The stable isotopic composition of water vapour provides information about moisture sources and processes difficult to obtain with traditional measurement techniques. Recently, it has been proposed that the D-excess of water vapour (d(v) = delta H-2 - 8 x delta O-18) can provide a diagnostic tracer of continental moisture recycling. However, D-excess exhibits a diurnal cycle that has been observed across a variety of ecosystems and may be influenced by a range of processes beyond regional-scale moisture recycling, including local evaporation (ET) fluxes. There is a lack of measurements of D-excess in evaporation (ET) fluxes, which has made it difficult to assess how ET fluxes modify the Dexcess in water vapour (d(v)). With this in mind, we employed a chamber-based approach to directly measure D-excess in ET (d(ET)) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime d(v) values. The low d(ET) observed here was sourced from a soil water pool that had undergone an extended drying period, leading to low D-excess in the soil moisture pool. A strong correlation between daytime d(v) and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime d(v) variability. During the early evening, ET fluxes into a shallow nocturnal inversion layer caused a lowering of d(v) values near the surface. In addition, transient mixing of vapour with a higher D-excess from above the nocturnal inversion modified these values, causing large variability during the night. These results indicate d(ET) can generally be expected to show large spatial and temporal variability and to depend on the soil moisture state. For long periods between rain events, common in semi-arid environments, ET would be expected to impose negative forcing on the surface d(v). Spatial and temporal variability of D-excess in ET fluxes therefore needs to be considered when using d(v) to study moisture recycling and during extended dry periods with weak moisture recycling may act as a tracer of the relative humidity at the oceanic moisture source.

1 Water Desalination and Reuse Centre, King Abdullah University of Science and Technology 8 (KAUST), Jeddah, Saudi Arabia 9 2 Department of Civil and Environmental Engineering, University of New South Wales, Sydney, 10 Australia 11 3 Australian Nuclear Science and Technology Organization, Sydney, New South Wales, 12 Australia 13 4 Department of Earth Sciences, Indiana University–Purdue University Indianapolis (IUPUI), 14 Indianapolis 15 16

Radon (Rn-222) is a powerful natural tracer of mixing and exchange processes in the atmospheric boundary layer. The authors present and discuss the main features of a unique dataset of 50 high-resolution vertical radon profiles up to 3500 m above ground level, obtained in clear and cloudy daytime terrestrial boundary layers over an inland rural site in Australia using an instrumented motorized research glider. It is demonstrated that boundary layer radon profiles frequently exhibit a complex layered structure as a result of mixing and exchange processes of varying strengths and extents working in clear and cloudy conditions within the context of the diurnal cycle and the synoptic meteorology. Normalized aircraft radon measurements are presented, revealing the characteristic structure and variability of three major classes of daytime boundary layer: 1) dry convective boundary layers, 2) mixed layers topped with residual layers, and 3) convective boundary layers topped with coupled nonprecipitating clouds. Robust and unambiguous signatures of important atmospheric processes in the boundary layer are identifiable in the radon profiles, including "top-down" mixing associated with entrainment in clear-sky cases and strongly enhanced venting and subcloud-layer mixing when substantial active cumulus are present. In poorly mixed conditions, radon gradients in the daytime atmospheric surface layer significantly exceed those predicted by Monin-Obukhov similarity theory. In two case studies, it is demonstrated for the first time that a sequence of vertical radon profiles measured over the course of a single day can consistently reproduce major structural features of the evolving boundary layer.

The stable isotopic composition of atmospheric water vapour is related to the hydrological processes that occuralong the back trajectory of an air mass, including evaporation at the moisture source, atmospheric mixing andprecipitation. Incorporation of stable water isotopes into weather and climate prediction models therefore has thepotential to aid in their evaluation, calibration and improvement, but this requires the availability of continuous timeseries of isotope data to compare with isotopically-enabled simulations. We present an analysis of a 15-month highresolution time series of atmospheric water vapour stable isotope measurements at a site near Sydney, Australia.The deuterium isotope exhibits a large amount of variability (-60 to -180 per mille), reflecting the range of weatherconditions encountered at this near-coastal measurement site. Using collocated meteorological and surface radonmeasurements, together with back trajectory analysis, we investigate major processes that contribute to variabilityin the stable isotope value of water vapour in the Sydney region. The analysis indicates that the lowest isotopevalues are generally associated with cold fronts passing over the Sydney region. When a cold front passes over ornear the measurement site, the deuterium isotope value can be observed to change by up to 100 per mille within thespace of a few hours. In addition, cold frontal passages with contrasting moisture source and precipitation historiesexhibit systematic differences in water vapour stable isotope signals as they pass over Sydney. On the other hand,higher and more slowly changing isotope values are generally associated with anticyclonic conditions. From thisstudy it is clear that the variations in the stable isotope values are strongly influenced by the hydrological historyof air parcels at a synoptic scale.

A time-dependent map of radon-222 flux density at the Australian land surface has been constructed with a spatial resolution of 0.05° and temporal resolution of one month. Radon flux density was calculated from a simple model utilising data from national gamma-ray aerial surveys; modelled soil moisture, available from 1900 in near real-time; and maps of soil properties. The model was calibrated against a data set of accumulation chamber measurements, thereby constraining it with experimental data. A notable application of the map is in atmospheric mixing and transport studies which use radon as a tracer, where it is a clear improvement on the common assumption of uniform radon flux density.

Estimation of the total amount of the natural radioactive tracer radon-222 (radon) in a vertical column through the troposphere is a critical step in the process of calculating regionally integrated emissions of important greenhouse gases using radon-calibrated budget techniques. As continuous long-term radon time series used for such calculations are typically gathered at sites located at or near the surface, a rigorous column radon estimate would require knowledge of the vertical distribution of radon through the atmospheric boundary layer (ABL). The most frequent approach to addressing this issue is to assume a uniform radon profile within the ABL, and no radon in the free atmosphere above. This study aims at refining these traditional assumptions by presenting vertical integrations of high-resolution radon profiles, gathered using a motorised glider in and above daytime boundary layers over rural inland Australia under a range of stability and cloud conditions.

The vertical distribution of the natural radioactive tracer radon‐222 through the atmospheric boundary layer (ABL) is a quantitative indicator of exchange and mixing between the terrestrial surface and the lower atmosphere. Radon is therefore a useful tool in the effort to reduce systematic errors in the representation of boundary layer processes in weather and climate prediction models. We present surface time series and vertical profiles of radon and meteorological quantities in daytime boundary layers over rural inland Australia, obtained during winter and summer field campaigns with samplers based on the ground and mounted on motorized gliders. Cases range from light-wind strong convection to high-wind near-neutral conditions, and from clear skies to moderately developed fair-weather cumulus and stratocumulus. Due to its 3.8-day half‐life, radon concentrations in the free atmosphere are constrained to be 1‐3 orders of magnitude lower than near-surface values. This ensures that a large radon jump is always maintained between the ABL and the air high above. As a consequence of the “top-down” mixing process, radon displays a range of gradients in the upper mixed layer of the ABL that are sensitive to the degree of exchange (entrainment) across the interface. In the presence of active boundary layer clouds, the venting of air from the sub-cloud layer is strongly enhanced, leading to radon concentrations that remain high within the main part of the cloud layer and only diminish towards its top. Vertical profiles from a number of flight case studies are related to the time series of surface radon concentrations, which exhibit a huge variability in diurnal amplitude between periods of light-wind clear-sky conditions (large amplitude) and high-wind cloudy conditions (small amplitude).