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The hygroscopic properties of atmospheric aerosols are highly relevant forthe quantification of radiative effects in the atmosphere, but also ofinterest for the assessment of particle health effects upon inhalation.This article reports measurements of aerosol particle hygroscopicity inthe highly polluted urban atmosphere of Beijing, China in January 2005.The meteorological conditions corresponded to a relatively cold and dryatmosphere. Three different methods were used: 1) A combination of HumidifyingDifferential Mobility Particle Sizer (H-DMPS) and Twin DifferentialMobility Particle Sizer (TDMPS) measurements, 2) AHygroscopic Tandem Differential Mobility Analyzer (H-TDMA), and 3) Asimplistic solubility model fed by chemical particle composition determinedfrom Micro Orifice Uniform Deposit Impactor (MOUDI) samples. From theH-DMPS and TDMPS particle number size distributions, a size-resolveddescriptive hygroscopic growth factor (DHGF) was determined for the relativehumidities (RH) 55%, 77% and 90%, and particle diameters between 30 and400 nm. In Beijing, the highest DHGFs were observed for accumulation modeparticles, 1.40 (±0.03) at 90% RH. DHGF decreased significantly withparticle size, reaching 1.04 (±0.15) at 30 nm. H-TDMA dataalso suggest a decrease in growth factor towards the biggest particlesinvestigated (350 nm), associated with an increasing fraction of nearlyhydrophobic particles. The agreement between the H-DMPS/TDMPS andH-TDMA methods was satisfactory in the accumulation mode size range(100–400 nm). In the Aitken mode range (<100 nm), the H-DMPS/TDMPSmethod yielded growth factors lower by up to 0.1 at 90% RH. The application ofthe solubility model based on measured chemical composition clearly reproducedthe size-dependent trend in hygroscopic particle growth observed by the othermethods. In the case of aerosol dominated by inorganic ions, the composition-derivedgrowth factors tended to agree (± 0.05) or underestimate (up to 0.1) thevalues measured by the other two methods. In the case of aerosol dominated byorganics, the reverse was true, with an overestimation of up to 0.2. The resultsshed light on the experimental and methodological uncertainties that are stillconnected with the determination of hygroscopic growth factors

Hygroscopic growth of urban aerosol particles in Beijing (China) during wintertime: a comparison of three experimental methods