Separating remote fetch and local mixing influences on vertical radon measurements in the lower atmosphere

TLDR: In this article, a 31-month data set of hourly radon measurements at 2 and 50 m is used to characterize the seasonality and diurnal variability of radon concentrations and gradients at a site near Sydney.

Abstract: Two-point radon gradients provide a direct, unambiguous measure of near-surface atmospheric mixing. A 31-month data set of hourly radon measurements at 2 and 50 m is used to characterize the seasonality and diurnal variability of radon concentrations and gradients at a site near Sydney. Vertical differencing allows separation of remote (fetch-related) effects on measured radon concentrations from those due to diurnal variations in the strength and extent of vertical mixing. Diurnal composites, grouped according to the maximum nocturnal radon gradient ( ΔC max ), reveal strong connections between radon, wind, temperature and mixing depth on subdiurnal timescales. Comparison of the bulk Richardson Number ( Ri B ) and the turbulence kinetic energy (TKE) with the radon-derived bulk diffusivity ( K B ) helps to elucidate the relationship between thermal stability, turbulence intensity and the resultant mixing. On nights with large  ΔC max ,  K B and TKE levels are low and  Ri B is well above the ‘critical’ value. Conversely, when  ΔC max is small,  K B and TKE levels are high and  Ri B is near zero. For intermediate  ΔC max , however,  Ri B remains small whereas TKE and  K B both indicate significantly reduced mixing. The relationship between stability and turbulence is therefore non-linear, with even mildly stable conditions being sufficient to suppress mixing. DOI: 10.1111/j.1600-0889.2011.00565.x