Abstract

Atmospheric processes, including both primary emissions and secondary formation, may exert complex effects on aerosol hygroscopicity, which is of significant importance in understanding and quantifying the effect of aerosols on climate and human health. In order to explore the influence of local emissions and secondary formation processes on aerosol hygroscopicity, we investigated the hygroscopic properties of submicron aerosol particles at a rural site in the North China Plain (NCP) in winter 2018. This was conducted by simultaneous measurements of aerosol hygroscopicity and chemical composition, using a self-assembled hygroscopic tandem differential mobility analyzer (HTDMA) and a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM). The hygroscopicity results showed that the particles during the entire campaign were mainly externally mixed, with a more hygroscopic (MH) mode and a less hygroscopic (LH) particles mode. The mean hygroscopicity parameter values (κmean) derived from hygroscopicity measurements for particles at 60, 100, 150, and 200 nm were 0.16, 0.18, 0.16, and 0.15, respectively. During this study, we classified two distinct episodes with different RH/T conditions, indicative of different primary emissions and secondary formation processes. It was observed that aerosols at all measured sizes were more hygroscopic under the high RH (HRH) episode than those under the low RH (LRH) episode. During the LRH, κ decreased with increasing particle size, which may be explained by the enhanced domestic heating at low temperature, causing large emissions of non- or less-hygroscopic primary aerosols. This is particularly obvious for 200 nm particles, with a dominant number fraction (> 50 %) of LH mode particles. Using O : C-dependent hygroscopic parameters of secondary organic compounds (κSOA), closure analysis between the HTDMA_measured κ and the ACSM_derived κ was carried out. The results showed that κSOA under the LRH episode was less sensitive to the changes in organic oxidation level, while κSOA under the HRH had a relatively stronger dependency on the organic O : C. This feature suggests that the different sources and aerosol evolution processes, partly resulting from the variation in atmospheric RH/T conditions, may lead to significant changes in aerosol chemical composition, which will further influence their corresponding physical properties.

Highlights

  • The hygroscopicity of aerosol, describing its tendency to absorb moisture from the environment, plays an important role in understanding and quantifying its effects on climate and human health (Martin et al, 2004; McFiggans et al, 2006; Swietlicki et al, 2008; Tao et al, 2012)

  • We aim to explore the effects of different atmospheric processes on the aerosol 105 hygroscopicity in the North China Plain (NCP)

  • 200 4.1 Overview of the measurement Figure 1 displays the time series of meteorological conditions, mass concentration of PM2.5 and BC, the averaged hygroscopicity parameter (κ) and κ probability density function (κ-PDF) for particles at dry sizes of 60, 100, 150 and 200 nm and the mass fractions of chemical components in PM1 measured by ACSM, 205 respectively

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Summary

Introduction

The hygroscopicity of aerosol, describing its tendency to absorb moisture from the environment, plays an important role in understanding and quantifying its effects on climate and human health (Martin et al, 2004; McFiggans et al, 2006; Swietlicki et al, 2008; Tao et al, 2012). Due to the absorbed water by aerosol particles, the 60 hygroscopicity of particles is strongly associated with heterogeneous and multiphase chemistry, regulating the abundance of different species in the gas and particle phases (Hennigan et al, 2009; Ye et al, 2011). 65 According to the Köhler theory, the propensity of aerosols to uptake water at a certain sub/supersaturation is mainly determined by the particle size and various properties of the species within the particles, such as solubility, molecular weight, density, that is to say, their chemical composition (Petters and Kreidenweis, 2008, 2007). Ambient aerosols with different chemical composition, which vary widely with their sources and atmospheric processes, may show significant difference in their

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