.7, 2.8 and 2.7 Bq m(-3), respectively. The low, spring radon concentration was especially attributed to the joint effect of enhanced vertical mixing due to the increasing solar irradiance and inhibited local emissions due to snow thawing. The lowest atmospheric radon concentration was observed with northwesterly winds and high radon concentrations with southeasterly winds, which were associated with the marine and continental origins of air masses, respectively. The atmospheric radon concentration was in general inversely proportional to the mixing layer height. However, the ambient temperature and small-scale turbulent mixing were observed to disturb this relationship. The evolution of turbulence within the mixing layer was expected to be a key explanation for the delay in the response of the atmospheric radon concentration to the changes in the mixing layer thickness. Radon is a valuable naturally-occurring tracer for studying boundary layer mixing processes and transport patterns, especially when the mixing layer is fully developed. However, complementing information, provided by understanding the variability of the atmospheric radon concentration, is of high necessity to be taken into consideration for realistically interpreting the evolution of air masses or planetary boundary layer.