Abstract
Abstract. Ambient aerosols are a complex mixture of particles with different physical and chemical properties and consequently distinct hygroscopic behaviour. The hygroscopicity of a particle determines its water uptake at subsaturated relative humidity (RH) and its ability to form a cloud droplet at supersaturated RH. These processes influence Earth's climate and the atmospheric lifetime of the particles. Cloud condensation nuclei (CCN) number size distributions (i.e. CCN number concentrations as a function of dry particle diameter) were measured close to Paris during the MEGAPOLI campaign in January–February 2010, covering 10 different supersaturations (SS = 0.1–1.0%). The time-resolved hygroscopic mixing state with respect to CCN activation was also derived from these measurements. Simultaneously, a hygroscopicity tandem differential mobility analyser (HTDMA) was used to measure the hygroscopic growth factor (ratio of wet to dry mobility diameter) distributions at RH = 90%. The aerosol was highly externally mixed and its mixing state showed significant temporal variability. The average particle hygroscopicity was relatively low at subsaturation (RH = 90%; mean hygroscopicity parameter κ = 0.12–0.27) and increased with increasing dry diameter in the range 35–265 nm. The mean κ value, derived from the CCN measurements at supersaturation, ranged from 0.08 to 0.24 at SS = 1.0–0.1%. Two types of mixing-state resolved hygroscopicity closure studies were performed, comparing the water uptake ability measured below and above saturation. In the first type the CCN counter was connected in series with the HTDMA and and closure was achieved over the whole range of probed dry diameters, growth factors and supersaturations using the κ-parametrization for the water activity and assuming surface tension of pure water in the Köhler theory. In the second closure type we compared hygroscopicity distributions derived from parallel monodisperse CCN measurements and HTDMA measurements. Very good agreement was found at all supersaturations, which shows that monodisperse CCN measurements are a reliable alternative to determine the hygroscopic mixing state of ambient aerosols.
Highlights
The GF probability density function (GF-PDF) at 90 % relative humidity (RH) measured by the hygroscopicity tandem differential mobility analyser (HTDMA) for different dry sizes were used to predict the complete AF(D,SS) size distributions that would be obtained by the monodisperse Cloud condensation nuclei (CCN) measurements
The CCNC was directly coupled to the HTDMA, which allowed selecting particles by their dry size and hygroscopicity before probing their CCN activity
It was shown that the κ-Kohler theory, with assuming surface tension of pure water, connects the hygroscopic growth of particles at 90 % RH with their CCN activity at supersaturation in a highly accurate manner
Summary
* at: Institute of Aerosol and Sensor Technology, University of Applied Sciences Northwestern Switzerland, Klosterzelgstrasse 2, 5210 Windisch, Switzerland. Received: 3 December 2012 – Published in Atmos.
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