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Juranyi, Z, Tritscher T, Gysel M, Laborde M, Gomes L, Roberts G, Baltensperger U, Weingartner E.  2013.  Hygroscopic mixing state of urban aerosol derived from size-resolved cloud condensation nuclei measurements during the MEGAPOLI campaign in Paris. Atmospheric Chemistry and Physics. 13:6431-6446.   10.5194/acp-13-6431-2013   AbstractWebsite

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 kappa = 0.12-0.27) and increased with increasing dry diameter in the range 35-265 nm. The mean kappa 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 kappa-parametrization for the water activity and assuming surface tension of pure water in the Kohler 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.

Wonaschutz, A, Coggon M, Sorooshian A, Modini R, Frossard AA, Ahlm L, Mulmenstadt J, Roberts GC, Russell LM, Dey S, Brechtel FJ, Seinfeld JH.  2013.  Hygroscopic properties of smoke-generated organic aerosol particles emitted in the marine atmosphere. Atmospheric Chemistry and Physics. 13:9819-9835.   10.5194/acp-13-9819-2013   AbstractWebsite

During the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. In this study, the hygroscopic properties and the chemical composition of the plume were studied at plume ages between 0 and 4 h in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs): between 1.05 and 1.09 for 30 nm and between 1.02 and 1.1 for 150 nm dry size at a relative humidity (RH) of 92 %, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10 000 cm(-3)), which lead to substantially increased cloud condensation nuclei (CCN) concentrations at super-saturations between 0.07 and 0.88 %. Ratios of oxygen to carbon (O : C) and water-soluble organic mass (WSOM) increased with plume age: from <0.001 to 0.2, and from 2.42 to 4.96 mu g m(-3), respectively, while organic mass fractions decreased slightly (similar to 0.97 to similar to 0.94). High-resolution aerosol mass spectrometer (AMS) spectra show that the organic fragment m/z 43 was dominated by C2H3O+ in the small, new particle mode and by C3H7+ in the large particle mode. In the marine background aerosol, GFs for 150 nm particles at 40% RH were found to be enhanced at higher organic mass fractions: an average GF of 1.06 was observed for aerosols with an organic mass fraction of 0.53, and a GF of 1.04 for an organic mass fraction of 0.35.