Radon-based atmospheric stability classification in contrasting sub-Alpine and sub-Mediterranean environments

Highlights: • A radon-based technique is used to classify atmospheric stability in summer for two contrasting topographic settings. • Meteorological conditions associated with each relative stability class differ markedly between basin and foothills sites. • Diurnal radon cycles show that the capability of the atmosphere to dilute urban emissions is less in a basin environment. • Radon exhalation from flysch and carbonate rocks is about two-fold higher than over sea and lake basin sediments. - Abstract: A recently-developed radon-based technique is used to investigate relative changes in summertime atmospheric stability at two sites in Slovenia with contrasting geographical settings. Although atmospheric stability for both sites (50 km apart) was shown to be governed by similar synoptic conditions, their contrasting settings caused differences in mixing conditions for each stability category. At the urban sub-Alpine site Ljubljana, situated within a topographic basin, wind speeds associated with the most stable conditions were 0.2–0.3 m s⁻¹. By comparison, corresponding wind speeds for the near-coastal sub-Mediterranean site Ajdovščina, located at the foothills of the Trnovski gozd barrier, were 0–0.2 m s⁻¹. The wind direction at Ljubljana under stable conditions (∼80°) was consistent with drainage flow into the basin along the Sava River valley. The corresponding wind direction at Ajdovščina was 20–40°, consistent with gentle katabatic drainage from the flanks of the Trnovski gozd barrier. After removing fetch effects on radon variability at each site, a large contrast in local contributions to the radon signal was noted: the diurnal amplitude of the local radon signal increased from ∼24 Bq m⁻³ at Ljubljana to ∼47 Bq m⁻³ at Ajdovščina. This difference was attributed to a greater nocturnal radon accumulation rate at Ajdovščina (3.5 Bq m⁻³ h⁻¹ vs 2.1 Bq m⁻³ h⁻¹) due to higher radon fluxes from flysch and carbonate rocks compared to the sea and lake sediments in the Ljubljana Basin. The ability of radon to consistently distinguish subtle changes in atmospheric mixing at sites with contrasting topographic settings indicates that it will be a powerful tool for characterising air quality in these complex environments. Specifically, diurnal radon cycles indicate that the capability of the atmosphere to dilute primary pollutants is considerably less in the basin environment.