Alastair G. Williams

The Vertical Distribution of Radon in Clear and Cloudy Daytime Terrestrial Boundary Layers

Abstract Radon (222Rn) 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 ...

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

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 (fetchrelated) 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 (Cmax), reveal strong connections between radon, wind, temperature and mixing depth on subdiurnal 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 Cmax, KB and TKE levels are low and RiB is well above the ‘critical’ value. Conversely, when Cmax is small, KB and TKE levels are high and RiB is near zero. For intermediate Cmax, 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.

Constraining annual and seasonal radon-222 flux density from the Southern Ocean using radon-222 concentrations in the boundary layer at Cape Grim

ABSTRACT Radon concentrations measured between 2001 and 2008 in marine air at Cape Grim, a baseline site in north-western Tasmania, are used to constrain the radon flux density from the Southern Ocean. A method is described for selecting hourly radon concentrations that are least perturbed by land emissions and dilution by the free troposphere. The distribution of subsequent radon flux density estimates is representative of a large area of the Southern Ocean, an important fetch region for Southern Hemisphere climate and air pollution studies. The annual mean flux density (0.27 mBq m−2 s−1) compares well with the mean of the limited number of spot measurements previously conducted in the Southern Ocean (0.24 mBq m−2 s−1), and to some spot measurements made in other oceanic regions. However, a number of spot measurements in other oceanic regions, as well as most oceanic radon flux density values assumed for modelling studies and intercomparisons, are considerably lower than the mean reported here. The reported radon flux varies with seasons and, in summer, with latitude. It also shows a quadratic dependence on wind speed and significant wave height, as postulated and measured by others, which seems to support our assumption that the selected least perturbed radon concentrations were in equilibrium with the oceanic radon source. By comparing the least perturbed radon observations in 2002–2003 with corresponding ‘TransCom’ model intercomparison results, the best agreement is found when assuming a normally distributed radon flux density with σ=0.075 mBq m−2 s−1.

Comparisons of aircraft, ship, and buoy radiation and SST measurements from TOGA COARE

Mean radiative fluxes and sea surface temperature measured by the five Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) boundary layer research aircraft were compared with each other and with surface measurements from moored buoys and ships. The basic data-processing techniques for radiative flux and sea surface temperature (SST) measurements from an aircraft were reviewed, and an empirical optimization method to calibrate an Eppley pyrgeometer was introduced. On the basis of aircraft wingtip-to-wingtip comparison periods, the processed aircraft downwelling shortwave and longwave irradiance and SST measurements were found to agree to 28±18 W m−2, 9±4 W m−2, and 0.7±0.4°C, respectively. By using the same comparison periods, empirical corrections that removed systematic errors in the aircraft data were determined. Application of these corrections improved the wingtip comparison accuracy to 3±16 W m−2, 1±4 W m−2, and 0.1±0.3°C, respectively. Comparisons between the (fully corrected) aircraft and the surface platform measurements revealed the aircraft data to be slightly greater for all three parameters. The agreement was around 3±37 W m−2, 3±6 W m−2, and 0.3±0.5°C for shortwave irradiance, longwave irradiance, and SST, respectively. (Detailed comparison results were provided for each individual ship and buoy.) After applying the aircraft empirical corrections the level of accuracy was near the COARE objectives.

Estimating the Asian radon flux density and its latitudinal gradient in winter using ground-based radon observations at Sado Island

Terrestrial radon-222 flux density for the Asian continent, integrated over distances of 4500 km, is estimated in two 20. latitudinal bands centred on 48.8.N and 63.2.N. The evaluation is based on three years of wintertime radon measurements at Sado Island, Japan, together with meteorological and trajectory information. A selection of 18% of observations are suitable for evaluation of an analytical expression for the continental surface flux. Various meteorological assumptions are discussed; it is found that there is a substantial effect of increased complexity of the formulation on the flux estimates obtained. The distribution of spatially integrated radon flux over the Asian landmass is reported for the first time. Expressed as geometric means and 1±-ranges, estimated fluxes are 14.1 mBq m.2 s.1 (1±-range: 18 mBq m-2 s-1) and 8.4 mBq m-2 s-1 (1±-range: 10 mBq m-2 s-1) for the lower and higher latitude bands. These results constitute an annual minimum in flux densities for these regions, and are higher than previously reported. The existence of a latitudinal gradient in the continental radon source function is confirmed; the present estimate for Asia (-0.39 mBq m-2 s-1 per degree of latitude) is in agreement with the northern hemisphere terrestrial radon flux gradient proposed previously.

Comparisons of aircraft, ship, and buoy meteorological measurements from TOGA COARE

Comparisons of mean ambient temperature, specific humidity, static pressure, and horizontal wind from the five Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) boundary layer aircraft were obtained from 38 two- and three-aircraft, close-formation, level runs. These, together with consideration of surface measurements from buoys and ships, led to proposed empirical corrections for the aircrafts temperature, humidity, and pressure measurements, minimizing the systematic errors between the aircraft data sets. The aircraft-measured winds were also compared. The TOGA COARE bulk flux algorithm was used to extrapolate the low-level aircraft data to the individual ship and buoy sensor heights for 267 overflight comparisons. In addition, all low-level aircraft data and corresponding ship and buoy data from boundary layer missions were extracted and adjusted to a 10-m reference height. The recommended aircraft corrections bring the aircraft-ship-buoy data sets into better agreement, resulting in a consistent data set for air-sea interaction analyses. Frequency distributions of the 10-m aircraft, ship, and buoy data from the boundary layer missions also agree.

