Separating remote fetch and local mixing influences on near-surface radon gradient mesaurements

Abstract

Predictions of weather and climate conditions are crucially reliant upon the fidelity of model parameterisations that represent the integrated behaviour of key physical processes responsible for transport and mixing in the atmospheric boundary layer. Distributions of trace gases and aerosols with respect to their natural or anthropogenic sources, as well as their removal through deposition, are also controlled by these processes. However, scientific understanding of many aspects of mixing and transport processes still requires substantial refinement, or even fundamental revision. In the stably stratified boundary layer vertical mixing processes remain poorly understood, particularly in very stable conditions when surface inversions can be extremely shallow and the thermodynamic structure of the lowest 50−100 m very complex. At the surface, for even simple investigations of atmospheric chemistry, there is a need to improve our understanding of the processes controlling the spatial/temporal variability in vertical exchange rates between the roughness elements (canopy/buildings) and the atmosphere above. Two-point radon gradients provide a direct, unambiguous measure of near-surface atmospheric mixing. A 31-month dataset of hourly radon measurements at 2 and 50 m is used to characterise 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. With the help of model-derived back trajectories and boundary layer depths, we were able to characterise the pronounced seasonal variability in afternoon surface radon concentrations in the Sydney region in terms of air mass fetch, contact time with land, ABL dilution and regional variability of the radon source function. Influences of coastal sea breeze circulations and the local topography were identified, superimposed upon the dominant seasonal variations in regional circulation patterns. Diurnal composites, grouped according to the maximum nocturnal radon gradient (ΔCmax), reveal strong connections between radon, wind, temperature and mixing depth on sub-diurnal timescales. Comparison of the bulk Richardson Number (RiB) and the turbulence kinetic energy (TKE) with the radon-derived bulk diffusivity (KB) helps to elucidate the relationship between thermal stability, turbulence intensity and the resultant mixing. On nights with large ΔCmax, KB and TKE levels are low and RiB is well above the “critical” value. Conversely, when ΔCmax is small, KB and TKE levels are high and RiB is near zero. For intermediate ΔCmax, however, RiB remains small whereas TKE and KB 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