A Volatility-Tandem-Differential-Mobility-Analyzer (VTDMA) and a Differential Mobility Particle Sizer (DMPS) were used to determine the number and mass concentration of externally mixed aerosol particles in urban background air. In the VTDMA the less-volatile (LV) particle fraction was measured at 300°C for particles in the size range 20–250 nm. The LV particle fraction was converted to the number concentration of LV particles (NLV) and the mass concentration (MLV). MLV was compared with the mass concentration of black carbon (MBC) measured by a Multi-Angle Absorption Photometer (MAAP). The DMPS and VTDMA data were used for calculating scattering and absorption coefficients (σSP and σ AP) with a Mie model and compared with σ SP and σ AP measured with a TSI nephelometer and the MAAP. The model was run by assuming external and internal mixing of absorbing and scattering aerosol. The best fit of measured and modeled σ SP and σ AP was sought by varying the refractive index. During periods dominated by local emissions LV particle fraction LV was high ( >0.2). In these cases, the MLV and the modeled σ AP assuming external mixing agreed well with the measured MBC and σ AP, respectively. For the long-range transported aerosol LV was small ( <0.1) and MBC was higher than MLV. For the whole period the average (± std) refractive index was 1.55 (± 0.09) – 0.04 (±0.02)i when internal mixing was assumed. When LV was >0.2 the average refractive index of LV particles was 1.96 – 0.8 (±0.18)i when σ AP was modeled assuming external mixing.