A large-eddy simulation of the phase transition of ammonium nitrate in a convective boundary layer

Authors

  • J. M. J. Aan de Brugh,

    Corresponding author
    1. Department of Meteorology and Air Quality, Wageningen University, Wageningen, The Netherlands
    2. TNO, Earth, Environment and Life Sciences, Research Group Climate, Air and Sustainability, Utrecht, The Netherlands
    3. Now at: Netherlands Institute of Space Research, Utrecht, The Netherlands
    • Corresponding author: J. M. J. Aan deBrugh,Department of Meteorology and Air Quality, Wageningen University, Bldg. 100, 6708 PB, Wageningen, The Netherlands. (joost.aandebrugh@wur.nl)

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  • H. G. Ouwersloot,

    1. Department of Meteorology and Air Quality, Wageningen University, Wageningen, The Netherlands
    2. Max Planck Institute for Chemistry, Mainz, Germany
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  • J. Vilà-Guerau de Arellano,

    1. Department of Meteorology and Air Quality, Wageningen University, Wageningen, The Netherlands
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  • M. C. Krol

    1. Department of Meteorology and Air Quality, Wageningen University, Wageningen, The Netherlands
    2. Institute for Marine and Atmospheric Research, Utrecht, The Netherlands
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Abstract

[1] Under warm and dry conditions, ammonium nitrate aerosol outgasses to form ammonia and nitric acid in the lower atmospheric boundary layer. In the upper boundary layer, where the temperature is lower and the relative humidity is higher, nitric acid and ammonia condense back to the aerosol phase. Measurements show that aerosol nitrate mixing ratios increase with altitude, confirming this phase transition. Since phase equilibrium is not reached instantaneously, updrafts transport aerosol-poor air from the surface to high altitudes and aerosol-rich air subsides from high altitudes to the surface under turbulent conditions. As a result, the partitioning deviates from equilibrium, so the horizontal and temporal variabilities of the aerosol nitrate mixing ratio are enhanced and a continuous downward aerosol nitrate flux emerges. We postulate that observations of this variability and flux should not be interpreted as instrument noise and deposition of nitrate. In an idealized large-eddy simulation (LES) experiment of a convective boundary layer, we find that the larger variability and flux occurred at about one third of the boundary layer height. Both are largest when the gas-aerosol partitioning time scale is assumed to be about half the time scale of turbulence. Our LES result shows negatively skewed nitrate mixing ratios. Under colder conditions, a smaller fraction of ammonium nitrate aerosol outgasses at the surface, so the absolute effect of nitrate repartitioning becomes smaller. However, dimensionless statistical properties do not change as long as the turbulent properties of the boundary layer remain similar. This indicates that the identified processes are also present under colder conditions.

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