R. Rosset

Description of the Mesoscale Nonhydrostatic Chemistry model and application to a transboundary pollution episode between northern France and southern England

[1]xa0The 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 lEnvironnement et de la Maitrise de lEnergie (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.

Development of a reduced chemical scheme for use in mesoscale meteorological models

Abstract A new Regional Lumped Atmospheric Chemical Scheme (ReLACS) is introduced. This mechanism is intended to be valid for clean to polluted conditions. ReLACS is derived from a new reactivity weighting approach which may be applied to any atmospheric chemical scheme. In this work, ReLACS is based upon the reduction of the regional atmospheric chemistry mechanism (RACM) (Stockwell et al., 1997. Journal of Geophysical Research D22, 25847–25849) on a given moderately polluted emission scenario. However, sensitivity tests around this scenario show that ReLACS compares favorably with RACM, not only for ozone but also for other important atmospheric oxidants. The gain obtained from this lumping in the number of species allows ReLACS to be suitable to perform, otherwise costly, three dimensional pollution studies when coupled with mesoscale meteorological models.

Internal and external mixing in atmospheric aerosols by coagulation: Impact on the optical and hygroscopic properties of the sulphate-soot system

Abstract A sectional aerosol model is used to study the impact of internal/external aerosol mixing on the optical and hygroscopic properties of two-component aerosol populations. Time evolution of the aerosol spectrum due to mixing by coagulation of two initially different particle populations is simulated. The impact of the state of mixing is determined through comparison of model results assuming either internal mixture (IM) only or both internal and external mixture (IEM). The model is first validated using the analytic solution for an idealized IEM problem and secondly against experimental data in an urban plume. Then, a preliminary application is made which consists in a scenario of mixing between a plume containing soot particles and its environment mainly loaded with accumulation-mode sulphate particles. Emphasis is put on the evolving state of mixing of the aerosols thus formed. Evolution of the optical properties is computed using Mie theory for both homogeneous and concentric spheres (coated aerosols). For this particular scenario, the IEM model is more light-diffusive and less light-absorbant than the IM model. The extinction coefficient is practically insensitive to the way of modelling of the state of mixing. Hygroscopic properties are also derived, based on empirical growth laws, showing more activation in the IEM than in the IM model. However, further such studies are necessary to determine more fully the variability range in the optical and hygroscopic properties of aerosols at different degrees of mixing.

An episode of photooxidant plume pollution over the Paris region

Abstract An ozone pollution episode typically at the mesoscale is studied for the period 17–20 July 1996 in the northern half of France. This episode has been documented through extra stations supplementing the regular French network in the southwest of the Paris region at large: the ozone threshold value of 90xa0ppb has been observed to be exceeded only at downwind rural stations at distances ranging between 25 and 110xa0km from downtown Paris. This episode has been simulated with the mesoscale model Meso-NH-C in which the meteorological model Meso-NH is coupled on-line with a chemistry module. Various assumptions are presented which must be made in order to run Meso-NH-C: xa0e.g. reduction of the chemical scheme to reduce the computational costs or definition of procedures to fill in the lack of emission inventory data. Meso-NH appears to realistically simulate the position, extent, average and peak ozone values within the pollution plume. Sensitivity analyses emphasize, in particular, the need for accurate simulation of the wind field to capture correct characteristics of this plume.

Mixing of boundary layer and upper tropospheric ozone during a deep convective event over Western Europe

Abstract Typically, during summer over Europe, pollution episodes in the boundary layer are interspersed with deep convective events which significantly redistribute all pollutants in the vertical. A 3D mesoscale model with an entraining/detraining plume model coupled on-line with gaseous chemistry (J. Geophys. Res., 2002, in press), is used to study the impact of deep convection upon the redistribution of ozone during a summer pollution episode over northern France combining both stratospheric ozone intrusion and enhanced upward transfers. The model reproduces well the ozone concentrations measured in the upper troposphere during two MOZAIC flights and, through sensitivity analyses, can clearly ascertain to convective transport a 110 ppb ozone peak at 6000 m . This study also emphasizes the impact of convective processes on the ozone spatial distribution near the surface. As a result, convective updrafts and downdrafts affect all chemical concentrations, particularly over a range of ±30 ppb in the ozone surface concentrations. At this stage, our conclusion is that deep convection not only modifies the ozone distribution in the mid and upper troposphere but also has a significant effect at the surface.

Air pollution modelling at a regional scale

Abstract A mesoscale simulation system Meso-NH-C allowing for on line coupling between dynamics and chemistry is presented. Further advantages of this system lie in high vertical resolution, nesting capabilities and a full set of parametrization schemes. This system is applied from 9 to 11 August 1997 for a pollution episode over Western Europe. In this episode, both a European and a regional scale analysis have been performed (Greater Paris area). The possibilities of such a modelling system have been emphasized, particularly regarding the evaluation of transboundary fluxes.