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

Abstract

Abstract. The hydroxyl radical (OH) is a highly reactive specie and plays a key role in the oxidative capacity of the atmosphere. The total OH reactivity, corresponding to the inverse of OH lifetime, may have a significant fraction non-attributable to commonly measured compounds. The turbulence-driven segregation of OH and its reactants can cause substantial modification of averaged reaction rates, and thus of the total OH reactivity, when compared to a perfectly mixed assumption. We study the impact of turbulent mixing on the OH reactivity with Large-Eddy Simulations from the Meso-NH model coupled on-line with a 5 detailed chemistry mechanism in two contrasted regimes. Our findings show that the non-mixing of isoprene (resp. aldehydes) and OH leads to 30% decrease (resp. 16% increase) of the mean reaction rate at the top of the boundary layer and consequently to 9% decrease (resp. 5% increase) of the OH total reactivity in a biogenic (resp. anthropogenic) environment. Moreover, the total OH reactivity is highest inside thermals in both cases.

Large Eddy Simulation for Investigating Coupled Forest Canopy and Turbulence Influences on Atmospheric Chemistry

“…Our estimates are also much lower than previous work estimating OH‐ISOP $${I}_{\mathrm{s}}$$ of up to −13% in a clear ABL using airborne measurements (Butler et al., 2008; Kaser et al., 2015). Other estimates with LES or airborne measurements suggests that heterogeneous isoprene emissions and clouds can alter OH‐ISOP $${I}_{\mathrm{s}}$$ (Brosse et al., 2018; Karl et al., 2007; Kaser et al., 2015; Kim et al., 2016; Krol et al., 2000; Li, Barth et al., 2016; Ouwersloot et al., 2011), but only clear‐sky conditions and a canopy with homogenous structure and composition are investigated here.…”