Hygroscopic behavior of multicomponent organic aerosols and their internal mixtures with ammonium sulfate

Bo Jing,Shengrui Tong,Qifan Liu,Weigang Wang,Yunhong Zhang,Kun Li,Maofa Ge

doi:10.5194/acp-16-4101-2016

Abstract

Abstract. Water-soluble organic compounds (WSOCs) are important components of organics in the atmospheric fine particulate matter. Although WSOCs play an important role in the hygroscopicity of aerosols, knowledge on the water uptake behavior of internally mixed WSOC aerosols remains limited. Here, the hygroscopic properties of single components such as levoglucosan, oxalic acid, malonic acid, succinic acid, phthalic acid, and multicomponent WSOC aerosols mainly involving oxalic acid are investigated with the hygroscopicity tandem differential mobility analyzer (HTDMA). The coexisting hygroscopic species including levoglucosan, malonic acid, and phthalic acid have a strong influence on the hygroscopic growth and phase behavior of oxalic acid, even suppressing its crystallization completely during the drying process. The phase behaviors of oxalic acid/levoglucosan mixed particles are confirmed by infrared spectra. The discrepancies between measured growth factors and predictions from Extended Aerosol Inorganics Model (E-AIM) with the Universal Quasi-Chemical Functional Group Activity Coefficient (UNIFAC) method and Zdanovskii–Stokes–Robinson (ZSR) approach increase at medium and high relative humidity (RH) assuming oxalic acid in a crystalline solid state. For the internal mixture of oxalic acid with levoglucosan or succinic acid, there is enhanced water uptake at high RH compared to the model predictions based on reasonable oxalic acid phase assumption. Organic mixture has more complex effects on the hygroscopicity of ammonium sulfate than single species. Although hygroscopic species such as levoglucosan account for a small fraction in the multicomponent aerosols, they may still strongly influence the hygroscopic behavior of ammonium sulfate by changing the phase state of oxalic acid which plays the role of "intermediate" species. Considering the abundance of oxalic acid in the atmospheric aerosols, its mixtures with hygroscopic species may significantly promote water uptake under high RH conditions and thus affect the cloud condensation nuclei (CCN) activity, optical properties, and chemical reactivity of atmospheric particles.

Highlights

Atmospheric aerosols can strongly affect the Earth’s climate and atmospheric processes (Ramanathan et al, 2001)
OA has a high vapor pressure, no evaporation losses for OA particles are observed in agreement with earlier hygroscopicity tandem differential mobility analyzer (HTDMA) studies, suggesting that the initial particles under dry conditions consisted of OA dihydrate or nonstoichiometric hydrates containing about two water molecules per oxalic acid molecule www.atmos-chem-phys.net/16/4101/2016/
OA in the initial mixed particles may exist in a liquid state and contributes to water uptake by mixed particles in the subsequent humidification (Prenni et al, 2001; Mikhailov et al, 2009), which leads to enhanced hygroscopic growth compared to predictions assuming crystalline solid oxalic acid in the mixture

Summary

Introduction

Atmospheric aerosols can strongly affect the Earth’s climate and atmospheric processes (Ramanathan et al, 2001). As one of the most important physicochemical properties, the hygroscopic properties determine the size, concentration, and phase state of aerosol particles and contribute to radiative forcing on the climate system, including both the direct forcing by absorbing or scattering light and indirect forcing through activation of cloud condensation nuclei (CCN) (Haywood and Boucher, 2000; Kanakidou et al, 2005). The water uptake of atmospheric aerosols influences their atmospheric lifetimes, reactivity, air quality, and even human health (Pandis et al, 1995; Krueger et al, 2003; Chan and Yao, 2008; Pöschl, 2005; Schneidemesser et al, 2015).

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