Radon as a tracer of atmospheric influences on traffic-related air pollution in a small inland city

One year of radon, benzene and carbon monoxide (CO) concentrations were analysed to characterise the combined influences of variations in traffic density and meteorological conditions on urban air quality in Bern, Switzerland. A recently developed radon-based stability categorisation technique was adapted to account for seasonal changes in day length and reduction in the local radon flux due to snow/ice cover and high soil moisture. Diurnal pollutant cycles were shown to result from an interplay between variations in surface emissions (traffic density), the depth of the nocturnal atmospheric mixing layer (dilution) and local horizontal advection of cleaner air from outside the central urban/industrial area of this small compact inland city. Substantial seasonal differences in the timing and duration of peak pollutant concentrations in the diurnal cycle were attributable to changes in day length and the switching to/from daylight-savings time in relation to traffic patterns. In summer, average peak benzene concentrations (0.62 ppb) occurred in the morning and remained above 0.5 ppb for 2 hours, whereas in winter average peak concentrations (0.85 ppb) occurred in the evening and remained above 0.5 ppb for 9 hours. Under stable conditions in winter, average peak benzene concentrations (1.1 ppb) were 120% higher than for well-mixed conditions (0.5 ppb). By comparison, summertime peak benzene concentrations increased by 53% from well-mixed (0.45 ppb) to stable nocturnal conditions (0.7 ppb). An idealised box model incorporating a simple advection term was used to derive a nocturnal mixing length scale based on radon, and then inverted to simulate diurnal benzene and CO emission variations at the city centre. This method effectively removes the influences of local horizontal advection and stability-related vertical dilution from the emissions signal, enabling a direct comparison with hourly traffic density. With the advection term calibrated appropriately, excellent results were obtained, with high regression coefficients in spring and summer for both benzene (r2 ~0.90–0.96) and CO (r2 ~0.88–0.98) in the two highest stability categories. Weaker regressions in winter likely indicate additional contributions from combustion sources unrelated to vehicular emissions. Average vehicular emissions during daylight hours were estimated to be around 0.503 (542) kg km−2 h−1 for benzene (CO) in the Bern city centre.

Nuclear tools for characterising radiological dispersion in complex terrain: evaluation of regulatory and emergency response models

Routine operations of a nuclear research reactor and its facilities offer opportunities for collection of rare environmental tracer datasets which can be used for atmospheric dispersion model evaluation studies. The HIFAR reactor near Sydney, Australia, routinely emits the radioactive noble gas 41Ar, and other radionuclides such as 133Xe and 135Xe are also emitted from nearby radiopharmaceutical production facilities. Despite extremely low emission levels of these gases, they are nevertheless detectable using state-of-the-art technology, and sensitive detectors have been placed at four locations in the surrounding region which features complex terrain. The high research potential of this unique dataset is illustrated in the current study, in which predictions from two atmospheric dispersion models used for emergency response are compared with 41Ar peak observations from the detector network under a range of stability conditions, and long-term integrated data is also compared with a routine impact assessment model.

Baseline characterisation of source contributions to daily-integrated PM2.5 observations at Cape Grim using Radon-222

We discuss 15 years (2000-2015) of daily-integrated PM2.5 samples from the Cape Grim Station. Ion beam analysis and positive matrix factorisation are used to identify six source-type fingerprints: fresh sea salt (57%); secondary sulfate (14%); smoke (13%); aged sea salt (12%); soil dust (2.4%); and industrial metals (1.5%). An existing hourly radon-only baseline selection technique is modified for use with the daily-integrated observations. Results were not significantly different for days on which >20 hours were below the baseline radon threshold compared with days when all 24 hours satisfied the baseline criteria. This relaxed daily baseline criteria increased the number of samples for analysis by almost a factor of two. Two radon baseline thresholds were tested: historic (100 mBq m-3), and revised (50 mBq m-3). Median aerosol concentrations were similar for both radon thresholds, but maximum values were higher for the 100 mBq m-3 threshold. Back trajectories indicated more interaction with southern Australia and the Antarctic coastline for air masses selected with the 100 mBq m-3 threshold. Radon-only baseline selection using the 50 mBq m-3 threshold was more selective of minimal terrestrial influence than a similar recent study using wind direction and back trajectories. The ratio of concentrations between terrestrial and baseline days for the primary sources soil, smoke and industrial metals was 3.4, 2.6, and 5.5, respectively. Seasonal cycles of soil dust had a summer maximum and winter minimum. Seasonal cycles of smoke were of similar amplitude for terrestrial and baseline events, but of completely different shape: peaking in autumn and spring for terrestrial events, compared to summer for baseline conditions. Seasonal cycles of industrial metals had a summer maximum and winter minimum. A significant fraction of the Cape Grim baseline smoke and industrial metal contributions appeared to be derived from long-term transport (>3 weeks since last terrestrial influence).