On the impact of the vertical resolution on chemistry-transport modelling

International audienceThis paper presents a sensitivity analysis of the modelling of air pollutant concentrations in the surface layer with the WRF/CHIMERE models. The influence of the vertical resolution near the surface is studied. The simulations are carried out over two periods (winter and summer 2009) over the Paris area. Three model configurations are used: (i) the CHIMERE mesh used for the PREVAIR forecast (8 levels from 995 to 500 hPa), (ii) a mesh refined along the whole vertical axis (20 levels from 995 to 500 hPa) and (iii) a mesh with a refinement near the surface (9 levels from 999 to 500 hPa). The results are discussed in terms of differences on surface concentrations between the reference case and an improved resolution. Adding a point close to the surface appears to be important mainly for high nocturnal concentrations in very stable boundary layers. Refining the vertical mesh, with 20 levels instead of 8, enables to model new structures in the well mixed boundary layer, but with a moderate impact at the surface. It is shown that the different model configurations lead to changes of a few mu g m(-3) at most, showing that the vertical mesh is not the most sensitive factor in chemistry-transport modelling when results are compared to surface measurements. This finding validates the fact that a simplified vertical mesh is suitable for air quality forecasting even if an improved vertical resolution close to the ground is important to take into account the urban increment

AerSett v1.0: A simple and straightforward model for the settling speed of big spherical atmospheric aerosol

This study introduces AerSett v1.0 (AERosol SETTling version 1.0), a model giving the settling speed of big spherical aerosols in the atmosphere without going through an iterative equation resolution. We prove that, for all spherical atmospheric aerosols with diameter D up to 1000 &micro;m, this direct and explicit method based on the drag coefficient formulation of Clift and Gauvin (1971) gives results within 2 % of the exact solution obtained from numerical resolution of a non-linear fixed-point equation. This error is acceptable considering the uncertainties on the drag coefficient formulations themselves. For D &lt; 100 &micro;m, the error is below 0.5 %. We hope that with this simple and straightforward model, more Chemistry-Tranport models and General Circulation models will be able to take into account large-particle correction to the settling speed of big spherical aerosol particles in the atmosphere, without performing an iterative and time-consuming calculation.</p

Source contributions to 2012 summertime aerosols in the Euro-Mediterranean region

International audienceIn the Mediterranean area, aerosols may originate from anthropogenic or natural emissions (biogenic, mineral dust, fire and sea salt) before undergoing complex chemistry. In case of a huge pollution event, it is important to know whether European pollution limits are exceeded and, if so, whether the pollution is due to anthropogenic or natural sources. In this study, the relative contribution of emissions to surface PM10, surface PM2.5 and total aerosol optical depth (AOD) is quantified. For Europe and the Mediterranean regions and during the summer of 2012, the WRF and CHIMERE models are used to perform a sensitivity analysis on a 50 km resolution domain (from −10° W to 40° E and from 30° N to 55° N): one simulation with all sources (reference) and all others with one source removed. The reference simulation is compared to data from the AirBase network and two ChArMEx stations, and from the AERONET network and the MODIS satellite instrument, to quantify the ability of the model to reproduce the observations. It is shown that the correlation ranges from 0.19 to 0.57 for surface particulate matter and from 0.35 to 0.75 for AOD. For the summer of 2012, the model shows that the region is mainly influenced by aerosols due to mineral dust and anthropogenic emissions (62 and 19 %, respectively, of total surface PM10 and 17 and 52 % of total surface PM2.5). The western part of the Mediterranean is strongly influenced by mineral dust emissions (86 % for surface PM10 and 44 % for PM2.5), while anthropogenic emissions dominate in the northern Mediterranean basin (up to 75 % for PM2.5). Fire emissions are more sporadic but may represent 20 % of surface PM2.5, on average, during the period near local sources. Sea salt mainly contributes for coastal sites (up to 29 %) and biogenic emissions mainly in central Europe (up to 20 %)

Impact of Surface Roughness and Soil Texture on Mineral Dust Emission Fluxes Modeling

Dust production models (DPM) used to estimate vertical fluxes of mineral dust aerosols over arid regions need accurate data on soil and surface properties. The Laboratoire Inter-Universitaire des Systemes Atmospheriques (LISA) data set was developed for Northern Africa, the Middle East, and East Asia. This regional data set was built through dedicated field campaigns and include, among others, the aerodynamic roughness length, the smooth roughness length of the erodible fraction of the surface, and the dry (undisturbed) soil size distribution. Recently, satellite-derived roughness length and high-resolution soil texture data sets at the global scale have emerged and provide the opportunity for the use of advanced schemes in global models. This paper analyzes the behavior of the ERS satellite-derived global roughness length and the State Soil Geographic data base-Food and Agriculture Organization of the United Nations (STATSGO-FAO) soil texture data set (based on wet techniques) using an advanced DPM in comparison to the LISA data set over Northern Africa and the Middle East. We explore the sensitivity of the drag partition scheme (a critical component of the DPM) and of the dust vertical fluxes (intensity and spatial patterns) to the roughness length and soil texture data sets. We also compare the use of the drag partition scheme to a widely used preferential source approach in global models. Idealized experiments with prescribed wind speeds show that the ERS and STATSGO-FAO data sets provide realistic spatial patterns of dust emission and friction velocity thresholds in the region. Finally, we evaluate a dust transport model for the period of March to July 2011 with observed aerosol optical depths from Aerosol Robotic Network sites. Results show that ERS and STATSGO-FAO provide realistic simulations in the region

Lidar signal simulation for the evaluation of aerosols in chemistry transport models

