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The accurate simulation of meteorological conditions, especially within the planetary boundary layer (PBL), is of major importance for air quality modeling. In the present work, we have used the Weather Research and Forecast (WRF) model coupled with the chemistry transport model (CTM) CHIMERE to understand the impact of physics parameterizations on air quality simulation during a short-term pollution episode on the Paris region. A lower first model layer with a 4 m surface layer could better reproduce the transport and diffusion of pollutants in a real urban environment. Three canopy models could better reproduce a 2 m temperature (T2) in the daytime but present a positive bias from 1 to 5 &deg, C during the nighttime, the multi-urban canopy scheme &ldquo, building effect parameterization&rdquo, (BEP) underestimates the 10 m windspeed (W10) around 1.2 m s&minus, 1 for the whole episode, indicating the city cluster plays an important role in the diffusion rate in urban areas. For the simulation of pollutant concentrations, large differences were found between three canopy schemes, but with an overall overestimation during the pollution episode, especially for NO2 simulation, the average mean biases of NO2 prediction during the pollution episode were 40.9, 62.2, and 29.7 &micro, g m&minus, 3 for the Bulk, urban canopy model (UCM), and BEP schemes, respectively. Meanwhile, the vertical profile of the diffusion coefficients and pollutants indicated an important impact of the canopy model on the vertical diffusion. The PBL scheme sensitivity tests displayed an underestimation of the height of the PBL when compared with observations issued from the Lidar. The YonSei University scheme YSU and Boulac PBL schemes improved the PBL prediction compared with the Mellor&ndash, Yamada&ndash, Janjic (MYJ) scheme. All the sensitivity tests, except the Boulac&ndash, BEP, could not fairly reproduce the PBL height during the pollution episode. The Boulac&ndash, BEP scheme had significantly better performances than the other schemes for the simulation of both the PBL height and pollutants, especially for the NO2 and PM2.5 (particulate matter 2.5 micrometers or less in diameter) simulations. The mean bias of the NO2, PM2.5, and PM10 (particulate matter 10 micrometers or less in diameter) prediction were &minus, 5.1, 1.2, and &minus, 8.6 &micro, 3, respectively, indicating that both the canopy schemes and PBL schemes have a critical effect on air quality prediction in the urban region.

Context Lineage and birth of the EU ETS Almost like a living organism, the European Union Emissions Trading Scheme (EU ETS) is one of the final lines of an elaborate and complex process. This process began years ago with the formal acknowledgment at international level “that change in the Earth's climate and its adverse effects are a common concern of humankind” (United Nations, 1992) and the recognition “that human activities have been substantially increasing the atmospheric concentrations of greenhouse gases, that these increases enhance the natural greenhouse effect, and that this will result on average in an additional warming of the Earth's surface and atmosphere and may adversely affect natural ecosystems and humankind” (United Nations, 1992). This recognition led to setting a goal, namely the “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” (United Nations, 1992). From this awareness and with this general objective in mind, a first operational step was taken with the highly publicized Kyoto Protocol (United Nations, 1997) which sets quantified emission reduction targets for each developed country. In order to fulfill these commitments while taking into account the technical, political and economic constraints, several tools were created including MRV procedures, flexibility mechanisms and a compliance committee. The role of the MRV procedures is to ensure that the developed Parties to the UNFCCC monitor and submit their annual emissions in a way which assures comparability and consistency with the rules of the Protocol (see Chapter 2 on national GHG inventories under the UNFCCC). The market-based mechanisms intend to provide Parties with the possibility to abate emissions where it is cheapest. The compliance committee ultimately judges whether Parties complied with the rules and mechanisms. In particular, it solves disputes between Parties, or between a Party and an expert review team (see Chapter 2 on national GHG inventories under the UNFCCC).