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Discussion papers
https://doi.org/10.5194/acp-2018-215
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-215
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Mar 2018

Research article | 13 Mar 2018

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This discussion paper is a preprint. A revision of the manuscript for further review has not been submitted.

Is Mass Transfer in Secondary Organic Aerosol Particles Intrinsically Slow? Equilibration Timescales of Engine Exhaust and α-Pinene SOA Under Dry and Humid Conditions

Khairallah Atwi1,a, Mohamad Baassiri1, Mariam Fawaz1,b, and Alan Shihadeh1 Khairallah Atwi et al.
  • 1Aerosol Research Laboratory, Department of Mechanical Engineering, American University of Beirut, Beirut, Lebanon
  • acurrent address: Air Quality and Climate Research Lab, University of Georgia, Athens, Georgia, USA
  • bcurrent address: University of Illinois at Urbana-Champaign, Urbana, Illinois, USA

Abstract. Semi-volatile secondary organic aerosols (SOA) comprise a major fraction of ambient particle pollutants. The partitioning of SOA in the atmosphere has commonly been assumed to be fast enough that it could be computed solely from thermodynamic equilibrium considerations e.g., using Raoult's Law. This simplifying assumption has been called into question by recent studies of single SOA particles evaporating in a zero-vapor concentration environment, which reported unexpectedly slow evaporation relative to atmospheric timescales. In this work we directly investigated the phase equilibration kinetics of systems of SOA particles under realistic atmospheric conditions. SOA was generated in an oxidation flow reactor (OFR) from engine exhaust or α-pinene and mixed with clean air in an atmospheric pressure smog chamber (32°C) to induce evaporation. The evolution of the particle size distribution was monitored over time as the aerosol system returned to phase equilibrium under different particle concentrations (2.5 and 5µgm−3) and humidity conditions (<10% and 60%). We found that under typical ambient conditions, and independent of relative humidity and precursor origin (engine exhaust vs. α-pinene), SOA reestablished equilibrium with the vapor phase within minutes, and that the evolution of particle size was well-fit by a computational model treating the particle phase as well-mixed. The effective thermodynamic saturation concentration of the SOA was found to be in the range 0.02–0.11µgm−3 at 20°C, assuming an enthalpy of vaporization of 150kJmol−1. The effective evaporation coefficient was found to be in the range 0.1–0.2 using a gas diffusion coefficient of 5×10−6m2s−1. Unlike previous single-particle studies, this data suggests that under most loading conditions, anthropogenic and biogenic SOA can rapidly attain phase equilibrium in the atmosphere and that their partitioning can be modeled assuming thermodynamic equilibrium.

Khairallah Atwi et al.
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Khairallah Atwi et al.
Khairallah Atwi et al.
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Short summary
Secondary organic aerosols (SOA) influence health and climate. A major simplifying assumption usually made in airshed models is that SOA particles attain instantaneous phase equilibrium with their surroundings. This assumption has been questioned by several recent studies. We generated engine exhaust and biogenic SOA and investigated their evaporation behavior upon dilution. Consistent with current practice, we found that SOA reestablish equilibrium on timescales small enough to be neglected.
Secondary organic aerosols (SOA) influence health and climate. A major simplifying assumption...
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