Abstract
Abstract. Aerosol hygroscopicity is crucial for understanding roles of aerosol particles in atmospheric chemistry and aerosol climate effects. Light-scattering enhancement factor f(RH, λ) is one of the parameters describing aerosol hygroscopicity, which is defined as f(RH, λ) = σsp(RH, λ)∕σsp(dry, λ), where σsp(RH, λ) or σsp(dry, λ) represents σsp at wavelength λ under certain relative humidity (RH) or dry conditions. Traditionally, an overall hygroscopicity parameter κ can be retrieved from measured f(RH, λ), hereinafter referred to as κf(RH), by combining concurrently measured particle number size distribution (PNSD) and mass concentration of black carbon. In this paper, a new method is proposed to directly derive κf(RH) based only on measurements from a three-wavelength humidified nephelometer system. The advantage of this newly proposed approach is that κf(RH) can be estimated without any additional information about PNSD and black carbon. This method is verified with measurements from two different field campaigns. Values of κf(RH) estimated from this new method agree very well with those retrieved by using the traditional method: all points lie near the 1 : 1 line and the square of correlation coefficient between them is 0.99. The verification results demonstrate that this newly proposed method of deriving κf(RH) is applicable at different sites and in seasons of the North China Plain and might also be applicable in other regions around the world.
Highlights
Atmospheric aerosol particles play vital roles in visibility, energy balance, and the hydrological cycle of the Earth– atmosphere system and have attracted a lot of attention in recent decades
black carbon (BC) is considered to be homogeneously mixed with other aerosol components, and the mass size distribution of BC used in Ma et al (2012), which is observed on the North China Plain (NCP), is used in this research to account for the mass distributions of BC at different particle sizes
Values of aerosol hygroscopicity parameter κ during the Wangdu campaign are first derived from measurements of f (RH) by combining measurements of BC and particle number size distribution (PNSD) in a dry state
Summary
Atmospheric aerosol particles play vital roles in visibility, energy balance, and the hydrological cycle of the Earth– atmosphere system and have attracted a lot of attention in recent decades. HTDMA systems are not capable of providing more details about the hygroscopicity of aerosol particles that contribute most to aerosol optical properties and aerosol liquid water contents (their diameters usually ranging from 200 nm to 1 μm) (Ma et al, 2012; Bian et al, 2014). Κ calculated from f (RH) measurements can be understood as an optically weighted κ and represents the overall hygroscopicity of ambient aerosol particles This κ is more suitable for being used to account for the influences of aerosol hygroscopic growth on aerosol optical properties compared to aerosol hygroscopicity derived from HTDMA measurements. Based on detailed analysis of the relationship between κf (RH) and κsca, a novel method for directly deriving κf (RH) based only on measurements from a humidified nephelometer system is proposed This newly proposed approach makes it more convenient and cheaper for researchers to conduct aerosol hygroscopicity research with f (RH) measurements
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