its principal source in the ground (rocks and soil) and distance from other sources that can locally or regionally affect ambient radon, such as bodies of water, mine or mill tailings, vegetation, and fossil-fuel combustion. The decrease in radon with height from the source is not simply tied to ground exhalation, nor is the variance a simple mathematical function. A number of studies have documented the decrease in ambient radon with increasing height above the ground and concluded that it is due predominantly to dilution by atmospheric mixing and turbulence (Gogolak and Beck 1980; Druilhet and others 1980; Bakulin and others 1970; Pearson and Jones 1966; Servant 1966; Moses and Pearson 1965; Pearson and Jones 1965). The ambient radon concentration can decrease by more than half in the first 10 m, but many studies show decreases of only one-tenth to one-third in the first 10 m. Concentrations of outdoor radon also change daily and seasonally in response to temperature, changes in atmospheric pressure, and precipitation.
Gesell (1983), Blanchard (1989), and Harley (1990) reviewed available studies of outdoor radon from around the world and observed consistent diurnal and seasonal trends. Generally, the diurnal pattern of outdoor radon concentration includes early morning and evening maxima related to cooling and air stability. Minimum concentrations typically occur in the afternoon because of