Seasonal cycle, size dependencies, and source analyses of aerosol optical properties at the SMEAR II measurement station in Hyytiälä, Finland

Abstract. Scattering and absorption were measured at the SMEAR II station in Hyytiälä, Finland, from October 2006 to May 2009. The average scattering coefficient σSP (λ=550 nm) 18 Mm−1 was about twice as much as at the Pallas GAW station in Finnish Lapland. The average absorption coefficient σAP (λ=550 nm) was 2.1 Mm−1. The seasonal cycles were analyzed from hourly-averaged data classified according to the measurement month. The ratio of the highest to the lowest average σSP and σAP was ~1.8 and ~2.8, respectively. The average single-scattering albedo (ω0) was 0.86 in winter and 0.91 in summer. σSP was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003–10 μm yielding PM10 mass scattering efficiency of 2.75 ± 0.01 g m−2 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 σSP(modelled)=1.04×σSP(measured) but there were also large deviations from the regression line: 10% of the σSP(modelled)-to-σSP(measured) ratios, calculated for each hour, were smaller than 0.9 and 10% of them were larger than 1.27. The scattering size distributions were bimodal, with a large submicrometer mode with geometric mean diameters Dg between ~300 and 400 nm and a smaller supermicrometer mode with Dg at ~1.5–1.9 μm. The contribution of submicrometer particles to scattering was ~90%. The Ångström exponent of scattering, αSP, 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 αSP 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 Ångström exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing αSP and CsMD did not have any clear relationship with αSP. Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean σSP and σAP associated with the grid cells in Eastern Europe were in the range 20–40 Mm−1 and 4–6 Mm−1, respectively. The respective geometric means of σSP and σAP in the grid cells over Norwegian Sea were in the range 5–10 Mm−1 and <1 Mm−1. The source areas associated with high αSP values were norther than those for σSP and σAP. The trajectory statistical approach and a simple wind sector classification agreed well.