The white-light humidified optical particle spectrometer (WHOPS) – a novel airborne system to characterize aerosol hygroscopicity

[摘要] Aerosol particles experience hygroscopic growth at enhanced relative humidity(RH), which leads to changes in their optical properties. We developed thewhite-light humidified optical particle spectrometer (WHOPS), a newinstrument to investigate the particles' hygroscopic growth. Here we presenta detailed technical description and characterization of the WHOPS inlaboratory and field experiments. The WHOPS consists of a differentialmobility analyzer, a humidifier/bypass and awhite-light aerosolspectrometer (WELAS) connected in series to provide fast measurements of particlehygroscopicity at subsaturated RH and optical properties on airborneplatforms. The WELAS employs a white-light source to minimize ambiguities inthe optical particle sizing. In contrast to other hygroscopicity instruments,the WHOPS retrieves information of relatively large particles (i.e., diameterD > 280 nm), therefore investigating the more optically relevant sizeranges.

The effective index of refraction of the dry particles is retrievedfrom the optical diameter measured for size-selected aerosol sampleswith a well-defined dry mobility diameter. The data analysisapproach for the optical sizing and retrieval of the index ofrefraction was extensively tested in laboratory experiments withpolystyrene latex size standards and ammonium sulfate particles ofdifferent diameters. The hygroscopic growth factor (GF) distributionand aerosol mixing state is inferred from the optical sizedistribution measured for the size-selected and humidified aerosolsample. Laboratory experiments with pure ammonium sulfate particlesrevealed good agreement with Köhler theory (mean bias of ~3% and maximal deviation of 8% for GFs at RH =95%).

During first airborne measurements in the Netherlands, GFs (meanvalue of the GF distribution) at RH = 95% between 1.79and 2.43 with a median of 2.02 were observed for particles with a dry diameter of500 nm. This corresponds to hygroscopicity parameters(κ) between 0.25 and 0.75 with a median of 0.38. The GFdistributions indicate externally mixed particles covering the wholerange of GFs between ~1.0 and 3.0.On average, ~74%of the 500 nm particles had GFs > 1.5, ~15% had GF < 1.1 andthe remaining ~1% showed values of 1.1 < GF < 1.5. The more hygroscopic mode sometimes peaked atGF > 2, indicating influence of sea-salt particles, consistentwith previous ground-based particle hygroscopicity measurements inthis area. The mean dry effective index of refraction for 500 nmparticles was found to be rather constant with a value of 1.42±0.04 (mean ± 1SD).

[学科分类] 几何与拓扑