A closure study of sub-micrometer aerosol particle hygroscopic behaviour

The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in n orthern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diamet ers between 35 and 265 nm at one of the sites upwind of the orographic clou d. An external mixture consisting of three groups of particles, each with d ifferent hygroscopic properties, was observed. These particle groups were d enoted less-hygroscopic, more-hygroscopic and sea spray particles and had a verage diameter growth factors of 1.11-1.15, 1.38-1.69 and 2.08-2.21 respec tively when taken from a dry state to a relative humidity of 90%. Average g rowth factors increased with dry particle size. A bimodal hygroscopic behav iour was observed for 74-87% of the cases depending on particle size. Paral lel measurements of dry sub-micrometer particle number size distributions w ere performed with a Differential Mobility Particle Sizer (DMPS). The inorg anic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impacto rs at ambient conditions. The number of ions collected on each impactor sta ge was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion numb er concentration was compared with the actual number of all positive and ne gative ions collected on the various impactor stages. For the impactor stag e which collected particles with aerodynamic diameters between 0.17-0.53 mu m at ambient relative humidity, and for which all pertinent data was avail able for the hygroscopic closure study, the predicted ion concentrations ag reed with the measured values within the combined measurement and model unc ertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemi cal activity which had probably produced hygroscopically active material ot her than inorganic ions, such as organic oxygenated substances. (C) 1999 El sevier Science B.V. All rights reserved.