## Materials and Methods
### Instrumentation  
eBC mass concentrations were measured using one multi-wavelength aethalometer (Magee Scientific aethalometer model AE33, Aerosol d.o.o, Slovenia), with a BGI, MiniPM(r) Inlet at a flow rate of 5 L min\({}^{-1}\). Data were recorded with a 1 min time resolution. Filter-based light attenuation by the deposited aerosol particles was measured at 7 wavelengths (370, 470, 520, 590, 660, 880 and 950 nm). The eBC mass concentration was calculated using the measurement at the 950 nm wavelength with a MAC of 7.19 m\({}^{2}\) g\({}^{-1}\)[9].  
The system used to carry out the measurements of UFP was one SMPS, operated in the scanning mode. The SMPS consists of an Electrostatic Classifier (Model 3080, TSI United Kingdom) and a DMA (Model 3081, TSI United Kingdom) connected to a Water Condensation Particle Counter (Model 3785, TSI United Kingdom). Only the data of Nt were used in this study. The sampled aerosol was dried using a Nafion(r) dryer. Additionally, a pre-impactor with 0.0514 cm nozzle was used. The SMPS was set with a sheath and a polydisperse aerosol flow rate of 10 and 11 pm, respectively, to scan the size range between 10 and 289 nm (although this range is not properly UFP (<100 nm); it is called this way in this paper since the range is closer to the ultrafine range than to the fine range (<1 \(\upmu\)m)). Every 5 min, the system sampled two 120 s scans per sample. Data were compensated for losses by diffusion and multiple charges with the Aerosol Instrument Manager(r) software (version 9.0.0., TSI, Inc., St Paul, MN, USA).  
Quality control protocols were considered for both instruments, including a weekly check for verifying a correct operation and data acquisition. Sampling flows were checked on a monthly basis for SMPS, and on an annual basis for aethalometer. Moreover, several tests were performed annually for this last instrument (e.g., leakage tests). Finally, SMPS national intercomparison exercises were performed in the framework of the Spanish Network of Environmental DMAs (REDMAAS) (26) in order to evaluate the accuracy, since there are no standards to verify the correct measurement of Nt.  
The sampling campaign considered in this paper was performed during the years 2015 and 2016, with data coverage close to 75% for the aethalometer and close to 55% for the SMPS.  
The source apportionment of eBC was performed using the two-component model described by [PERSON] et al. (8), using light absorption measurements at 470 nm and 950 nm (27; 28), since eBC from fossil fuel has a weak dependence on wavelength (i.e., AAE \(\sim\) 1), whereas eBC from biomass burning features a stronger absorption spectral dependence and shows enhanced absorption at a shorter wavelength (i.e., AAE > 1) (29; 30). The absorption at 470 nm was used, instead of the UV channel at 370 nm, to minimize the interferences introduced by types of organic compounds, based on the sensitivity of the aethalometer model due to different wavelength combinations carried out by [PERSON] et al. (31). Even though 880 nm is considered the standard channel for eBC measurement by aethalometers, the 950 nm wavelength was used in this research according to the results obtained in the sensitivity of the aethalometer model using different pairs of wavelengths carried out by [PERSON] et al. (31). Therefore, the analysis here conducted was based on the wavelength range 470-950 nm.