Phase state of ambient aerosol linked with water uptake and chemical aging in the southeastern US

Aki Pajunoja,Brett B Palm,Annele Virtanen,Yu J Leong,Weiwei Hu,Pasi Miettinen,Nathan F Taylor,Jose L Jimenez,Santtu Mikkonen,Don R Collins

doi:10.5194/acp-16-11163-2016

Abstract

Abstract. During the summer 2013 Southern Aerosol and Oxidant Study (SOAS) field campaign in a rural site in the southeastern United States, the effect of hygroscopicity and composition on the phase state of atmospheric aerosol particles dominated by the organic fraction was studied. The analysis is based on hygroscopicity measurements by a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA), physical phase state investigations by an Aerosol Bounce Instrument (ABI) and composition measurements using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). To study the effect of atmospheric aging on these properties, an OH-radical oxidation flow reactor (OFR) was used to simulate longer atmospheric aging times of up to 3 weeks. Hygroscopicity and bounce behavior of the particles had a clear relationship showing higher bounce at elevated relative humidity (RH) values for less hygroscopic particles, which agrees well with earlier laboratory studies. Additional OH oxidation of the aerosol particles in the OFR increased the O : C and the hygroscopicity resulting in liquefying of the particles at lower RH values. At the highest OH exposures, the inorganic fraction starts to dominate the bounce process due to production of inorganics and concurrent loss of organics in the OFR. Our results indicate that at typical ambient RH and temperature, organic-dominated particles stay mostly liquid in the atmospheric conditions in the southeastern US, but they often turn semisolid when dried below ∼ 50 % RH in the sampling inlets. While the liquid phase state suggests solution behavior and equilibrium partitioning for the SOA particles in ambient air, the possible phase change in the drying process highlights the importance of thoroughly considered sampling techniques of SOA particles.

Highlights

Atmospheric secondary organic aerosols (SOAs) result from gas-phase oxidation of volatile organic compounds (VOCs) (Hallquist et al, 2009), which are emitted from anthropogenic and biogenic sources
For oxidation flow reactor (OFR) data, fOA has negative and fABS positive correlation with O : C which is due to loss of organics at high OH exposures
Based on Aerosol Bounce Instrument (ABI) measurements, we found that the phase transition from semisolid to liquid phase starts in the relative humidity (RH)

Summary

Introduction

Atmospheric secondary organic aerosols (SOAs) result from gas-phase oxidation of volatile organic compounds (VOCs) (Hallquist et al, 2009), which are emitted from anthropogenic and biogenic sources. The semisolid or solid phase state of the SOA particles can limit the diffusion of condensable gas-phase molecules from the surface into the particle bulk (Koop et al, 2011; Shiraiwa et al, 2011; Riipinen et al, 2012; Kuwata and Martin, 2012; Lienhard et al, 2014) This may affect inner mixing and disturb the equilibrium in gas–particle partitioning and result in slower evaporation of the particles than expected (Vaden et al, 2011; Saleh et al, 2011; Perraud et al, 2012; Abramson et al, 2013). It should be noted that despite the recent SOA viscosity studies, most of the current regional and global aerosol models treat particles as liquid droplets considering no particle phase diffusion limitations

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