AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Low Hygroscopic Scattering Enhancement of Boreal Aerosol and the Implications for a Columnar Optical Closure Study
PAUL ZIEGER, Pasi Aalto, Veijo Aaltonen, Mikko Äijälä, John Backman, Juan Hong, Mika Komppula, Radek Krejci, Laborde Marie, Janne Lampilahti, Gerrit de Leeuw, Anne Pfüller, Bernadette Rosati, Matthias Tesche, Peter Tunved, Riikka Väänänen, Tuukka Petäjä, Stockholm University
Abstract Number: 159 Working Group: Remote and Regional Atmospheric Aerosols
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 importance for radiative forcing calculations and is also needed for the comparison of remote sensing or model results with in-situ measurements. Specifically, the 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.
Here, we present results of an intensive field campaign carried out in summer 2013 at Hyytiälä, Finland. Ground-based and airborne measurements of aerosol optical, chemical and microphysical properties were conducted. The f(RH) measured at ground is found to be in general lower 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. 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.
By combining the ground-based measurements with aircraft measurements of the particle number size distribution, columnar values of the particle extinction coefficient are determined and compared to direct 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 depth (AOD) clearly correlates with directly measured values, but is significantly lower compared to the Sun photometer AOD. 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, elevated layers above 3 km, and losses in the in-situ sampling lines.