Organic aerosol volatility and viscosity in the North China Plain: contrast between summer and winter

Weiqi Xu,Zhijie Li,Zhiqiang Zhang,Yele Sun,Zifa Wang,Ying Li,Wanyun Xu,Jiang Zhu,Eleni Karnezi,Nan Ma,Conghui Xie,Yanmei Qiu,Pingqing Fu,Jiaxing Sun,Chun Chen,Douglas R Worsnop,Nga Lee Ng,Spyros N Pandis

doi:10.5194/acp-21-5463-2021

Abstract

Abstract. Volatility and viscosity have substantial impacts on gas–particle partitioning, formation and evolution of aerosol and hence the predictions of aerosol-related air quality and climate effects. Here aerosol volatility and viscosity at a rural site (Gucheng) and an urban site (Beijing) in the North China Plain (NCP) in summer and winter were investigated by using a thermodenuder coupled with a high-resolution aerosol mass spectrometer. The effective saturation concentration (C*) of organic aerosol (OA) in summer was smaller than that in winter (0.55 µg m−3 vs. 0.71–0.75 µg m−3), indicating that OA in winter in the NCP is more volatile due to enhanced primary emissions from coal combustion and biomass burning. The volatility distributions varied and were largely different among different OA factors. In particular, we found that hydrocarbon-like OA (HOA) contained more nonvolatile compounds compared to coal-combustion-related OA. The more oxidized oxygenated OA (MO-OOA) showed overall lower volatility than less oxidized OOA (LO-OOA) in both summer and winter, yet the volatility of MO-OOA was found to be relative humidity (RH) dependent showing more volatile properties at higher RH. Our results demonstrated the different composition and chemical formation pathways of MO-OOA under different RH levels. The glass transition temperature (Tg) and viscosity of OA in summer and winter are estimated using the recently developed parameterization formula. Our results showed that the Tg of OA in summer in Beijing (291.5 K) was higher than that in winter (289.7–290.0 K), while it varied greatly among different OA factors. The viscosity suggested that OA existed mainly as solid in winter in Beijing (RH = 29 ± 17 %), but as semisolids in Beijing in summer (RH = 48 ± 25 %) and Gucheng in winter (RH = 68 ± 24 %). These results have the important implication that kinetically limited gas–particle partitioning may need to be considered when simulating secondary OA formation in the NCP.

Highlights

Organic aerosol (OA) accounts for a substantial mass fraction of atmospheric fine particulate matter (Jimenez et al, 2009)
We found that the volatile properties of fossil-fuel-related OA were quite different between urban and rural sites, likely due to variations in oxidation during transport, different coal fuels and combustion conditions
We found that the volatility of more oxidized OOA (MOOOA) was relative humidity (RH) dependent, with higher volatility at higher RH levels

Summary

Introduction

Organic aerosol (OA) accounts for a substantial mass fraction of atmospheric fine particulate matter (Jimenez et al, 2009). Volatility and viscosity are two important properties of OA They have substantial impacts on the gas–particle partitioning of oxidized compounds (Shiraiwa and Seinfeld, 2012; Liu et al, 2018) and the formation and evolution of OA, which further contributes to the uncertainty in predictions of aerosol-related air quality and climate effects (Glasius and Goldstein, 2016; Shrivastava et al, 2017). The evaporation kinetics of limonene SOA particles at low RH levels (< 5 % and 50 %) are nearly the same, while a slightly larger fraction evaporates at higher RH (90 %) (Wilson et al, 2015; Lee et al, 2011b). Huang et al (2019)

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Conclusion