Abstract. The Model of Atmospheric Transport and Chemistry (MATCH) is used tosimulate the transport of 222Rn using both European Centre for Medium-Range Weather Forecasts (ECMWF) winds and National Center for Environmental Prediction/National Center for Atmospheric Research (hereafter referred to as NCEP) reanalysis winds. These winds have the advantage of being based on observed winds but have the disadvantage that the subgrid-scale transport processes are not routinely archived. MATCH derives subgrid-scale mixing rates for the boundary layer using a nonlocal scheme and for moist convective mixing using one of two parameterizations (Tiedtke [1989] or Pan and Wu [1997]). This paper describes the ability of the model to recreate mixing rates of 222Rn MATCH increases Rn concentrations in the upper troposphere by 50% compared to not having moist convective mixing, while surface concentrations do not appear to be very sensitive to moist convection. In addition, differences between the upper tropospheric concentrations of radon predicted using the ECMWF and NCEP winds can be 30% for large areas of the globe, due to either differences in the forecast center winds themselves or the moist convective mixing schemes used in conjunction with them. This has implications for model simulations of radiatively and chemically important trace species in the atmosphere.

Abstract. A new design of two-filter radon detector has been developed for measurement of extremely low levels of radon in the harsh environments on board ships and remote islands. These were needed for the First Aerosol Characterization (ACE 1) multiplatform experiment in the Southern Ocean. By employing an internal recirculation system and a wire mesh screen as the second filter it has been possible to reduce the power consumption by as much as a factor of 10 and the weight and cost by a factor of 2 compared to current designs of comparable sensitivity. A very high efficiency of 0.38 count Review of the Two-Filter Radon DetectorThe new radon detectors for shipboard use belong to the class of instrument known as two-filter detectors. The name is derived from the mode of operation: an air sample is drawn through one filter which removes all radon and thoron decay products ("daughters"), then through a delay chamber in which some daughters are produced. Finally, the air passes through a second filter which retains the daughters. The daughters on the second filter have been produced in controlled conditions, so their number is proportional to the radon and thoron concentrations.There are many variations of the two-filter detector. The simplest design has an easily removable second filter. After a prescribed sampling period, the filter is removed and placed in an alpha particle detector, based typically on a zinc sulphide scintillatot [Thomas and LeClare, 1970]. The instrument responds to both radon (222Rn, half-life 3.82 days) and thoron (222Rn, half-life 55.6 s). If thoron is present, it decays to 2•2pb which has a half-life of 10.64 hours and causes an unwanted background count in an instrument intended to detect radon. It is possible to lower the thoron background count to an acceptable level by delaying air in the inlet by a few minutes. Since thoron has a half-life of less than a minute, it decays before entering the main delay chamber.This simplest form of two-filter detector can be automated by adding a filter changing mechanism. Hutter et al. [1990] have done this for an application where there was effectively no thoron in the air and continuous operation was essential. Their detector retains the well-defined time resolution of the simple detector. Its sensitivity per unit delay chamber volume was

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The accumulator technique was used to measure radon and thoron flux density at a variety of locations throughout Australia. This is the first such systematic study of Australia and, in the case of thoron, one of few such studies of any large land mass. Seasonally adjusted arithmetic mean flux densities from Australian soils were estimated to be 22 mBq m−2 s−1 (1.05 atom cm−2 s−1) for radon and 1.7 Bq m−2 s−1 (0.0135 atom cm−2 s−1) for thoron. Consideration of statistical sampling error, and systematic error with the accumulator method, leads to an error estimate of about ±20% for these numbers; projection of total flux to the atmosphere requires consideration of additional sources of error. Only modest correlations with variables easily measured in the field were observed. The strongest correlation was a positive one between flux density and gamma dose rate 1 m above ground. Weaker correlations were seen with soil temperature (positive) and soil moisture (negative at higher moistures). Radon and thoron flux density were strongly correlated, but only a weak correlation (negative) existed between them and vegetation. The amount of radon isotope released to the pore space seems particularly important for controlling the wide variation in observed flux densities, but it remains difficult to predict flux densities based on simple field measurements or information in conventional soil and geological maps.

Methane measurements, radon measurements, and air mass trajectories calculated for Mauna Lea Observatory (MLO) am examined to determine relationships among methane source/sink regions, flow pattems for MLO, and methane variations on the synoptic-to-seasonal scale. We present evidence that the methane seasonal cycle observed at MLO is in large part driven by seasonal variations in transport. Furthermore, the variability in methane mixing ratio at MLO is higher in winter than in summer because of greater variability in flow pattems. Ten-day back trajectories are classified according to wind speed and direction using cluster analysis to determine six typical transport regimes. The methane data are then grouped according to transport cluster. The median methane mixing ratio corresponding to tradewind flow was 17.2 ppbv (parts per billion by volume) lower than that corresponding to strong westerly flow. This difference is attributed to transport from source/sink regions, flow across the methane latitudinal gradient, and seasonality of flow pattems. Case studies utilizing individual trajectories and radon measurements to determine probable air parcel origins illustrate the effects of long-range transport on the methane mixing ratio at MLO. Changes in flow pattem from sink to source origins can result in a 50 ppbv rise in methane mixing ratio over a period of a few days. During winter, altemation of westerly winds, tradewinds and anticyclonically curving flows contributes to the large variability in the methane mixing ratio. During summer this variability is reduced with the cessation of strong westerly flows from methane source regions. In July and August, air parcels originate far from methane source regions and in the area of highest modeled OH concentration. At the same time, methane mixing ratios decrease to the lowest values for the year. In this way, the seasonality of flow patterns exerts a major influence on the observed seasonal cycle of methane at MLO.