Climatic and seasonal influences on radon time series in an environment of low anthropogenic activity

Between June 2003 and March 2008, radon concentrations were measured at hourly intervals in an environmentally-stable, rarely-visited basement, subject to minimal anthropogenic activity. Measurement periods totalled 1025 days (59% of the total investigation period), the longest continuous measurement period and the longest interval between measurement periods being 335 days (19%) and 271 days (15%) respectively. Mean daily, monthly (calendar) and annual concentrations were generated. For each calendar month, global averages (mean of measurements in corresponding months of at least three separate years during the study) were derived. Similar means were generated from meteorological data collected 10km from the measurement site. Overall mean and standard deviation (SD) radon concentrations were 73.0 Bq.m-3 (95% CI 0.8 Bq.m-3) and 62.7 Bq.m-3 respectively, corresponding mean and SD internal temperatures being 26.2 C (95% CI 0.02 C) and 1.7 C, respectively. Cross-correlation confirmed monthly mean radon concentrations as essentially independent of external temperature, atmospheric pressure, wind-speed, rainfall and relative humidity, with Correlation Coefficients, R2, all less than 0.06. Statistically-derived Seasonal Correction Factors (SCFs) are conventionally used to convert a radon concentration measurement over a sub-year period to a notional annual mean. It is recognised that this approach is not universally valid, a recent proposal using mean external temperature to derive a localised SCF, subject to positive temperature difference between the interior of the building under investigation and the environment. To test this, SCF sets were derived using monthly mean radon concentration, external temperature and internal/external temperature difference, using mean variance between these and the UK SCF set to quantise goodness-of-fit. Deriving SCFs from external temperature provides significantly better fit to the UK SCF set (mean variance = 0.09) than do radon concentration or temperature difference (mean variance = 0.48 and 0.47 respectively). Details of the experimental method and analysis results will be presented, together with further observations on seasonal variability of radon concentrations.