Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 15, 7247-7267, 2015
https://doi.org/10.5194/acp-15-7247-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
02 Jul 2015
Low hygroscopic scattering enhancement of boreal aerosol and the implications for a columnar optical closure study
P. Zieger1, P. P. Aalto2, V. Aaltonen3, M. Äijälä2, J. Backman2,a, J. Hong2, M. Komppula4, R. Krejci1,2, M. Laborde5,6, J. Lampilahti2, G. de Leeuw2,3, A. Pfüller4, B. Rosati7, M. Tesche1,b, P. Tunved1, R. Väänänen2, and T. Petäjä2 1Stockholm University, Department of Environmental Science and Analytical Chemistry & Bolin Centre for Climate Research, Stockholm, Sweden
2University of Helsinki, Department of Physics, Helsinki, Finland
3Finnish Meteorological Institute, Climate Research Unit, Helsinki, Finland
4Finnish Meteorological Institute, Atmospheric Research Centre of Eastern Finland, Kuopio, Finland
5AerosolConsultingML GmbH, Lausanne, Switzerland
6Ecotech Pty Ltd., Melbourne, Australia
7Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, Villigen, Switzerland
anow at: Finnish Meteorological Institute, Atmospheric Composition Research, Helsinki, Finland
bnow at: University of Hertfordshire, School of Physics, Astronomy and Mathematics, Herts AL10 9AB, UK
Abstract. Ambient aerosol particles can take up water and thus change their optical properties depending on the hygroscopicity and the relative humidity (RH) of the surrounding air. Knowledge of the hygroscopicity effect is of crucial importance for radiative forcing calculations and is also needed for the comparison or validation of remote sensing or model results with in situ measurements. Specifically, particle light scattering depends on RH and can be described by the scattering enhancement factor f(RH), which is defined as the particle light scattering coefficient at defined RH divided by its dry value (RH <30–40 %).

Here, we present results of an intensive field campaign carried out in summer 2013 at the SMEAR II station at Hyytiälä, Finland. Ground-based and airborne measurements of aerosol optical, chemical and microphysical properties were conducted. The f(RH) measured at ground level by a humidified nephelometer is found to be generally lower (e.g. 1.63±0.22 at RH = 85 % and λ = 525 nm) than observed at other European sites. One reason is the high organic mass fraction of the aerosol encountered at Hyytiälä to which f(RH) is clearly anti-correlated (R2≈0.8). A simplified parametrization of f(RH) based on the measured chemical mass fraction can therefore be derived for this aerosol type. A trajectory analysis revealed that elevated values of f(RH) and the corresponding elevated inorganic mass fraction are partially caused by transported hygroscopic sea spray particles. An optical closure study shows the consistency of the ground-based in situ measurements.

Our measurements allow to determine the ambient particle light extinction coefficient using the measured f(RH). By combining the ground-based measurements with intensive aircraft measurements of the particle number size distribution and ambient RH, columnar values of the particle extinction coefficient are determined and compared to columnar measurements of a co-located AERONET sun photometer. The water uptake is found to be of minor importance for the column-averaged properties due to the low particle hygroscopicity and the low RH during the daytime of the summer months. The in situ derived aerosol optical depths (AOD) clearly correlate with directly measured values of the sun photometer but are substantially lower compared to the directly measured values (factor of ~ 2–3). The comparison degrades for longer wavelengths. The disagreement between in situ derived and directly measured AOD is hypothesized to originate from losses of coarse and fine mode particles through dry deposition within the canopy and losses in the in situ sampling lines. In addition, elevated aerosol layers (above 3 km) from long-range transport were observed using an aerosol lidar at Kuopio, Finland, about 200 km east-north-east of Hyytiälä. These elevated layers further explain parts of the disagreement.


Citation: Zieger, P., Aalto, P. P., Aaltonen, V., Äijälä, M., Backman, J., Hong, J., Komppula, M., Krejci, R., Laborde, M., Lampilahti, J., de Leeuw, G., Pfüller, A., Rosati, B., Tesche, M., Tunved, P., Väänänen, R., and Petäjä, T.: Low hygroscopic scattering enhancement of boreal aerosol and the implications for a columnar optical closure study, Atmos. Chem. Phys., 15, 7247-7267, https://doi.org/10.5194/acp-15-7247-2015, 2015.
Publications Copernicus
Download
Short summary
The effect of water uptake (hygroscopicity) on aerosol light scattering properties is generally lower for boreal aerosol due to the dominance of organic substances. A columnar optical closure study using ground-based and airborne measurements of aerosol optical, chemical and microphysical properties was conducted and the implications and limitations are discussed.
The effect of water uptake (hygroscopicity) on aerosol light scattering properties is generally...
Share