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In situ measurements of aerosol optical properties and number size distributions in a coastal region of Norway during the summer of 2008

Mogo S, Cachorro V, Lopez J, Montilla E, Torres B, Rodríguez E, Bennouna Y, De Frutos A ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 13 (2012) pp. 5841-5857

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Abstract

In situ measurements of aerosol optical properties and particle size
distributions were made in the summer of 2008 at the ALOMAR station
facility (69 degrees 16 ` N, 16 degrees 00 ` E), located in a rural site
in the north of the island of Andoya (VesterAyenlen 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). Our goal was to characterize the aerosols of this
sub-Arctic area, which are frequently transported to the Arctic region.
Data from 13 June to 26 August 2008 were available and the statistical
data for all instruments were calculated based on the hourly averages.
The overall data coverage was approximately 72%. The hourly mean values
of the light-scattering coefficient, Sigma(s), and the light-absorption
coefficient, Sigma(a), at 550 nm were 5.41 Mm(-1) (StD = 3.55 Mm(-1))
and 0.40 Mm(-1) (StD = 0.27 Mm(-1)), respectively. The
scattering/absorption Angstrom exponents, alpha(s,a), were used in a
detailed analysis of the variations of the spectral shape of Sigma(s,a).
While alpha(s) indicates the presence of two particle sizes
corresponding to two types of aerosols, alpha(a) indicates only one type
of absorbing aerosol particle. alpha(a) values greater than 1 were not
observed. The single-scattering albedo, omega(0), ranged from 0.62 to
0.99 (mean = 0.91, StD = 0.05), and the relationships between this
parameter and the absorption/scattering coefficients and the Angstrom
exponents are presented. Any absorption value may lead to the lowest
values of omega(0), whereas only the lowest scattering values were
observed in the lowest range of omega(0). For a given absorption value,
lower omega(0) were observed for smaller alpha(s). The submicrometer,
micrometer and total concentrations of the particles presented hourly
mean values of 1277 cm(-3) (StD = 1563 cm(-3)), 1 cm(-3) (StD = 1
cm(-3)) and 2463 cm(-3) (StD = 4251 cm(-3)), respectively, and the modal
correlations were also investigated. The optical and microphysical
parameters, as well as their relationship with each other, are reported.
Sigma(s) correlated strongly with the number concentration of
accumulation mode particles and more strongly with the micrometer
fraction of particles, but weak correlations were observed for the
Aitken and nucleation modes. The origins and pathways of the air masses
were examined, and based on sector classification, a relationship
between the air mass origin, the optical parameters and the size
distributions was established.
The low values of the optical and microphysical parameters indicate that
the predominant regional aerosol is mostly clean and the shape of the
size distribution is characterized by bimodal median size distributions.
However, the relationships between the air mass origins and the
parameters studied allow us to describe two characteristic situations:
the one of the northern and western air masses, which were predominantly
composed of marine aerosols and presented the lowest optical and
microphysical values observed, indicating predominantly non-absorbent
and coarser particles; and the one of the eastern and southern air
masses, in which continental aerosols were predominant and exhibited
higher values for all parameters, indicating the presence of smaller
absorbent particles. The north-northeastern air masses presented the
strongest Aitken mode, indicating more recently formed particles, and
the southeastern air masses presented the strongest accumulation mode
(however, the southeastern air masses were the least common, accounting
for only 3% of occurrences).

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