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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Virkkula, A., Backman, J., Aalto, P. P., Hulkkonen, M., Riuttanen, L., Nieminen, T., Maso, M., Sogacheva, L., Leeuw, G., Kulmala, M.
• Format: Text
• Language: English
• Published: 2018
• Subjects: Norwegian Sea; Lapland
• Online Access: https://doi.org/10.5194/acp-11-4445-2011; https://www.atmos-chem-phys.net/11/4445/2011/

Scattering and absorption were measured at the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II) station in Hyytiälä, Finland, from October 2006 to May 2009. The average scattering coefficient σ SP (λ = 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 σ AP (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 σ SP and σ AP was ~1.8 and ~2.8, respectively. The average single-scattering albedo (ω 0 ) was 0.86 in winter and 0.91 in summer. σ SP was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003–10 μm. Assuming that the particle density was 1.5 g cm −3 , the PM 10 mass scattering efficiency was 3.1 ± 0.9 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 σ SP (modelled) = 1.046 × σ SP (measured) for the data with the low nephelometer sample volume relative humidity (RH NEPH = 30 ± 9 %) and σ SP (modelled) = 0.985 × σ SP (measured) when RH NEPH = 55 ± 4 %. The effective complex refractive index was obtained by an iterative approach, by matching the measured and the modelled σ SP and σ AP . The average effective complex refractive index was (1.517 ± 0.057) + (0.019 ± 0.015) 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 Ångström exponent of scattering, σ 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 α 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 Ångström exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing α SP and CsMD did not have any clear relationship with α SP . Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean σ SP and σ 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 σ SP and σ 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 α SP values were norther than those for σ SP and σ AP . The trajectory statistical approach and a simple wind sector classification agreed well.