A European aerosol phenomenology – 6: scattering properties of atmospheric aerosol particles from 28 ACTRIS sites

Abstract

This paper presents the light-scattering properties<br> of atmospheric aerosol particles measured over the<br> past decade at 28 ACTRIS observatories, which are located<br> mainly in Europe. The data include particle light<br> scattering (sp) and hemispheric backscattering (bsp) coefficients,<br> scattering Ångström exponent (SAE), backscatter<br> fraction (BF) and asymmetry parameter (g). An increasing<br> gradient of sp is observed when moving from remote environments<br> (arctic/mountain) to regional and to urban environments.<br> At a regional level in Europe, sp also increases<br> when moving from Nordic and Baltic countries and from<br> western Europe to central/eastern Europe, whereas no clear<br> spatial gradient is observed for other station environments.<br> The SAE does not show a clear gradient as a function of the<br> placement of the station. However, a west-to-east-increasing<br> gradient is observed for both regional and mountain placements,<br> suggesting a lower fraction of fine-mode particle in<br> western/south-western Europe compared to central and eastern<br> Europe, where the fine-mode particles dominate the scattering.<br> The g does not show any clear gradient by station<br> placement or geographical location reflecting the complex<br> relationship of this parameter with the physical properties of<br> the aerosol particles. Both the station placement and the geographical<br> location are important factors affecting the intraannual<br> variability. At mountain sites, higher sp and SAE values<br> are measured in the summer due to the enhanced boundary<br> layer influence and/or new particle-formation episodes.<br> Conversely, the lower horizontal and vertical dispersion during<br> winter leads to higher sp values at all low-altitude sites<br> in central and eastern Europe compared to summer. These<br> sites also show SAE maxima in the summer (with corresponding<br> g minima). At all sites, both SAE and g show<br> a strong variation with aerosol particle loading. The lowest<br> values of g are always observed together with low sp values,<br> indicating a larger contribution from particles in the smaller<br> accumulation mode. During periods of high sp values, the<br> variation of g is less pronounced, whereas the SAE increases<br> or decreases, suggesting changes mostly in the coarse aerosol<br> particle mode rather than in the fine mode. Statistically significant<br> decreasing trends of sp are observed at 5 out of the<br> 13 stations included in the trend analyses. The total reductions<br> of sp are consistent with those reported for PM2:5 and<br> PM10 mass concentrations over similar periods across Europe.