Influence of turbulent mixing and air circulation in the lower atmosphere on fetch areas of selected WMO Global Atmosphere Watch baseline air pollution stations

The World Meteorological Organisation's Global Atmosphere Watch Programme (WMO GAW) coordinates activities of more than 20 ground stations around the world where atmospheric trace gases and aerosols of interest, as well as a range of meteorological variables, are monitored. The choice of recorded species reflects current interests in climate and pollution studies. Concentrations of recorded species are influenced by meteorological conditions as well as the species' surface emissions. Such influences vary on seasonal, synoptic and diurnal time scales, and local and regional emissions as well as the local orography are of primary concern. The combined influence of terrestrial source distribution, species' entrainment into air parcels analysed at the stations, and the parcels' transport in the lower atmosphere results in station-specific geographic areas (fetch areas) which are over- and under-represented in stations' records. Hence, interpreting species' variability requires that (at least) extremes of fetch areas be independently delineated and their temporal variability understood and documented. We present an approach to the definition of such fetch areas which utilises radon-222 (radon) hourly observations combined with back trajectories corresponding to these observations. Radon, a passive, naturally occurring radioactive tracer is well suited to delineate fetch areas as its half-life (3.8d) is, on one hand, longer than mixing processes in the lower atmosphere, and, on the other hand, short enough not be accumulated in the lower atmosphere. Compared with other trace gases, radon surface source is relatively constant on relevant temporal and spatial scales. Also, radon land emissions are at least two orders of magnitude higher than oceanic emissions thus allowing for an independent differentiation between air parcels predominantly influenced by land and oceanic emissions. The site selection included two long-established sites at Mauna Loa, Hawaii and Cape Grim, Tasmania, as well as a relatively new station located at Gosan, Jeju Island, South Korea. We have used hourly radon concentrations in air, which were measured in 2002-2006. The presented fetch areas have been derived from back trajectories corresponding to strong and weak radon signals (higher than 90th percentile and lower than 10th percentile of the corresponding radon concentration distributions). The figure shows an example of a strong land fetch for the Mauna Loa and Jeju sites. We discuss inter-annual and seasonal variations of fetch areas and report on the site-specific features of importance for the sampling strategy and interpretation of the signal variability. The trajectory-based fetch analysis is complemented by a cluster analysis which identifies major air paths and their contribution to identified fetch regions.