MEGAPOLI - PARIS : Pollution des AéRosols: Impact sur la qualité de l'air et quantification des Sources – MEGAPOLI-PARIS

Very dense pollutant emissions over megacities (large urban agglomerations with more than about 7 millions of inhabitants) are strongly affecting air quality on an urban scale and chemical composition on a regional and even global scale. This ultimately affects both public health and regional to global climate. In order to assess the overall effects of megacity emissions, all links between emissions, transport, chemical transformation and deposition processes should be well understood, which is currently not yet true. In particular, despite its adverse effects on health and their climate effect, primary and secondary sources of particulate matter, and in particular of organic aerosol in the urban atmosphere are not well qualified and quantified. The project MEGAPOLI-PARIS aims at gaining an increasing comprehension of major processes affecting the abundance of particulate matter in a polluted atmosphere, in particular : ' to better assess primary sources of carbonaceous aerosols, ' to better assess secondary sources of organic aerosols through gas-to-particle conversion, ' using this knowledge, to evaluate and improve process and air quality models. The first major objective of the project deals with closing identified knowledge gaps and to provide experimental data for model improvement and evaluation. This concerns in particular formation processes of primary and secondary aerosol at regional scale, with a specific focus on organic matter, for which urban sources are not well quantified. A number of specific objectives will be pursued: ' to document the aerosol composition and properties, and gaseous precursor concentrations, within a large agglomeration and in its plume, with a focus on the chemical speciation of organic carbon; the defined target area is the Ile de France region, ' to provide source apportionment of elemental and organic carbon PM, in order to improve regional scale emission inventories, ' to document pathways of secondary organic aerosol build-up from gaseous precursors within the agglomeration and in its plume, ' to use experimental data obtained through the project and already existing data bases in order to evaluate and to improve process (0D) and air quality (3D) models for a large range of conditions (effective emission strength, meteorological variables). Greater Paris has been chosen as a playground for investigating theses aspects, because it is a major and densely populated pollution source (more than 10 million inhabitants), surrounded by rural areas and relatively flat terrain. Therefore, a dedicated field campaign is designed over the Greater Paris to allow for the characterisation of the aerosol pollution in large urban areas. In addition, the facility of campaign logistics (established co-operations for access to campaign sites, vicinity of research labs, presence of a very well established air quality survey network), as compared to an extra-European megacity, was taken into account for the choice. During the campaign, two (one urban and one sub-urban) super-sites will be set-up during one summer and winter month each. In addition, two mobile laboratories will allow sampling the pollution plume and upwind conditions. Finally, dedicated aircraft measurements will allow following the chemical composition of the polluted plume. Given the variety of platforms and the state of the art character of deployed instrumentation, this is the most detailed aerosol characterisation experiment perform so far in an urban environment over Europe. As a second overall objective, the system's sensitivity with respect to anthropogenic forcing and the impact of mitigation strategies will be evaluated. This work will be performed with 0D process and 3D air quality models, which will have been carefully evaluated and improved using the in particular the campaign observations. Sensitivity analysis will address different chemical families of aerosol and size ranges, and will investigate dependence on both emission flux from individual sources and on meteorological variables. This is prerequisite to elaborate and to evaluate mitigation strategies to control the health risk of particulate matter pollution under different scenarios. Considering the generally trans-boundary nature of particulate pollution episodes, it is relevant to consider emission reduction strategies that should be applied at the European, in addition to specific urban scale scenarios.