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Abstract. Two ozone episodes (occurring in June 2001 and June 2003) in the air quality monitoring network of the Basque Country (BC) are analyzed. The population information threshold was exceeded in many stations (urban, urbanbackground and rural). During this type of episodes, forced by a blocking anticyclone over the British Isles, ozone background concentrations over the area increase after the import of pollution from both, the continental Europe and the western Mediterranean areas (Gangoiti et al., 2002). For the present analysis, emphasis is made in the search for transport mechanisms, pathways and area sources contributing to the build-up of the episodes. Contributions from a selection of 17 urban and industrial conglomerates in the western European Atlantic (WEA) and the western Mediterranean (WM) are shown after the results of a coupled RAMS-HYPACT modelling system. Meteorological simulations are tested against both the high-resolution wind data recorded at the BC coastal area by a boundary layer wind-profiler radar (Alonso et al., 1998) and the wind soundings reported by the National Centres of Meteorology at a selection of European and northAfrican sites. Results show that during the accumulation phase of the episodes, background ozone concentrations increase in the whole territory as a consequence of transport from the Atlantic coast of France and the British Channel. For the peak phase, intrusions from new sources, located at the Western Mediterranean, Southern France, Ebro Valley, and, occasionally, the area of Madrid are added, resulting in a further increase in the ozone concentrations. Direct day and night transport within the north-easterly winds over the sea from the WEA source region, and night-time transport within the residual layer over continental areas (southern France, the Ebro Valley, and central Iberia) modulate the import sequence of pollutants and the local increase of ozone concentrations. The alternative direct use of low resolution meteoCorrespondence to: G. Gangoiti (g.gangoiti@ehu.es) rological data for the estimation of back-trajectories shows a more simple transport scheme with no contributions neither from the Western Mediterranean nor from the Madrid area.

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Odour complaints are frequent nowadays, particularly nearby industrial facilities emitting odorous compounds. Among all compounds susceptible of causing odour annoyance, reduced sulphur compounds (RSC) were studied, due to their unpleasant odour and low odour threshold. RSC ambient air mixing ratios, meteorological conditions and population complaints were analysed in an area of complex topography in order to identify their potential sources. Mixing ratios of three compounds, dimethyl sulphide (DMS), carbon disulphide (CDS) and dimethyl disulphide (DMDS), were continuously monitored using an online gas chromatograph coupled with a mass spectrometer detector (GC-MSD), which was placed in a mobile air quality monitoring unit. Measurement campaigns were performed during 2012 and 2013 for periods of 7-25 days in an urban area (four campaigns, N = 1368) and an urban area surrounded by industrial activities (three campaigns, N = 564). During such campaigns, RSC mixing ratios were frequently above their odour thresholds, which did not always involve citizen complaints. Average RSC ambient air mixing ratios tended to be lower in the urban area (DMS 0.06-0.33, CDS 0.05-0.10, DMDS 0.07-0.30 μg m) than in the industry surrounded one (DMS 0.30-2.39, 0.05-0.18, DMDS 0.09-0.62 μg m). The DMS/DMDS mixing ratio was frequently above 1, being a paper mill one of the main sources of RSC in the area. DMS/DMDS ratios below 1 were also recorded, suggesting a waste treatment plant as the RSC source or older air masses coming from the paper mill.

The RAMS-CALMET-CALPUFF modelling system, together with observations, has been used to analyse the benzene impacts of a coke plant located in a narrow river valley over a nearby urban area in the estuary of Bilbao. The initial aim of this study was to set up a methodology suitable for dispersion studies in very complex areas, where pollutant dynamics are highly affected by mesoscale processes. Emphasis was put on the validation and improvement of the simulated meteorology. High spatio-temporal resolution meteorological simulations were performed with the non-hydrostatic mesoscale meteorological model RAMS, initialized with NCEP reanalysis data, for two ten-day periods. Results were validated against data recorded both at surface stations and by a wind profiler radar (WPR). Comparisons against the WPR revealed inaccurate NCEP data for one of the periods. Alternative nudging with ECMWF ERA-Interim data improved the results. The RAMS output was downscaled from 1 km to 250 m resolution with the CALMET diagnostic model. The main flows that affect dispersion in the area were mostly well simulated, but important disagreements with observed drainage flows were found for some days. The assimilation of surface meteorological observations into CALMET, greatly improved the results. The meteorological fields were input into the CALPUFF non-steady-state puff dispersion model for dispersion simulations. The actual daily cycles of ground level benzene were well reproduced but concentration levels were underestimated. The availability of good meteorological observations in the area and good emission inventory has shown to be of prime importance.