LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa
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Published:2018-05-09
Issue:9
Volume:18
Page:6601-6624
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Brosse FabienORCID, Leriche Maud, Mari Céline, Couvreux Fleur
Abstract
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.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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