Application of mathematical models to simulate an extreme air pollution episode in the Bulgarian city of Stara Zagora

The application of complex atmospheric transport and transformation mesoscale models is becoming more common for study complex environmental problems. In this contribution, we present a pollution episode occurred in the area of Stara Zagora (Bulgaria), where a large SO2 industrial emitter is present. This industrial emitter accounts for more than 75% of the total SO2 emissions in Bulgaria. Two monitoring stations located in Stara Zagora and Mogila observed high SO2 concentration peaks for 7–11 July 2007. We have used the MM5–CMAQ air quality modelling system with CBM-IV chemical mechanism. We have used two different emission modelling tools such as SMOKE and EMIMO. In the first simulation SMOKE is used. In the second simulation, we have used EMIMO including biogenic and anthropogenic sources coming from traffic, domestic sources, etc. Additionally, in the second simulation we have used a so-called ON–OFF approach, which means that we have run two time the modelling system with and without the emissions of the large industrial plant “Maritza-Iztok”, located in the surrounding area of the two monitoring stations. The use of MM5–CMAQ air quality modelling system with two different approaches, parameterizations, spatial resolution, emission modelling systems and vertical grid levels produce two significantly different results. The results show that the parameterizations and approaches for using a complex air quality modelling system such as MM5–CMAQ, are crucial for obtaining some similarities between observed and modelled data.