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Volume 18, issue 9 | Copyright

Special issue: Results of the project "Dynamics–aerosol–chemistry–cloud...

Atmos. Chem. Phys., 18, 6601-6624, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 May 2018

Research article | 09 May 2018

LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa

Fabien Brosse1, Maud Leriche1, Céline Mari1, and Fleur Couvreux2 Fabien Brosse et al.
  • 1Laboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, France
  • 2CNRM, Météo-France & CNRS, Toulouse, France

Abstract. The hydroxyl radical (OH) is a highly reactive species and plays a key role in the oxidative capacity of the atmosphere. We explore the potential impact of a convective boundary layer on reconciling the calculation–measurement differences for OH reactivity (the inverse of OH lifetime) attributable to the segregation of OH and its reactants by thermals and the resulting modification of averaged reaction rates. The large-eddy simulation version of the Meso-NH model is used, coupled on-line with a detailed chemistry mechanism to simulate two contrasted biogenic and urban chemical regimes. In both environments, the top of the boundary layer is the region with the highest calculated segregation intensities but with the opposite sign. In the biogenic environment, the inhomogeneous mixing of isoprene and OH leads to a maximum decrease of 30% of the mean reaction rate in this zone. In the anthropogenic case, the effective rate constant for OH reacting with aldehydes is 16% higher than the averaged value. OH reactivity is always higher by 15 to 40% inside thermals in comparison to their surroundings as a function of the chemical environment and time of the day. Since thermals occupy a small fraction of the simulated domain, the impact of turbulent motions on domain-averaged total OH reactivity reaches a maximum decrease of 9% for the biogenic case and a maximum increase of 5% for the anthropogenic case. Accounting for the segregation of air masses by turbulent motions in regional and global models may increase OH reactivity in urban environments but lower OH reactivity in biogenic environments. In both cases, segregation alone is insufficient for resolving the underestimation between observed and modeled OH reactivity.

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The cleansing capacity of the atmosphere is studied through the hydroxyl radical (OH) chemical reactivity in numerical simulations of natural and urban environments. Turbulence-driven segregation of chemical compounds in the atmospheric boundary layer is explored and may partially explain discrepancies between observed and modeled OH reactivity in both environments.
The cleansing capacity of the atmosphere is studied through the hydroxyl radical (OH) chemical...