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 , Artiñano Begoña , Backman John , Baltensperger Urs , Bonasoni Paolo , Bukowiecki Nicolas , Collaud Coen Martine , Conil Sebastian , 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 , Stamenov Dimiter B. , 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: October 2017

Abstract. 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 (σsp) and hemispheric backscattering (σbsp) 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 σsp 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 σsp was measured in summer mainly because of the enhanced boundary layer influence. Conversely, less horizontal and vertical dispersion in winter led to higher σsp 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 σsp 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.