A European aerosol phenomenology-6: Scattering properties of atmospheric aerosol particles from 28 ACTRIS sites

This paper presents the light scattering properties of atmospheric aerosol particles measured over the past decade at 28 ACTRIS observatories, located mainly in Europe. The data include particle light scattering (?sp) and hemispheric backscattering (?bsp) coefficients, scattering Ångström exponent (SAE), backscatter fraction (BF) and asymmetry parameter (g). A large range of ssp was observed across the network. Low ssp values were on average measured in Nordic and Baltic countries and in Western Europe whereas the highest ?sp were measured at regional sites in eastern and central Europe. In these regional areas the SAE was also high indicating the predominance of fine-mode particles. On average, the SAE was lower in the Nordic and Baltic, western and southern countries suggesting a lower fraction of fine-mode particle compared to central and eastern Europe. An increasing gradient of ssp was observed when moving from mountain to regional and to urban sites. Conversely, the mass-independent SAE and g parameters did not show the same gradient. At all sites, both SAE and g varied greatly with aerosol particle loading. The lowest values of g were always observed under low ssp indicating a larger contribution from particles in the smaller accumulation mode. Then, g steeply increased with increasing ssp indicating a progressive shift of the particle size distribution toward the larger end of the accumulation mode. Under periods of high particle mass concentrations, the variation of g was less pronounced whereas the SAE increased or decreased suggesting changes mostly in the coarse aerosol particles mode rather than in the fine mode. The station placement seemed to be the main parameter affecting the intra-annual variability. At mountain sites, higher ?sp was measured in summer mainly because of the enhanced boundary layer influence. Conversely, less horizontal and vertical dispersion in winter led to higher ?sp at all low altitude sites in central and eastern Europe compared to summer. On average, these sites also showed SAE maxima in summer (and correspondingly g minima). Large intra-annual variability of SAE and g was observed also at Nordic and Baltic countries due to seasonal-dependent transport of different air masses to these remote sites. Statistically significant decreasing trends of ?sp were observed at 5 out of 13 stations included in trend analyses. The total reductions of ssp were consistent with those reported for PM2.5 and PM10 mass concentrations over similar periods across Europe.