Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2010
10.5194/acpd-10-3659-2010
http://www.atmos-chem-phys-discuss.net/10/3659/2010/
http://www.atmos-chem-phys-discuss.net/10/3659/2010/acpd-10-3659-2010.html
http://www.atmos-chem-phys-discuss.net/10/3659/2010/acpd-10-3659-2010.pdf
3659
3698
2010-02-09
Effects of relative humidity on aerosol light scattering in the Arctic
P. Zieger
R. Fierz-Schmidhauser
M. Gysel
J. Ström
S. Henne
K. E. Yttri
U. Baltensperger
E. Weingartner
ernest.weingartner@psi.ch
Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, 5232 Villigen, Switzerland
Norwegian Polar Institute, Polarmiljøsenteret, 9296 Tromsø, Norway
Empa, Laboratory for Air Pollution and Environmental Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Norwegian Institute for Air Research, Dept. Atmospheric and Climate Research, P.O. Box 100, 2027 Kjeller, Norway
Aerosol particles experience hygroscopic growth in the ambient
atmosphere. Their optical properties – especially the aerosol light
scattering – are therefore strongly dependent on the ambient relative
humidity (RH). In-situ light scattering measurements of long-term
observations are usually performed under dry conditions
(RH<30–40%). The knowledge of this RH effect is of eminent
importance for climate forcing calculations or for the comparison of
remote sensing with in-situ measurements. This study combines
measurements and model calculations to describe the RH effect on
aerosol light scattering for the first time of aerosol particles
present in summer and fall at the high Arctic. For this purpose,
a field campaign was carried out from July to October 2008 at the
Zeppelin station in Ny-Ã…lesund, Svalbard. The aerosol light
scattering coefficient σ<sub>sp</sub>(λ) was measured at
three distinct wavelengths (λ=450, 550, and 700 nm) at
dry and at various, predefined RH conditions between 20% and 95%
with a recently developed humidified nephelometer (WetNeph) and with
a second nephelometer measuring at dry conditions (DryNeph). In
addition, the aerosol size distribution and the aerosol absorption
coefficient were measured. The scattering enhancement factor
<i>f</i>(RH,λ) is the key parameter to describe the RH effect on
σ<sub>sp</sub>(λ) and is defined as the RH dependent
σ<sub>sp</sub>(RH,λ) divided by the corresponding dry
σ<sub>sp</sub>(RH<sub>dry</sub>,λ). During our campaign the
average <i>f</i>(RH=85%, λ=550 nm) was
3.24±0.63 (mean ± standard deviation), and no clear
wavelength dependence of <i>f</i>(RH,λ) was observed. This
means that the ambient scattering coefficients at
RH=85% were on average about three times higher than the dry
measured in-situ scattering coefficients. The RH dependency of the
recorded <i>f</i>(RH,λ) can be well described by an empirical
one-parameter equation. We used a simplified method to retrieve an
apparent hygroscopic growth factor <i>g</i>, defined as the aerosol
particle diameter at a certain RH divided by the dry diameter, using
the WetNeph, the DryNeph, the aerosol size distribution measurements
and Mie theory. With this approach we found on average for <i>g</i> values
of 1.61±0.12 (mean ± standard deviation). No clear seasonal
shift of <i>f</i>(RH,λ) was observed during the 3-month period,
while aerosol properties (size and chemical composition) clearly
changed with time. While the beginning of the campaign was mainly
characterized by smaller and less hygroscopic particles, the end was
dominated by larger and more hygroscopic particles. This suggests that
compensating effects of hygroscopicity and size determined the
temporal stability of <i>f</i>(RH,λ). During sea salt
influenced periods, distinct deliquescence transitions were
observed. At the end we give a method on how to transfer the dry
in-situ measured aerosol scattering coefficients to ambient values for
the aerosol measured during summer and fall at this location.
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