In situ measurements of aerosols optical properties and number size distribution in a subarctic coastal region of Norway

Sandra Mogo, Victoria Cachorro, Juan F. López, Elena Montilla, Benjamin Torres, Edith Rodriguez, Yasmine Bennouna and Angel de Frutos


In situ measurements of aerosol optical properties were made in the summer of 2008
at the ALOMAR station facility (69_16 N, 16_00 E), located at a rural site in the north of
the island of Andøya (Vester°alen archipelago), approximately 300 km north of the Arctic
Circle. The extended three-month campaign was part of the POLARCAT Project (Polar
Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate,
Chemistry, Aerosols, and Transport) of the International Polar Year (IPY-2007-2008).
Its goal was to characterize the aerosols of this sub-Arctic area, which are frequently
transported to the Arctic region. The ambient light-scattering coefficient, s (550 nm),
at ALOMAR had a measured hourly mean value of 5.41Mm−1 (StD=3.55Mm−1), and
the light-absorption coefficient, a (550 nm), had a measured hourly mean value of
0.40Mm−1 (StD=0.27Mm−1). The scattering/absorption A° ngstro¨m exponents, s,a,
are used for a detailed analysis of the variations of the spectral shape of s,a. Whereas
s demonstrates the presence of two particle sizes corresponding to two types of
15 aerosols, the a demonstrates only one type of absorbing aerosol particles. Values
of a above 1 were not observed. The single-scattering albedo, 0, ranged from 0.62
to 0.99 (mean=0.91, StD=0.05), and the relationships of this property to the absorption/
scattering coefficients and the A° ngstro¨m exponents are presented. The concentration
of the particles was monitored using a scanning mobility particle sizer (SMPS),
20 an aerodynamic particle sizer (APS) and an ultrafine condensation particle counter
(UCPC). The shape of the median size distribution of the particles in the submicrometer
fraction was bimodal, and the submicrometer, micrometer and total concentrations
presented hourly mean values of 1277cm3 (StD=1563 cm3), 1cm3 (StD=1 cm3) and
2463 cm3 (StD=4251 cm3), respectively. The modal correlations were investigated,
and the concentration of particles sized between 30 and 100 nm (Aitken mode) are
presented as a function of the concentration of the particles sized between 100 and
390nm (accumulation mode). The optical and the microphysical parameters are related
to each other, and the results are presented. The origins and pathways of air
masses were examined by computing the back-trajectories in a trajectory model (HYSPLIT).
Six geographical sectors were defined to classify the air masses, and, based
on the sector classification, the linkage between the air mass origin and the optical
parameters was established. Aerosol size distributions were also evaluated in relation
to the air masses. The relationships between the air mass origins and other parameters,
especially those related to the single scattering albedo, allow us to describe two
characteristic situations: northern and western air masses, which had predominantly
marine aerosols, presented lower optical parameter values, indicating predominantly
coarser and non-absorbent particles; and eastern and southern air masses, in which
continental aerosols were predominant, presented higher values for all optical parameters,
indicating the presence of smaller absorbent