## 3 Results and Discussion  
### eBC Mass Concentrations  
The mean eBC value \(\pm\) standard deviation obtained at this site during the period of study was 0.85 \(\pm\) 0.83 \(\upmu\)m\({}^{-3}\). The measured values were lower than those measured in other sites around Europe (Table 1). In Figure 2a,b a combination of several variations is shown: hourly, weekly and monthly plots of eBC concentrations divided by season. With regard to the hourly variations, two peaks were detected during the day: one in the early hours and another in the evening, with differences in the mean concentrations (values were higher in the evening peak during winter than in the other seasons). These differences are likely to be related to the combination of anthropogenic emissions and boundary layer dynamics. An increase in eBC concentrations during the early hours can be explained by the increase of anthropogenic emissions and due to the formation of a nocturnal boundary layer (NBL), which favors the accumulation of pollutants. A decrease in eBC concentrations during the middle hours of the day is likely to be associated with a reduction of these emissions (from domestic heating and traffic) and due to a gradual increase in boundary layer height and wind speed. This fact enhances the vertical and horizontal diffusion of pollutants. A similar diurnal variation in eBC concentrations were observed in Ostrava [40] and urban sites of Barcelona and Granada [14]. A marked difference between workdays and weekends was also observed, pointing to traffic as the most important emission source of eBC in this area. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Site** & **eBC (\(\upmu\)g m\({}^{-3}\))** & **Reference** \\ \hline Barcelona–CSIC (urban) & 2.1 & [14] \\ Granada University (urban background) & 2.6 & [4] \\ Madrid–Escuelas Aguirre (urban traffic) & 3.70 & [38] \\ Madrid–CIEMAT (urban background) & 2.33 & [38] \\ Jaen-Villanueva (rural) & 2.61 & [38] \\ Urban site–Helsinki metropolitan area & 1.69 & [39] \\ Suburban site–Helsinki metropolitan area & 0.88 & [39] \\ Ostrava (Czech Republic) & 3.48 & [40] \\ Rural, suburban, urban background, urban center and & 0.45, 1.47, 1.34, 1.83 and 9.72, respectively & [41] \\ kerbside sites in UK & 1.92 & [42] \\ Suburban site located in metropolitan Milan (Bareggio) & 2.76 & [42] \\ Urban background in Rome & 2.6 & [19] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of equivalent black carbon (eBC) (\(\upmu\)g m\({}^{-3}\)) in different sites across Europe. In order to study the possible influence of near emission sources, data were represented as a percentile rose plot of eBC concentrations (Figure 3). The percentile intervals are shaded and shown by wind direction. The plot shows the variation by season and whether it is right-time or daylight hours (note that the open-air package estimates sunrise and sunset to give either daylight or night-time. This estimation, valid for dates between 1901 and 2099, is made using the location, date, time and astronomical algorithms to estimate the relative positions of the Sun and the location on the Earth's surface. For the sampling site here considered, for example, daylight hours in January range from 8 a.m. to 5 p.m. UTC). eBC concentrations are higher on average in the autumn and winter and when the wind comes from the south. The reason is most likely due to the presence of single family homes with heating systems based on wood combustion in this wind direction. eBC concentrations are somewhat higher at night-time in spring compared to daylight hours and higher during daylight hours in summer when the wind comes from the south-east. In this case, the influence of road traffic could be the main source of this pollutant in this wind direction. Figure 2: (**a**) Day of the week variation of median equivalent black carbon (eBC) concentrations. (**b**) Hour of day variation of median eBC concentrations. The shading shows the extent to the 25/75 th and 5/95 th quantiles.