A European aerosol phenomenology – 6: scattering properties of atmospheric aerosol particles from 28 ACTRIS sites
Authors: Pandolfi Marco , Alados-Arboledas Lucas , Alastuey Andrés , Andrade Marcos , Angelov Christo , Artiñano Begoña , Backman John , Baltensperger Urs , Bonasoni Paolo , Bukowiecki Nicolas , Collaud Coen Martine , Conil Sébastien , Coz Esther , Crenn Vincent , Dudoitis Vadimas , Ealo Marina , Eleftheriadis Kostas , Favez Olivier , Fetfatzis Prodromos , Fiebig Markus , Flentje Harald , Ginot Patrick , Gysel Martin , Henzing Bas , Hoffer Andras , Holubova Smejkalova Adela , Kalapov Ivo , Kalivitis Nikos , Kouvarakis Giorgos , Kristensson Adam , Kulmala Markku , Lihavainen Heikki , Lunder Chris , Luoma Krista , Lyamani Hassan , Marinoni Angela , Mihalopoulos Nikos , Moerman Marcel , Nicolas José , O'Dowd Colin , Petäjä Tuukka , Petit Jean-Eudes , Pichon Jean Marc , Prokopciuk Nina , Putaud Jean-Philippe , Rodríguez Sergio , Sciare Jean , Sellegri Karine , Swietlicki Erik , Titos Gloria , Tuch Thomas , Tunved Peter , Ulevicius Vidmantas , Vaishya Aditya , Vana Milan , Virkkula Aki , Vratolis Stergios , Weingartner Ernest , Wiedensohler Alfred , Laj Paolo 
Published: June 2018

Abstract. 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 (σsp) and hemispheric backscattering
(σbsp) coefficients, scattering Ångström exponent
(SAE), backscatter fraction (BF) and asymmetry parameter (g). An increasing
gradient of σsp is observed when moving from remote
environments (arctic/mountain) to regional and to urban environments. At a
regional level in Europe, σsp 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 intra-annual
variability. At mountain sites, higher σsp 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 σsp 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 σsp values, indicating a larger contribution from
particles in the smaller accumulation mode. During periods of high
σsp 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 σsp are observed at 5 out of the
13 stations included in the trend analyses. The total reductions of
σsp are consistent with those reported for PM2.5
and PM10 mass concentrations over similar periods across Europe.