Modelling non-equilibrium secondary organic aerosol formation and evaporation with the aerosol dynamics, gas- and particle-phase chemistry kinetic multilayer model ADCHAM

  • Roldin P
  • Eriksson A
  • Nordin E
 et al. 
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We have developed the novel Aerosol Dynam-ics, gas-and particle-phase chemistry model for laboratory CHAMber studies (ADCHAM). The model combines the detailed gas-phase Master Chemical Mechanism version 3.2 (MCMv3.2), an aerosol dynamics and particle-phase chem-istry module (which considers acid-catalysed oligomeriza-tion, heterogeneous oxidation reactions in the particle phase and non-ideal interactions between organic compounds, wa-ter and inorganic ions) and a kinetic multilayer module for diffusion-limited transport of compounds between the gas phase, particle surface and particle bulk phase. In this arti-cle we describe and use ADCHAM to study (1) the evap-oration of liquid dioctyl phthalate (DOP) particles, (2) the slow and almost particle-size-independent evaporation of α-pinene ozonolysis secondary organic aerosol (SOA) parti-cles, (3) the mass-transfer-limited uptake of ammonia (NH 3) and formation of organic salts between ammonium (NH + 4) and carboxylic acids (RCOOH), and (4) the influence of chamber wall effects on the observed SOA formation in smog chambers. ADCHAM is able to capture the observed α-pinene SOA mass increase in the presence of NH 3 (g). Organic salts of ammonium and carboxylic acids predominantly form during the early stage of SOA formation. In the smog chamber ex-periments, these salts contribute substantially to the initial growth of the homogeneously nucleated particles. The model simulations of evaporating α-pinene SOA par-ticles support the recent experimental findings that these par-ticles have a semi-solid tar-like amorphous-phase state. AD-CHAM is able to reproduce the main features of the ob-served slow evaporation rates if the concentration of low-volatility and viscous oligomerized SOA material at the par-ticle surface increases upon evaporation. The evaporation rate is mainly governed by the reversible decomposition of oligomers back to monomers. Finally, we demonstrate that the mass-transfer-limited up-take of condensable organic compounds onto wall-deposited particles or directly onto the Teflon chamber walls of smog chambers can have a profound influence on the observed SOA formation. During the early stage of the SOA forma-tion the wall-deposited particles and walls themselves serve as an SOA sink from the air to the walls. However, at the end of smog chamber experiments the semi-volatile SOA mate-rial may start to evaporate from the chamber walls. With these four model applications, we demonstrate that several poorly quantified processes (i.e. mass transport limi-tations within the particle phase, oligomerization, heteroge-neous oxidation, organic salt formation, and chamber wall effects) can have a substantial influence on the SOA forma-tion, lifetime, chemical and physical particle properties, and their evolution. In order to constrain the uncertainties related to these processes, future experiments are needed in which as many of the influential variables as possible are varied. ADCHAM can be a valuable model tool in the design and analysis of such experiments. Published by Copernicus Publications on behalf of the European Geosciences Union. 7954 P. Roldin et al.: Modelling non-equilibrium secondary organic aerosol formation

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  • P. Roldin

  • A. C. Eriksson

  • E. Z. Nordin

  • E. Hermansson

  • D. Mogensen

  • A. Rusanen

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