Abstract
Abstract. Scattering of solar radiation by aerosol particles is highly dependent on relative humidity (RH) as hygroscopic particles take up water with increasing RH. To achieve a better understanding of the effect of aerosol hygroscopic growth on light scattering properties and radiative forcing, the aerosol scattering coefficients at RH in the range of 40 to ~ 90 % were measured using a humidified nephelometer system in the Yangtze River Delta of China in March 2013. In addition, the aerosol size distribution and chemical composition were measured. During the observation period, the mean and standard deviation (SD) of enhancement factors at RH = 85 % for the scattering coefficient (f(85 %)), backscattering coefficient (fb(85 %)), and hemispheric backscatter fraction (fβ(85 %)) were 1.58 ± 0.12, 1.25 ± 0.07, and 0.79 ± 0.04, respectively, i.e., aerosol scattering coefficient and backscattering coefficient increased by 58 and 25 % as the RH increased from 40 to 85 %. Concurrently, the aerosol hemispheric backscatter fraction decreased by 21 %. The relative amount of organic matter (OM) or inorganics in PM1 was found to be a main factor determining the magnitude of f(RH). The highest values of f(RH) corresponded to the aerosols with a small fraction of OM, and vice versa. The relative amount of NO3− in fine particles was strongly correlated with f(85 %), which suggests that NO3− played a vital role in aerosol hygroscopic growth during this study. The mass fraction of nitrate also had a close relationship to the curvature of the humidograms; higher mass fractions of nitrate were associated with humidograms that had the least curvature. Aerosol hygroscopic growth caused a 47 % increase in the calculated aerosol direct radiative forcing at 85 % RH, compared to the forcing at 40 % RH.
Highlights
Hygroscopic aerosols take up water as humidity increases (Engelhart et al, 2011; Pilinis et al, 1989; Hänel, 1976; Covert et al, 1972)
Accurate performance of nephelometers and relative humidity (RH) sensors is crucial to retrieve reliable enhancement factors (f (RH,λ), fb(RH,λ) and fβ (RH,λ)), since they are defined as the aerosol scattering coefficient/backscattering coefficient/hemispheric backscatter fractions at a higher RH than those at a low RH
In March, the hourly averaged aerosol scattering coefficient, measured under dry conditions (Fig. 3c), varied from 21 to 1067 M m−1, and the maximum occurred on 16 March, when severe haze occurred
Summary
Hygroscopic aerosols take up water as humidity increases (Engelhart et al, 2011; Pilinis et al, 1989; Hänel, 1976; Covert et al, 1972). In order to better understand the aerosol light scattering properties and their dependency on relative humidity in the Yangtze River Delta, both the scattering and backscattering coefficients under dry (RH < 40 %) conditions and controlled, elevated relative humidity were measured, along with the chemical composition and particle number size distribution. The impact of relative humidity on the aerosol light scattering coefficient is called the scattering enhancement factor f (RH,λ), defined as f (RH, λ) = σsp (RH, λ ) /σsp (dry, λ) ,. Where σsp(dry,λ) and σsp(RH,λ) represent scattering coefficients at wavelength λ in dry conditions and at a defined higher relative humidity, respectively. Where σbsp(dry,λ) and σbsp(RH,λ) represent backscattering coefficients at wavelength λ in dry conditions and at a defined relative humidity, respectively. All the parameters discussed in this study are based on the measurements at 550 nm wavelength only
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