Differences of near-ground atmospheric Rn-222 concentration between urban and rural area with reference to microclimate diversity

Abstract

Abstract A unique 4-year dataset of hourly near-surface meteorological and atmospheric Radon-222 measurements made simultaneously at adjacent rural and urban sites in central Poland are presented and discussed. The annual, seasonal and diurnal variations of Rn-222 concentration were analysed. The magnitude of the urban heat island effect (UHI: difference in 2 m air temperature between the urban and rural regions) was found to be reasonably well correlated with traditional atmospheric stability indicators (i.e. wind speed and near-surface temperature gradient). To better quantify the influence of the UHI on the strength/depth of nocturnal mixing, the rural radon observations were used to classify the nocturnal mixing state regionally on a nightly basis, enabling a comparison between the rural and urban observations over four stability categories ranging from near-neutral to stable. Averaged over the entire dataset, near-neutral nocturnal conditions were characterised by 2 m wind speeds, U 2m , of ∼1.3 m s −1 and 2–0.2 m temperature gradients, ΔT, of ∼0.5 °C in the rural region, compared to U 2m = 1.15 m s −1 and ΔT = −0.24 °C in the urban region. By comparison, under regionally stable conditions U 2m = 0.6 m s −1 and ΔT = 1.5–2.0 °C in the rural region and U 2m = 0.8 m s −1 and ΔT = −0.25 °C in the urban region. Between near-neutral to stable conditions, the nocturnal UHI varied from ∼0.8 to 2.4 °C. The higher wind speeds under regionally stable conditions in the urban centre compared to the rural region (25 km distant) indicate that the slightly unstable conditions and UHI = 2.4 °C are sufficient to sustain a meso-scale circulation cell. The effect that the nocturnal urban heating has locally on the atmosphere's ability to dilute primary pollutants at night is indicated by the contrasting diurnal amplitude of radon concentration at each site under regionally stable conditions: ∼9 Bq m −3 in the rural region and ∼2 Bq m −3 in the urban region.