International audienceWe present an adaptable tool, the OPTSIM (OPTical properties SIMulation) software, for the simulation of optical properties and lidar attenuated backscattered profiles (beta') from aerosol concentrations calculated by chemistry transport models (CTM). It was developed to model both Level 1 observations and Level 2 aerosol lidar retrievals in order to compare model results to measurements: the level 2 enables to estimate the main properties of aerosols plume structures, but may be limited due to specific assumptions. The level 1, originally developed for this tool, gives access to more information about aerosols properties (beta') requiring, at the same time, less hypothesis on aerosols types. In addition to an evaluation of the aerosol loading and optical properties, active remote sensing allows the analysis of aerosols' vertical structures. An academic case study for two different species (black carbon and dust) is presented and shows the consistency of the simulator. Illustrations are then given through the analysis of dust events in the Mediterranean region during the summer 2007. These are based on simulations by the CHIMERE regional CTM and observations from the CALIOP space-based lidar, and highlight the potential of this approach to evaluate the concentration, size and vertical structure of the aerosol plumes

Pollution atmosphérique et climat

National audienceClimate change and air quality are closely related: through the policy measures implemented to mitigate these major environmental threats but also through the geophysical processes that drive them. We designed, developed and implemented a comprehensive regional air quality and climate modelling system to investigate future air quality in Europe taking into account the combined pressure of future climate change and long range transport. Using the prospective scenarios of the last generation of pathways for both climate change (emissions of well mixed greenhouse gases) and air pollutants, we can provide a quantitative view into the possible future air quality in Europe. We find that ozone pollution will decrease substantially under the most stringent scenario but the efforts of the air quality legislation will be adversely compensated by the penalty of global warming and long range transport for the business as usual scenario. For particulate matter, the projected reduction of emissions efficiently reduces exposure levels.Changement climatique et qualité de l'air sont intimement liés : à travers les politiques de gestion mises en oeuvre pour atténuer ces menaces environnementales majeures mais aussi à travers les processus géophysiques qui les gouvernent. Afin de pouvoir étudier l'évolution de la pollution atmosphérique en Europe en prenant en compte l'influence conjointe du changement climatique et du transport à longue distance, nous avons conçu, développé et mis en oeuvre un système complet de modélisation régionale du climat et de la qualité de l'air. En utilisant des scénarios prospectifs de dernière génération relatifs au changement climatique (émissions de gaz à effet de serre) mais aussi pour les polluants à courte durée de vie, nous avons pu proposer une quantification de l'évolution future de la qualité de l'air en Europe. D'après le scénario le plus volontariste, la pollution liée à l'ozone sera réduite de manière substantielle mais les efforts positifs induits par les politiques de gestion de la qualité de l'air seront contrebalancés par le changement climatique et le transport à longue distance pour le scénario statu-quo. En ce qui concerne les particules, les réductions d'émissions futures réduiront de manière efficace les niveaux d'exposition

PREV'AIR : un système opérationnel de prévision et de cartographie de la qualité de l'air en France et en Europe

National audienceLe système PREV'AIR de prévision et de surveillance de la qualité de l'air en France et en Europe est mis en oeuvre à l'Institut National de l'Environnement Industriel et des Risques (INERIS) depuis le printemps 2003, en coopération avec l'Institut Pierre-Simon Laplace (IPSL) du Centre National de la Recherche Scientifique (CNRS); Météo France et l'Agence De l'Environnement et de Maîtrise de l'Energie (ADEME). Le système a pour objectif de délivrer quotidiennement une information (sous la forme de prévisions et de cartographies) relative à la qualité de l'air en France et en Europe. Il est reconnu comme un complément du dispositif national de surveillance de la qualité de l'air assurée au niveau local par une quarantaine d'Associations Agréées de Surveillance de la Qualité de l'Air (AASQAs). Ce dispositif repose sur plus de 2000 analyseurs dédiés à la mesure d'indicateurs de pollution (SO2, O3, NO2, particules...), répartis sur plus de 700 sites fixes. Les modèles déterministes tridimensionnels de simulation de la qualité de l'air, ou modèles de Chimie-Transport (CTM), généralement couplés à un modèle météorologique, permettent désormais de répondre en grande partie aux objectifs de la prévision, grâce à leur fiabilité croissante et aux progrès réalisés dans le domaine de l'informatique scientifique. La diminution des temps de calculs, l'augmentation des capacités de stockage et le développement des technologies de l'information (accès rapide par Internet à de nombreuses bases de données) rendent désormais possible la mise en oeuvre quotidienne ("opérationnelle") de ces modèles, et la diffusion de l'information qui en découle. Le système PREV'AIR s'appuie sur un ensemble d'outils numériques (modèles et post traitement des sorties de modèles; modules d'interface pour la gestion des entrées / sorties de données). Les données numériques générées par le système (concentrations en polluants atmosphériques) sont exploitées afin de 1) réaliser des études spécifiques pour le compte du Ministère de l'Ecologie et du Développement Durable - MEDD- (bilan de qualité de l'air, analyses de tendances et prospectives); 2) fournir des prévisions de concentrations de polluants atmosphériques (ozone, dioxyde d'azote et particules) à trois jours d'échéance; 3) élaborer des "analyses" des concentrations de polluants (ozone) - c'est-à-dire des cartographies réalisées a posteriori en intégrant les résultats de modélisation et les observations disponibles. Les prévisions et les analyses sont délivrées quotidiennement via Internet sous forme de données numériques et de cartes (http://www.prevair.org). L'objet du présent article est de décrire brièvement le système PREV'AIR tel qu'il fonctionne depuis l'été 2004 ainsi que les produits de sortie délivrés par le système. Un état des performances du système sera également proposé