Over a 10-year record of aerosol optical properties at SMEAR II

Abstract

Aerosol optical properties (AOPs) describe the ability of aerosols to scatter and absorb radiation at different wavelengths. Since aerosol particles interact with the sun's radiation, they impact the climate. Our study focuses on the long-term trends and seasonal variations of different AOPs measured at a rural boreal forest site in northern Europe. To explain the observed variations in the AOPs, we also analyzed changes in the aerosol size distribution. AOPs of particles smaller than 10 μ m (PM 10) and 1 μ m (PM 1) have been measured at SMEAR II, in southern Finland, since 2006 and 2010, respectively. For PM 10 particles, the median values of the scattering and absorption coefficients, single-scattering albedo, and backscatter fraction at λ Combining double low line550 nm were 9.8 Mm -1, 1.3 Mm -1, 0.88, and 0.14. The median values of scattering and absorption Ångström exponents at the wavelength ranges 450-700 and 370-950 nm were 1.88 and 0.99, respectively. We found statistically significant trends for the PM 10 scattering and absorption coefficients, single-scattering albedo, and backscatter fraction, and the slopes of these trends were -0.32 Mm -1, -0.086 Mm -1, 2.2 × 10 - 3 , and 1.3 × 10 - 3 per year. The tendency for the extensive AOPs to decrease correlated well with the decrease in aerosol number and volume concentrations. The tendency for the backscattering fraction and single-scattering albedo to increase indicates that the aerosol size distribution consists of fewer larger particles and that aerosols absorb less light than at the beginning of the measurements. The trends of the single-scattering albedo and backscattering fraction influenced the aerosol radiative forcing efficiency, indicating that the aerosol particles are scattering the radiation more effectively back into space.