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Scattering and absorption were measured at the Station for Measuring Ecosystemâ€“Atmosphere Relations (SMEAR II) station in HyytiÃ¤lÃ¤, Finland, from October 2006 to May 2009. The average scattering coefficient σ<sub>SP</sub> (λ = 550 nm) 18 Mm<sup>âˆ’1</sup> was about twice as much as at the Pallas Global Atmosphere Watch (GAW) station in Finnish Lapland. The average absorption coefficient σ<sub>AP</sub> (λ = 550 nm) was 2.1 Mm<sup>âˆ’1</sup>. The seasonal cycles were analyzed from hourly-averaged data classified according to the measurement month. The ratio of the highest to the lowest average σ<sub>SP</sub> and σ<sub>AP</sub> was ~1.8 and ~2.8, respectively. The average single-scattering albedo (ω<sub>0</sub>) was 0.86 in winter and 0.91 in summer. σ<sub>SP</sub> was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003â€“10 Î¼m. Assuming that the particle density was 1.5 g cm<sup>âˆ’3</sup>, the PM<sub>10</sub> mass scattering efficiency was 3.1 Â± 0.9 g m<sup>âˆ’2</sup> at λ = 550 nm. Scattering coefficients were also calculated from the number size distributions by using a Mie code and the refractive index of ammonium sulfate. The linear regression yielded σ<sub>SP</sub>(modelled) = 1.046 × σ<sub>SP</sub>(measured) for the data with the low nephelometer sample volume relative humidity (RH<sub>NEPH</sub> = 30 ± 9 %) and σ<sub>SP</sub>(modelled) = 0.985 × σ<sub>SP</sub>(measured) when RH<sub>NEPH</sub> = 55 ± 4 %. The effective complex refractive index was obtained by an iterative approach, by matching the measured and the modelled σ<sub>SP</sub>and σ<sub>AP</sub>. The average effective complex refractive index was (1.517 Â± 0.057) + (0.019 Â± 0.015)<i>i</i> at λ = 550 nm. The iterated imaginary part had a strong seasonal cycle, with smallest values in summer and highest in winter. The contribution of submicron particles to scattering was ~90 %. The Ã…ngstrÃ¶m exponent of scattering, σ<sub>SP</sub>, was compared with the following weighted mean diameters: count mean diameter (CMD), surface mean diameter (SMD), scattering mean diameter (ScMD), condensation sink mean diameter (CsMD), and volume mean diameter (VMD). If α<sub>SP</sub> is to be used for estimating some measure of the size of particles, the best choice would be ScMD, then SMD, and then VMD. In all of these the qualitative relationship is similar: the larger the Ã…ngstrÃ¶m exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing α<sub>SP</sub> and CsMD did not have any clear relationship with α<sub>SP</sub>. Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean σ<sub>SP</sub> and σ<sub>AP</sub> associated with the grid cells in Eastern Europe were in the range 20â€“40 Mm<sup>âˆ’1</sup> and 4â€“6 Mm<sup>âˆ’1</sup>, respectively. The respective geometric means of σ<sub>SP</sub> and σ<sub>AP</sub> in the grid cells over Norwegian Sea were in the range 5â€“10 Mm<sup>âˆ’1</sup> and <1 Mm<sup>âˆ’1</sup>. The source areas associated with high α<sub>SP</sub> values were norther than those for σ<sub>SP</sub> and σ<sub>AP</sub>. The trajectory statistical approach and a simple wind sector classification agreed well.

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_{2}and particulate matter from the perspective of a Finnish measurement station in 1996â€“2008, Rep. Ser. Aer. Sci., 109, available at: www.atm.helsinki.fi/FAAR/reportseries/, 2010.]]>

^{nd}Aerosol Characterization Experiments, Tellus B, 52, 239â€“257, 2000.]]>