[1] Classical aerosol schemes use either a sectional (bin) or lognormal approach. Both approaches have particular capabilities and interests: the sectional approach is able to describe every kind of distribution, whereas the lognormal one makes assumption of the distribution form with a fewer number of explicit variables. For this last reason we developed a three-moment lognormal aerosol scheme named ORILAM to be coupled in three-dimensional mesoscale or CTM models. This paper presents the concept and hypothesis of a range of aerosol processes such as nucleation, coagulation, condensation, sedimentation, and dry deposition. One particular interest of ORILAM is to keep explicit the aerosol composition and distribution (mass of each constituent, mean radius, and standard deviation of the distribution are explicit) using the prediction of threemoment (m0, m3, and m6). The new model was evaluated by comparing simulations to measurements from the Escompte campaign and to a previously published aerosol model. The numerical cost of the lognormal mode is lower than two bins of the sectional one.Citation: Tulet, P., V. Crassier, F. Cousin, K. Suhre, and R. Rosset (2005), ORILAM, a three-moment lognormal aerosol scheme for mesoscale atmospheric model: Online coupling into the Meso-NH-C model and validation on the Escompte campaign, J. Geophys.

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[1] The mesoscale air quality Mesoscale Nonhydrostatic Chemistry (Meso-NH-C) model is applied to a complex pollution episode over Western Europe during the period 11 to 12 August 1997. As observed in satellite pictures and as simulated, the complexity of this episode is related to the presence of anticyclonic clear-sky areas and regions with deep convective activity in the simulation domain. A brief presentation of the model is made that covers in particular the on-line coupling capability for calculating meteorological and chemical concentration fields at each time step. Then, emphasis is put upon the simulation of transboundary pollution fluxes from London to northern France in a zone of large horizontal wind gradients. Comparison with data from the French Agence De l'Environnement et de la Maitrise de l'Energie (ADEME) pollution network indicates that ozone concentrations and time of arrival of the pollution plume are correctly predicted at surface stations in northern France. A sensitivity analysis relying upon local ozone production and pollution transport has shown that $30% of ozone maxima levels could be attributed to regional transboundary fluxes.

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