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A new methodology for the determination of the changes due to hygroscopicgrowth with relative humidity of the number size distribution and opticalproperties of polydispersed aerosols is described. This method uses thesimulation chamber CESAM where the hygroscopic properties of polydispersedaerosol particles can be investigated in situ by exposing them to RH ranging from 0to 100% for approximately 1 h. In situ humidification is used to providesimultaneous information on the RH-dependence of the particle size and thecorresponding scattering coefficient (σ<sub>scat</sub>), and that for theentire size distribution. Optical closure studies, based on integratednephelometer and aethalometer measurements, Mie scattering calculations andmeasured particle size distributions, can therefore be performed to yieldderived parameters such as the complex refractive index (CRI) at λ= 525 nm. The CRI can also be retrieved in the visible spectrum bycombining differential mobility analyzer (DMA) and white light aerosolspectrometer (Palas Welas®) measurements.  We have applied this methodology to ammonium sulfate particles, which havewell known optical and hygroscopic properties. The CRI obtained from the twomethods (1.54–1.57) compared favourably to each other and are also inreasonable agreement with the literature values. The particle's growth wascompared to values obtained for one selected size of particles (150 nm) witha H-TDMA and the effect of the residence time for particles humidificationwas investigated. When the humidification was performed in the chamber for afew minutes, a continuous increase of the ammonium sulfate particle's sizeand σ<sub>scat</sub> was observed from RH values as low as 30% RH.Comparison of the measured and modelled values based on Köhler and Mietheories shows that layers of water are adsorbed on ammonium sulfateparticles below the deliquescence point. In contradiction, the particle'sgrowth reported with H-TDMAs showed a prompt deliquescence of ammoniumsulfate particles with no continuous growth in size at low RH. These findingshighlight the need to allow sufficient time for particle-water vapourequilibrium in investigating the aerosols hygroscopic properties. H-TDMAinstruments induce limited residence time for humidification and seem to beinsufficient for water adsorption on ammonium sulfate particles

A new experimental approach to study the hygroscopic and optical properties of aerosols: application to ammonium sulfate particles