Troposphère

Résumé

vignette| gauche| Une photographie de la troposphère, prise depuis un avion. La troposphère est la première et la plus basse couche de l'atmosphère de la Terre, comprise entre la surface du globe et la stratosphère. Sa limite supérieure, la tropopause, se situe à une altitude d'environ selon la latitude et la saison. La troposphère est plus épaisse à l'équateur qu'aux pôles : la hauteur moyenne de la troposphère est de dans les tropiques ; dans les latitudes moyennes ; et dans les hautes latitudes des régions polaires en hiver. vignette|Évolution de la température dans l’atmosphère. vignette| upright=0.4|Schéma des couches de l'atmosphère (à l'échelle). La stratosphère (~) ne compte que pour environ 1 % de l'atmosphère. On considère que l'exosphère (observable depuis l'espace) s'étend jusqu'à ~. Le terme troposphère dérive des mots grecs tropos (rotation) et sphaira (sphère) indiquant que la rotation turbulence mélange les couches d'air et ainsi détermine la structure et les phé

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https://fr.wikipedia.org/wiki/Troposph%C3%A8re

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Ozone (O3) and aerosols are harmful to human health. Long-range transport sources contribute to the background levels upon which local pollution builds. The goal of this thesis is to describe and quantify the respective contribution of local and long-range transported sources to the O3 and aerosol budget in the framework of European air quality management. This issue is examined using a global model of chemistry and transport, the GEOS-Chem model, constrained by several experimental datasets (e.g. trace gases and aerosol distributions, aerosol optical depth, particulate matter concentrations) taken from field experiments, ground-based sites and satellite. The capabilities of the model to reproduce O3 and aerosol patterns is first examined. The model reproduces well in general the distribution of O3 and related species over the North Atlantic/Europe area. The simulation of particulate matter smaller than 2.5 μm (PM2.5) and aerosol optical depth (AOD) over Europe is also satisfying in general, but this may reflect some compensating errors between individual components (e.g. underestimate of organic matter, and overestimate of sulphate and dust). Intercomparison between the GEOS-Chem and another global model (MOZECH) also reveals possible problems in the water vapour distribution associated with the processes which drive the vertical lifting of pollutant over continental boundary layers. Despite these (relative) deficiencies, the model provides anyway insights about the contribution of long-range transport on the O3 and aerosol loads over Europe. The impact of continental outflow on the O3 chemical tendencies over oceanic regions is quantified. Net O3 photochemical production in polluted air masses travelling over oceanic areas under the influence of continental outflow is enhanced all-year round compared to the background marine environment by 2 to 6 ppbv/day in the boundary layer and by 1 to 3 ppbv/day in the upper troposphere. The origin of O3 long-range sources and their impact on the European O3 budget, as well as the role of changing emissions on this budget over the pasts decades is further investigated. Import of North American O3 into Europe is mainly controlled by meteorological patterns and by photochemical production over the North Atlantic and thus reaches a maximum in summer. In addition, Asia contributes to long-range transported O3 pollution by the Indian summer monsoon easterly winds during summer. During the monsoon period, convection is strong and associated NOx lightning emissions also contribute to high O3 levels, especially in the Mediterranean basin. North American and Asian anthropogenic pollution contribute substantially to the annual O3 burden (integrated over the whole tropospheric column) over Europe, accounting for 11% and 8%, respectively while the European contribution only accounts for 9%. The increase in Asian emissions from 1980 to 1997 have compensated the local reduction of O3 precursors, especially in the free troposphere. Finally, the aerosol load over Europe and the contribution of long and medium-range sources including anthropogenic and biomass burning pollution from North America and mineral dust from North Africa is examined. The model was used to interpret MODIS (Moderate Resolution Imaging Spectroradiometer) AOD and observations provided by the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over the North Atlantic and European area. Trans-Atlantic transport of aerosol is controlled by meteorological patterns and scavenging processes and reaches a maximum in spring. North American uxes of aerosols reach Europe at flower altitudes than that of O3 because a larger fraction of aerosols are scavenged in the venting from the boundary layer by frontal passages and deep convection. During high vegetation fire events in the boreal forest of Alaska and Canada, biomass burning pollution could also reach Europe at high altitudes, because a fraction of fire emissions are emitted directly in the free troposphere. Finally, the Saharan region also contributes significantly to the aerosol load over Europe. European sources contribute by 58% and 48% to the surface PM2.5 and column AOD over Europe. The second main sources are the mineral dust which represent in average 20% of the surface PM2.5 and AOD. In summer, North American anthropogenic and biomass burning emissions represent between 2 to 5% of the surface PM2.5 and AOD. The PM2.5 daily standard levels of the World Health organization (WHO) (10 μg/m3) and of the U.S. Environmental Protection Agency (EPA) (65 μg/m3) are often exceeded during dust outbreaks, especially in southern Europe. Long-range transport of anthropogenic and natural pollution is thus an important issue which should be considered in the definition of air quality standards and regulation treaties.

