Measurement of relative humidity dependent light scattering of aerosols

Atmospheric Measurement Techniques Discussions 01/2009; 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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    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.
    Aerosol Science and Technology 12/2012; · 2.78 Impact Factor
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    ABSTRACT: A laboratory-based closure study was completed to compare measured and modeled optical properties and their dependence on controlled relative humidity (RH) for inorganic salts, dicarboxylic acids, and their mixtures. The closure between measured and modeled values of the light scattering coefficients were evaluated by calculating the average relative difference (ARD) values, which revealed agreement within 8.0% for the total scattering (ssp) and 14.8% for the back scattering (sbsp) at dry RH conditions for all test aerosols. These ARD values were less than the total relative uncertainty based on the measurement and modeling approaches, indicating the achievement of closure for ssp and sbsp. Optical properties derived from ssp including: (1) the hygroscopic growth factor, fσsp, (2) the backscatter ratio, b, and (3) the Ångström exponent, å, were also compared with measured values. The ARD values between corresponding measured and modeled results for these derived optical parameters ranged from 0.1% to 30.8%. The impact of particulate organic matter (POM) on optical and hygroscopic properties of the aerosols tested here was compared to the aerosol optical and composition measurements that occurred during the New England Air Quality Study-Intercontinental Transport and Chemical Transformation field campaign. Such comparison confirmed that a larger POM mass fraction resulted in less hygroscopicity for both the ambient and the laboratory aerosols. This study evaluated closure between laboratory measurements and model calculations and validated the reliability of the model with the closure analysis. Therefore, in the future the Mie-Lorentz model can be used to calculate the optical properties and their dependence on RH for other aerosols with more confidence.
    Atmospheric Environment 09/2013; 81:177-187. · 3.11 Impact Factor
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    ABSTRACT: The relative humidity (RH) dependence of aerosol light scattering is an essential parameter for accurate estimation of the direct radiative forcing induced by aerosol particles. On account of the insufficient information of aerosol hygroscopicity in climate models, more details of the parameterized hygroscopic growth factors are urgently required. In this paper, a retrieval method to calculate the aerosol hygroscopicity parameter, κ, is proposed based on the in situ measured aerosol light scattering enhancement factor, namely f(RH), and particle number size distribution (PNSD) obtained from the HaChi (Haze in China) campaign. Measurements show that f(RH) sharply increases with the ascending RH, and the variation range of f(RH) is much wider at higher RH. Sensitivity study reveals that the f(RH) is more sensitive to the aerosol hygroscopicity than PNSD. f(RH) for polluted cases is distinctly higher than that for clean periods at a specific RH. The derived equivalent κ, combining with the PNSD measurements, is applied in the prediction of the CCN number concentration. Comparison between the predicted CCN number concentration with the derived equivalent κ and the measured ones agrees well, especially at high supersaturations. The proposed calculation algorithm of κ with the f(RH) measurements is demonstrated to be reasonable and can be widely used.
    01/2014; 14(3).

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