Measurement of relative humidity dependent light scattering of aerosols

Atmospheric Measurement Techniques Discussions 01/2009; 3(1). DOI: 10.5194/amtd-2-2161-2009
Source: DOAJ

ABSTRACT Relative humidity (RH) influences the water content of aerosol particles and therefore has an important impact on the particles' ability to scatter visible light. The RH dependence of the particle light scattering coefficient (σsp) is therefore an important measure for climate forcing calculations. We built a humidification system for a nephelometer which allows the measurement of σsp at a defined RH in the range of 40–90%. This RH conditioner consists of a humidifier followed by a dryer, which enables us to measure the hysteresis behavior of deliquescent aerosol particles. In this paper we present the set-up of a new humidified nephelometer, a detailed characterization with well defined laboratory generated aerosols, and a first application in the field by comparing our instrument to another humidified nephelometer. Monodisperse ammonium sulfate and sodium chloride particles were measured at four different dry particle sizes. Agreement between measurement and prediction based on Mie theory was found for both σsp and f(RH)=σsp(RH)/σsp(dry) within the range of uncertainty. The two humidified nephelometers measuring at a rural site in the Black Forest (Germany) often detected different f(RH), probably caused by the aerosol hysteresis behavior: when the aerosol was metastable, therefore was scattering more light, only one instrument detected the higher f(RH).

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Active GaAs MMIC filters in K-band, around 22 GHz, have been designed, realised and characterised. Simulation and measurement results are compared and agree. The best performances measured for a band-pass filter centered at 21.98 GHz are a bandwidth of 0.75 GHz and a noise figure of 1.95 dB.
    Microwave Conference, 2000. 30th European; 11/2000
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The goal of this study is to investigate the effects of relative humidity on aerosol optical properties under maritime and arctic conditions. For this, we installed a newly developed humidified nephelometer (WetNeph, Schmidhauser et al. 2008), an aethalometer (AE31), a scanning mobility particle sizer (SMPS) and an optical particle counter (OPC) for three months from July to October 2008 at the Zeppelin station in Ny-Ålesund, Spitsbergen. The WetNeph, as our main instrument, measured the aerosol scattering coefficient at controlled relative humidity (RH). This instrument allows the determination of the scattering enhancement factor f(RH), which is defined as the scattering coefficient at a certain RH divided by the dry scattering coefficient measured by a second nephelometer running synchronously at low RH. The aethalometer measured the aerosol absorption coefficient at seven wavelengths, while the SMPS and OPC measured the aerosol size distribution between 15 nm and 22.5 µm. These measurements are needed for the verification and improvement of our aerosol modeling activities. Here, the light scattering and absorption coefficients are modeled (as a function of relative humidity) with an advanced model using Mie theory. The results can then be used to recalculate long-term time series of light scattering and absorption coefficients, which are generally performed under dry conditions (RH
    Geochmica et Cosmochimica Acta 06/2009;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ambient relative humidity (RH) determines the water content of atmospheric aerosol particles and thus has an important influence on the amount of visible light scattered by particles. The RH dependence of the particle light scattering coefficient (σsp) is therefore an important variable for climate forcing calculations. We used a humidification system for a nephelometer which allows for the measurement of σsp at a defined RH in the range of 20–95%. In this paper we present measurements of light scattering enhancement factors f(RH)=σsp(RH)/σsp(dry) from a 1-month campaign (May 2008) at the high alpine site Jungfraujoch (3580 m a.s.l.), Switzerland. Measurements at the Jungfraujoch are representative for the lower free troposphere above Central Europe. For this aerosol type hardly any information about the f(RH) is available so far. At this site, f(RH=85%) varied between 1.2 and 3.3. Measured f(RH) agreed well with f(RH) calculated with Mie theory using measurements of the size distribution, chemical composition and hygroscopic diameter growth factors as input. Good f(RH) predictions at RH<85% were also obtained with a simplified model, which uses the Ångström exponent of σsp(dry) as input. RH influences further intensive optical aerosol properties. The backscatter fraction decreased by about 30% from 0.128 to 0.089, and the single scattering albedo increased on average by 0.05 at 85% RH compared to dry conditions. These changes in σsp, backscatter fraction and single scattering albedo have a distinct impact on the radiative forcing of the Jungfraujoch aerosol.
    Atmospheric Chemistry and Physics 03/2010; 10(5):2319-2333. DOI:10.5194/acpd-9-20063-2009 · 4.88 Impact Factor
Show more