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Discussion papers
https://doi.org/10.5194/acp-2019-76
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-76
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Feb 2019

Research article | 05 Feb 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Timescales of Secondary Organic Aerosols to Reach Equilibrium at Various Temperatures and Relative Humidities

Ying Li and Manabu Shiraiwa Ying Li and Manabu Shiraiwa
  • Department of Chemistry, University of California, Irvine, California, USA

Abstract. Secondary organic aerosols (SOA) account for a substantial fraction of air particulate matter and SOA formation is often modeled assuming rapid establishment of gas-particle equilibrium. Here, we estimate the characteristic timescale for SOA to achieve gas−particle equilibrium under a wide range of temperatures and relative humidities using a state-of-the-art kinetic flux model. Equilibration timescales were calculated by varying particle phase state, size, mass loadings, and volatility of organic compounds. Model simulations suggest that the equilibration timescale for semi-volatile compounds is on the order of seconds or minutes for most conditions in the planetary boundary layer, but it can be longer than one hour if particles adopt glassy or amorphous solid states with high glass transition temperature at low relative humidity. In the free troposphere with lower temperatures it can be longer than hours or days even at moderate or relatively high RH due to kinetic limitations of bulk diffusion in highly viscous particles. The timescale of partitioning of low-volatile compounds is shorter compared to semi-volatile compounds, as it is largely determined by condensation sink due to very slow re-evaporation. These results provide critical insights into thermodynamic or kinetic treatments of SOA partitioning for accurate predictions of gas- and particle-phase concentrations of semi-volatile compounds in regional and global chemical transport models.

Ying Li and Manabu Shiraiwa
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Status: open (until 02 Apr 2019)
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Ying Li and Manabu Shiraiwa
Ying Li and Manabu Shiraiwa
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Short summary
Timescales for secondary organic aerosols to reach equilibrium were estimated under various temperatures and relative humidities. Equilibration timescales in the free troposphere can be longer than hours or days even at moderate or relatively high relative humidity. These results provide critical insights into thermodynamic or kinetic treatments of SOA partitioning for accurate predictions of gas- and particle-phase concentrations of semi-volatile compounds in chemical transport models.
Timescales for secondary organic aerosols to reach equilibrium were estimated under various...
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