EPFL2006

Chargement

Long-Range Transport to Europe : Seasonal Variations and Implications for the European Ozone Budget

Marion Auvray, Isabelle Bey

We use a chemical transport model (GEOS-CHEM) to quantify the contribution of long-range transported pollution to the European ozone ({O}{3}) budget for the year 1997. The model reproduces the main features observed over Europe for {O}{3}, carbon monoxide and nitrogen dioxides, as well as two events of enhanced {O}{3} of North American origin over the eastern North Atlantic and over Europe. North American {O}{3} fluxes into Europe experience a maximum in spring and summer, reflecting the seasonal variation in photochemical activity and in export pathways. In summer, North American {O}_{3} enters Europe at higher altitudes and lower latitudes because of deep convection, and because the flow over the North Atlantic is mostly zonal in that season. The low-level inflow is only important in spring, when loss rates in the boundary layer over the North Atlantic are weaker. Asian O3 arrives mainly via the westerlies, and usually at higher altitudes than North American O3 because of stronger deep convection over Asia. In addition, Asian O3 fluxes are at a maximum in summer during the monsoon period because of enhanced convection over Asia, increased nitrogen oxides sources from lightning and direct transport towards Europe via the monsoon easterlies. Over Europe, total background accounts for 30 ppbv at the surface. North American and Asian O3 contribute substantially to the annual O3 budget over Europe, accounting for 10.9% and 7.7%, respectively,while the European contribution only accounts for 9.4%. We find that in summer, at the surface, O3 decreases over Europe from 1980 to 1997, reflecting the reduction of European O3 precursor emissions. In the free troposphere, this decrease is compensated by the increase in O3 due to increasing Asian emissions. This may explain the lack of trends observed over most of the European region, especially at mountain sites.

2005

Chargement

A model investigation of tropospheric ozone chemical tendencies in long-range transported pollution plumes

Marion Auvray, Isabelle Bey

The impact of continental outflow on the ozone chemical tendencies (i.e., production and loss rates) is quantified in the North Atlantic and northwest Pacific regions using the GEOS-Chem and the MOZECH global models of chemistry and transport. The ozone tendencies simulated by these global models are compared to box model simulations constrained by observations in different regions and seasons. The two global models generally capture the seasonal and regional variations of the ozone tendencies. The largest discrepancies between the ozone tendencies from the box model and those from the global models are found in the lower troposphere of the eastern North Atlantic during the ACSOE campaign and are attributed to differences between chemical schemes and too strong NO x concentrations in the global models. The background and plume (i.e., impacted by continental outflow) environments are differentiated over the oceanic areas using criteria based on simulated daily mean CO concentrations. The ozone tendencies in the plume environment differ from that in the background over the entire column in North Atlantic and northwest Pacific at all seasons. According to the models, net ozone production is enhanced by 2 to 6 ppbv/day in the boundary layer and by 1 to 3 ppbv/day in the upper troposphere, whereas the effect of pollution ranges from −1 ppbv/day to +1 ppbv/day in the middle troposphere (3–7 km), depending on the model used. The different responses of the two models are determined by differences in the water vapor distributions relative to those of pollutants in the plumes. In particular, GEOS-Chem tends to transport pollution in a drier sector of cyclones than MOZECH.

2007

Chargement

Long-Range Transport to Europe : Seasonal Variations and Implications for the European Ozone Budget

Marion Auvray, Isabelle Bey

2005

EPFL2006