Long-term variability of aerosol optical properties in northern Finland

We studied the optical properties in continental and marine air masses, including seasonal cycles and long-term trends using 10-year data on aerosol scattering properties and 5-year data on absorption and combined aerosol optical properties. The average (median) scattering coefficient, backscattefing fraction, absorption coefficient and single scattering albedo at the wavelength of 550 nm were 7.9 (4.4) Mm(-1), 0.13 (0.12), 0.74 (0.35) Mm(-1) and 0.92 (0.93), respectively. We observed clear seasonal cycles in these variables, the scattering coefficient having high values during summer and low in autumn, and absorption coefficient having high values during winter and low in autumn. We found that the high values of the absorption coefficient and low values of the single scattering albedo were related to continental air masses from lower latitudes. These aerosols can induce an additional effect on the surface albedo and melting of snow. We observed the signal of the Arctic haze in marine (northern) air masses during March and April. The haze increased the value of the absorption coefficient by almost 80% and that of the scattering coefficient by about 50% compared with the annual-average values. We did not observe any long-term trend in the scattering coefficient, while our analysis showed a clear decreasing trend in the backscattering fraction and scattering Angstrom exponent during winter. A possible reason for this feature is the gradual change in the relative contributions of different emission sources. However, this remains to be confirmed by future studies.