Effects of turbulence and heterogeneous emissions on photochemically active species in the convective boundary layer

Photochemistry is studied in a convective atmospheric boundary layer. The e ssential reactions that account for the ozone formation and depletion are i ncluded in the chemical mechanism which, as a consequence, contains a wide range of timescales. The turbulent reacting flow is modeled with a large-ed dy simulation (LES) code. The deviations from chemical equilibrium that are caused by turbulent motions are investigated in terms of the intensity of segregation For the studied cases it is found that the volume-averaged conc entrations calculated with the LES code agree well with the concentrations calculated with a box model. The reaction rate between RH (a generic hydroc arbon emitted at the surface) and OR is most strongly affected (3% slower t han in the box:model). However, if RH is emitted nonuniformly at the surfac e, or if the RH-OB reaction rate is increased, the volume-averaged RH destr uction by OH may be slowed down by as much as 30% compared to a box model. Sensitivity studies showed that the intensity of segregation between RH and OH not only depends on the strength and spatial distribution of the RH emi ssions but also on the way NO is emitted in the model atmosphere. The resul ts obtained indicate that the assumption that; localized emissions of react ive hydrocarbons, for example, isoprene or terpenes, are instantaneously mi xed may lead to an underestimation of their atmospheric lifetime.