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).

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    • "It should be noted that different authors used various reference RHs, ranging from 10 to 45% for the " dry " value, and 62 to 90% for the " wet " value (Kotchenruther and Hobbs 1998; Sheridan et al. 2002; Howell et al. 2006; Fierz-Schmidhauser et al. 2010), which makes direct comparison of the results difficult and/or confusing. The scattering factor F can also be assessed from the two or three parameter fit (Equation (1)); here, we will base our assessment of F on the most frequently used " dry " and " wet " RH values of 40 and 85% respectively, and the three parameter "
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    ABSTRACT: Modeling and measurements of aerosol properties is complicated by the hygroscopic behavior of the aerosols adding significant uncertainty to our best estimates of the direct effect aerosols exert on the radiative balance of the atmosphere. Airborne measurements of aerosol hygroscopicity are particularly challenging but critically needed. This motivated the development of a newly designed system which can measure the dependence of the aerosol light scattering coefficient (σsp) on relative humidity (RH), known as f(RH), in real-time at a rapid rate (<10 s) on an aerial platform. The new system has several advantages over existing systems. It consists of three integrating nephelometers and humidity conditioners for simultaneous measurement of the σsp at three different RHs. The humidity is directly controlled in exchanger cells without significant temperature disturbances and without particle dilution, heating or loss of volatile compounds. The single-wavelength nephelometers are illuminated by LED-based light sources thereby minimizing heating of the sample stream. The flexible design of the RH conditioners, consisting of a number of specially designed exchanger cells (driers or humidifiers), enables us to measure f(RH) under hydration or dehydration conditions (always starting with the aerosol in a known state) with a simple system re-configuration. These exchanger cells have been characterized for losses of particles using latex spheres and laboratory generated ammonium sulfate aerosols. Residence times of 6 - 9 s in the exchangers and subsequent lines is sufficient for most aerosols to attain equilibrium with the new water vapor content. The performance of this system has been assessed aboard DOE’s G-1 research aircraft during test flights over California, Oregon, and Washington.
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    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
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