Aerosol hygroscopicity at a regional background site (Ispra) in Northern Italy European Commission, Joint Research Centre, Institute for Environment and Sustainability, 21027, Ispra, Italy
02 Jul 2012
Received: 16 December 2011 – Published in Atmos. Chem. Phys. Discuss.: 17 February 2012 Abstract. This study focuses on the aerosol hygroscopic properties as determined from
ground-based measurements and Mie theory. Usually, aerosol ground-based
measurements are taken in dry conditions in order to have a consistency
within networks. The dependence of the various aerosol optical
characteristics (e.g. aerosol absorption, scattering, backscattering or
extinction coefficients) on relative humidity has therefore to be
established in order to determine their values in the atmosphere, where
relative humidity can reach high values.
Revised: 25 May 2012 – Accepted: 04 June 2012 – Published: 02 July 2012
We calculated mean monthly diurnal values of the aerosol hygroscopic growth
factor at 90% relative humidity GF(90) based on measurements performed at
the atmospheric research station in Ispra (Italy) with a Hygroscopicity
Tandem Differential Mobility Analyzer over eight months in 2008 and 2009.
Particle hygroscopicity increases with particle dry diameter ranging from 35
to 165 nm for all seasons. We observed a clear seasonal variation in
GF(90) for particles larger than 75 nm, and a diurnal cycle in spring and
winter for all sizes. For 165 nm particles, GF(90) averages 1.32 ± 0.06.
The effect of the particle hygroscopic growth on the aerosol optical
properties (scattering, extinction, absorption and backscatter coefficients,
asymmetry parameter and backscatter faction) was computed using the Mie
theory, based on data obtained from a series of instruments running at our
station. We found median enhancement factors (defined as ratios between the
values of optical variables at 90% and 0% relative humidity) equal
to 1.1, 2.1, 1.7, and 1.8, for the aerosol absorption, scattering,
backscattering, and extinction coefficients, respectively. All except the
absorption enhancement factors show a strong correlation with the
hygroscopic growth factor. The enhancement factors observed at our site are
among the lowest observed across the world for the aerosol scattering
coefficient, and among the highest for the aerosol backscatter fraction.
Citation: Adam, M., Putaud, J. P., Martins dos Santos, S., Dell'Acqua, A., and Gruening, C.: Aerosol hygroscopicity at a regional background site (Ispra) in Northern Italy, Atmos. Chem. Phys., 12, 5703-5717, doi:10.5194/acp-12-5703-2012, 2012.