Characterizing atmospheric controls on winter urban pollution in a topographic basin setting using Radon-222

Using hourly meteorological, air quality and radon observations over two winters (2016–2017 and 2017–2018), we combine diurnal and synoptic timescale radon-based mixing classification techniques, for the first time, to better understand air quality variability in the sub-Alpine city of Ljubljana. Of the two winters, only the second experienced significant snow cover (>1 month). A total of six mixing classes were defined for each winter: 5 diurnal and 1 synoptic (“persistent temperature inversion”, PTI); the latter being internally categorised as “strong” or “weak”. Diurnal and synoptic changes in mixing state played an important role in air quality variability in both winters. Little diurnal accumulation of local pollutants (CO, NO2, PM10 and BC) was observed for wind speeds ≥1.5 m s−1 (mixing classes #1 to #3). Under these windy conditions significant remote contributions to SO2 and O3 concentrations in Ljubljana were observed, primarily from the Po Valley region. The most stable conditions (class #5 and PTI days), characterized by nocturnal wind speeds ≤0.5 m s−1 or rates of 2 m air temperature change ≥1.5 °C h−1, were associated with the worst air quality conditions. Only under class #5 and PTI conditions did daily-mean PM10 concentrations exceed both EU and WHO guideline values. The absence of snow cover in the first winter resulted in stronger thermodynamic stability and higher pollution concentrations. Other contributing factors included a 28% larger traffic source near the monitoring site and a 17% greater consumption of heating fuel that winter in Ljubljana. [Display omitted] •Diurnal and synoptic timescale radon-based atmospheric mixing classification methods are combined for the first time.•New insights are gained regarding the atmospheric mixing conditions responsible for the poorest air quality in the basin.•The radon-based methods can distinguish local from remote contributions to changing winter air quality in an Alpine basin.