Exploring atmospheric boundary layer characteristics in a severe SO₂ episode in the north-eastern Adriatic

Abstract

Abstract. Stable atmospheric conditions are often connected with the occurrence of high pollutant episodes especially in urban or industrial areas. In this work we investigate a severe pollution SO2 episode observed on 3–5 February 2002 in a coastal industrial town of Rijeka, Croatia. The episode occurred under anticyclonic high pressure conditions during which a fog, low wind speeds and very high daily associated mean SO2 concentration of 353.5 μg m−3 were observed. First, the EMEP model was used to evaluate the long-range transport and its contribution to the local SO2 concentrations. The comparison between the EMEP modelled regional SO2 concentrations and measured ones in Rijeka showed that the episode was caused predominately by local sources. Furthermore, using three-dimensional, higher-order turbulence closure mesoscale models (WRF and MEMO), the wind regimes and thermo-dynamical structure of the lower troposphere above the greater Rijeka area were examined in detail. The obtained results suggest several factors responsible for elevated SO2 concentrations. The polluted air is transported towards Rijeka from nearby industrial areas where major pollution sources are located. This transport is associated with strong, ground-based temperature inversion and with a corresponding very low, mixing layer (below 140 m). Light winds or almost calm conditions in Rijeka town were another causative factor of the episode. Also, a vertical circulation cell formed between the mainland and a nearby island, causing the air subsidence and stability increase in the lowermost layer south of Rijeka.