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. The hydroxyl radical (OH) is a highly reactive species and plays a key rolein the oxidative capacity of the atmosphere. We explore the potential impactof a convective boundary layer on reconciling the calculation–measurementdifferences for OH reactivity (the inverse of OH lifetime) attributable tothe segregation of OH and its reactants by thermals and the resultingmodification of averaged reaction rates. The large-eddy simulation version ofthe Meso-NH model is used, coupled on-line with a detailed chemistrymechanism to simulate two contrasted biogenic and urban chemical regimes. Inboth environments, the top of the boundary layer is the region with thehighest calculated segregation intensities but with the opposite sign. In thebiogenic environment, the inhomogeneous mixing of isoprene and OH leads to amaximum decrease of 30 % of the mean reaction rate in this zone. In theanthropogenic case, the effective rate constant for OH reacting withaldehydes is 16 % higher than the averaged value. OH reactivity is alwayshigher by 15 to 40 % inside thermals in comparison to their surroundingsas a function of the chemical environment and time of the day. Since thermalsoccupy a small fraction of the simulated domain, the impact of turbulentmotions on domain-averaged total OH reactivity reaches a maximum decrease of9 % for the biogenic case and a maximum increase of 5 % for theanthropogenic case. Accounting for the segregation of air masses by turbulentmotions in regional and global models may increase OH reactivity in urbanenvironments but lower OH reactivity in biogenic environments. In both cases,segregation alone is insufficient for resolving the underestimation betweenobserved and modeled OH reactivity.

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