Measurement report: The effect of aerosol chemical composition on light scattering due to the hygroscopic swelling effect

Rongmin Ren,Peng Yan,Hao Wu,Yanan Li,Yuying Wang,Xiao'Ai Jin,Maureen Cribb,Dongmei Zhang,Zhanqing Li,Wei Wang

doi:10.5194/acp-21-9977-2021

Abstract

Abstract. Liquid water in aerosol particles has a significant effect on their optical properties, especially on light scattering, whose dependence on chemical composition is investigated here using measurements made in southern Beijing in 2019. The effect is measured by the particle light scattering enhancement f(RH), where RH denotes the relative humidity, which is found to be positively and negatively impacted by the proportions of inorganic and organic matter, respectively. Black carbon is also negatively correlated. The positive impact is more robust when the inorganic matter mass fraction was smaller than 40 % (R=0.93, R: the Pearson's correlation coefficient), becoming weaker as the inorganic matter mass fraction gets larger (R=0.48). A similar pattern was also found for the negative impact of the organic matter mass fraction. Nitrate played a more significant role in aerosol hygroscopicity than sulfate in Beijing. However, the deliquescence point of ambient aerosols was at about RH = 80 % when the ratio of the sulfate mass concentration to the nitrate mass concentration of the aerosol was high (mostly higher than ∼ 4). Two schemes to parameterize f(RH) were developed to account for the deliquescent and non-deliquescent effects. Using only one f(RH) parameterization scheme to fit all f(RH) processes incurs large errors. A piecewise parameterization scheme is proposed, which can better describe deliquescence and reduces uncertainties in simulating aerosol hygroscopicity.

Highlights

Atmospheric aerosols have impacts on visibility, the earth– atmosphere radiation budget, clouds, and precipitation via direct and indirect effects (IPCC, 2013). Both effects are associated with the hygroscopic properties of aerosols and relative humidity (RH) of the atmosphere
The instruments used in this field experiment include a dual-nephelometer system (Aurora 3000, Ecotech), an aerosol chemical speciation monitor (ACSM; Aerodyne Research Inc.), and a seven-wavelength aethalometer (AE33, Magee Scientific)
We assume that the aerosol is in the dry state when RH < 40 %

Summary

Introduction

Atmospheric aerosols have impacts on visibility, the earth– atmosphere radiation budget, clouds, and precipitation via direct and indirect effects (IPCC, 2013). The particle light scattering enhancement factor, f (RH, λ), is the ratio of the scattering coefficient at an elevated RH level to that under a fixed low RH level (usually RH < 40 %) at a certain light wavelength (λ). It has been characterized during international field experiments (Fierz-Schmidhauser et al, 2010a, b; Zieger et al, 2010, 2014) and in particular China It has been characterized during international field experiments (Fierz-Schmidhauser et al, 2010a, b; Zieger et al, 2010, 2014) and in particular China (Yan et al, 2009; Zhang et al, 2015; Kuang et al, 2016; L. Liu et al, 2018; C. Zhao et al, 2019; Zhao, 2019; Wu et al, 2020)

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