Radon and particles: Complementary tracers of polluted air masses at coastal and island sites

Condensation nucleus (particle) and radon concentrations have been measured continuously at a number of coastal and island sites in programs measuring 'baseline' concentrations of trace constituents of the atmosphere. The definition of 'baseline' is somewhat species-dependent, but it is generally accepted that air samples recently perturbed by passage over large land masses, cities, etc. cannot be accepted as 'baseline'. It is shown that the addition of particle and radon concentrations to other criteria for characterization of an air mass, substantially improves the objectivity of 'baseline' selection. Wind direction and particle concentration are both useful for indicating likelihood of pollution from sources within a few km. Beyond that the fetch over land is enough for radon to be useful. For sources more than a few hundred km away, wind direction becomes a poor indicator of the source position, whereas radon and particle concentrations still yield reliable indications of land contact. This is evident at Macquarie Is, 2,000 km from Australia. For transport over distances of the order of 6,000 km, as from Asia to Hawaii, particles may be subject to production and scavenging processes which reduces their value as a pollution indicator. However radon, being inert, is unaffected and can still be detected. A combination of the three parameters, wind direction, radon and particle concentration, is therefore a reliable indicator of pollution of air samples over a wide range of distances