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

Scattering and absorption were measured at the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II) station in Hyytiala, Finland, from October 2006 to May 2009. The average scattering coefficient sigma(SP) (lambda = 550 nm) 18 Mm(-1) was about twice as much as at the Pallas Global Atmosphere Watch (GAW) station in Finnish Lapland. The average absorption coefficient sigma(AP) (lambda = 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 sigma(SP) and sigma(AP) was similar to 1.8 and similar to 2.8, respectively. The average single-scattering albedo (omega(0)) was 0.86 in winter and 0.91 in summer. sigma(SP) was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003-10 mu m. Assuming that the particle density was 1.5 g cm(-3), the PM10 mass scattering efficiency was 3.1 +/- 0.9 g m(-2) at lambda = 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 sigma(SP)(modelled) = 1.046 x sigma(SP)(measured) for the data with the low nephelometer sample volume relative humidity (RHNEPH = 30 +/- 9 %) and sigma(SP)(modelled) = 0.985 x sigma(SP)(measured) when RHNEPH = 55 +/- 4 %. The effective complex refractive index was obtained by an iterative approach, by matching the measured and the modelled sigma(SP) and sigma(AP). The average effective complex refractive index was (1.517 +/- 0.057) + (0.019 +/- 0.015)i at lambda = 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 similar to 90 %. The Angstrom exponent of scattering, alpha(SP), 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 alpha(SP) 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 Angstrom exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing alpha(SP) and CsMD did not have any clear relationship with alpha(SP). Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean sigma(SP) and sigma(AP) 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 sigma(SP) and sigma(AP) 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 alpha(SP) values were norther than those for sigma(SP) and sigma(AP). The trajectory statistical approach and a simple wind sector classification agreed well.