Estimation of secondary organic aerosol viscosity from explicit modeling of gas-phase oxidation of isoprene and &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-pinene

Tommaso Galeazzo,Richard Valorso,Manabu Shiraiwa,Ying Li,Bernard Aumont,Marie Camredon

doi:10.5194/acp-21-10199-2021

Abstract

Abstract. Secondary organic aerosols (SOA) are major components of atmospheric fine particulate matter, affecting climate and air quality. Mounting evidence exists that SOA can adopt glassy and viscous semisolid states, impacting formation and partitioning of SOA. In this study, we apply the GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere) model to conduct explicit chemical modeling of isoprene photooxidation and α-pinene ozonolysis and their subsequent SOA formation. The detailed gas-phase chemical schemes from GECKO-A are implemented into a box model and coupled to our recently developed glass transition temperature parameterizations, allowing us to predict SOA viscosity. The effects of chemical composition, relative humidity, mass loadings and mass accommodation on particle viscosity are investigated in comparison with measurements of SOA viscosity. The simulated viscosity of isoprene SOA agrees well with viscosity measurements as a function of relative humidity, while the model underestimates viscosity of α-pinene SOA by a few orders of magnitude. This difference may be due to missing processes in the model, including autoxidation and particle-phase reactions, leading to the formation of high-molar-mass compounds that would increase particle viscosity. Additional simulations imply that kinetic limitations of bulk diffusion and reduction in mass accommodation coefficient may play a role in enhancing particle viscosity by suppressing condensation of semi-volatile compounds. The developed model is a useful tool for analysis and investigation of the interplay among gas-phase reactions, particle chemical composition and SOA phase state.

Highlights

Secondary organic aerosols (SOA) are ubiquitous in the atmosphere and represent a major component of fine particulate matter, affecting air quality, climate and public health (Jimenez et al, 2009; Pöschl and Shiraiwa, 2015)
SOA generated by isoprene ozonolysis adopt a semi-solid state for relative humidity (RH) < 40 % and a liquid state for RH > 60 %
We applied the Tg parameterizations and viscosity prediction method in the GECKO-A box model to simulate the evolution of viscosity and composition of SOA generated via α-pinene and isoprene oxidation

Summary

Introduction

Secondary organic aerosols (SOA) are ubiquitous in the atmosphere and represent a major component of fine particulate matter, affecting air quality, climate and public health (Jimenez et al, 2009; Pöschl and Shiraiwa, 2015). Due to their complexity, SOA represent a large source of uncertainty in current understanding of global climate change and air pollution (Tsigaridis et al, 2014; Ciarelli et al, 2019). The GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere) model is to date the most extensive generator of gas-phase chemical schemes; based on established reaction pathways and structure–activity relationships, it automatically generates detailed gas-phase mechanisms involving thousands to millions of oxidation products from a given VOC precursor (Aumont et al, 2005, 2012; Lee-Taylor et al, 2011)

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