## 3 Results and Discussion
### Source Apportionment of eBC  
The measured diurnal and hourly average cycles of the AAE during the period of study are shown in Figure 8a,b. AAE presents a strong diurnal variation during winter and autumn, with values in the evening hours until dawn consistent with an increase of biomass burning activity due to the use of the domestic wood stoves. AAE shows a clear seasonal cycle, with the largest values during winter and the lowest during summer. Large AAE values during winter are related to an increase in the contribution of UV-absorbing particles, likely BrC from biomass burning emissions [39]. Furthermore, the large variability observed in AAE values indicate changes in the contribution of different absorbing aerosol types. This variability could be due to the impact of different sources of absorbing aerosols (i.e., fossil fuel (ff) and biomass burning (bb)) with a distinct absorption spectral dependence. The AAE average value was 1.26 \(\pm\) 0.22 which is similar to the one obtained in a suburban area of Helsinki (1.25 \(\pm\) 0.11) [39] and was slightly lower than the AAE average values found in Rome (1.41 \(\pm\) 0.21) [19]. Thus, this supports the hypothesis that the highest eBC levels observed during cold seasons are strongly affected by wood combustion sources. The lowest AAE values obtained in summer suggests a low contribution from non-BC absorbing components to the absorption process, as [PERSON] et al. commented [49] for a remote site in Spain. Applying the model used in [PERSON] et al. [8], it was found that fossil fuel combustion was responsible for 69% of the eBC in 2015 and 64% in 2016. These values are slightly higher than the ones obtained in winter in three sites of Granada (52-68%) [50]. When applying this model, it is necessary to take into account, and should thus be considered carefully, that eBCff comprises traffic emissions as well as carbonaceous aerosols originating from fuel oil and natural gas combustion but excludes coal-burning organic aerosols. Another limitation might be the presence of mineral dust particles (notably containing iron oxides) [51,52], which also absorb light at near-UV wavelengths [53]. Figure 7: Percentile rose plot of Nt concentrations (particles cm\({}^{-3}\)). The average diurnal cycles of eBCff and eBCbb are summarized in Figure 9a,b. The eBCff is practically the same as that of the total measured eBC and higher than eBCbb concentrations. As expected, higher contributions to black carbon from biomass burning were observed in colder periods, which can be explained by the higher intensity of domestic heating. The decrease in eBCbb concentrations during the warmer seasons is likely explained by the decrease in local and/or regional emissions from domestic heating by wood stoves. This evidently affects the eBCbb concentration levels especially at this site, wherein the wood burning emissions are dominated by local appliances during winter. However, a great variation in eBCbb concentrations was found in summer probably due to episodes of biomass burning fires. The eBCff did not show a distinct seasonal dependency. Higher eBCff on weekdays during autumn and winter could be due to trips made by car to second homes when there is good weather (i.e., more trips appear to go to schools, to work, shopping, etc.). Figure 8: (**a**) Day of the week variation of absorption Ångström exponent (AAE). (**b**) Hour of day variation of AAE. The shading shows the extent of the 25/75 th and 5/95 th quantiles. During weekdays, the previous figure shows that the eBCff concentration was characterized by two peaks, one in the morning and another in the late afternoon/early evening, coinciding with peak traffic times. On the weekend, these peaks are less pronounced. A relatively weak morning peak and a strong evening peak characterize the average diurnal cycle of eBCbb. In winter, the contribution of eBCbb is higher during night-time, which can be explained by the typical operating time pattern of domestic heating appliances in the coldest months.