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

Abstract

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.

Seasonality of aerosol optical properties in the Arctic

“…In contrast, SUM shows the opposite seasonal cycle in asymmetry parameter, with g highest in the late summer and lowest in the late winter. PAL and ZEP g seasonalities are in agreement with those presented in Pandolfi et al (2018).…”

“…Note that y axes are different on each plot. SAE values) in the spring and larger aerosols in the late summer, in accordance with the Arctic haze phenomenon and in agreement with seasonal cycle of SAE at ZEP presented in Pandolfi et al (2018). Mean SAE at ZEP (see Table 2) is 1.15, which is slightly higher than the SAE median of just less than 1 presented in Pandolfi et al (2018), which used ZEP data from 2010 to 2014.…”

“…PAL has a different seasonality, with highest SAE values in the summer and lowest SAE values in the winter and early spring, which agrees with findings from Aaltonen et al (2006) and Lihavainen et al (2015). The statistics of SAE in Table 2 show an average SAE of 1.66 at PAL, which is close to the average of 1.7±0.7 that is reported in Lihavainen et al (2015) and the median of 1.8 reported by Pandolfi et al (2018). SUM statistical values of SAE are not directly comparable to the other Arctic sites due to its 2.5 µm size cut inlet, which limits measurements of large particles that would yield smaller SAE values.…”

“…At BRW, scattering data show a strong seasonality with values that are highest in the winter and spring during Arctic haze season, and lowest in late summer (Bodhaine, 1983(Bodhaine, , 1995Delene and Ogren, 2002;Quinn et al, 2007). At ZEP, a study from several decades ago also finds higher scattering coefficients in the winter and lower scattering coefficients in the summer (Heintzenberg, 1982), and a study by Pandolfi et al (2018) Lihavainen et al (2015), and Pandolfi et al (2018). In winter the scattering values at PAL are similar to values observed at ALT, BRW, TIK, and ZEP, but in summer PAL measures notably higher scattering.…”

“…However, Delene and Ogren (2002) did present the seasonality of backscatter fraction (b) for BRW; because g is expected to vary inversely with b, and because they find that b is highest in the summer and lowest in the winter, their results are also consistent with those reported here. Andrews et al (2011) and Pandolfi et al (2018) report a general tendency for g to increase as σ sp increases for mountain sites and European ACTRIS sites, respectively. The same tendency was found for the Arctic sites here (not shown).…”