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