Assessing the role of megacities on atmospheric CO2: results for Paris from the CO2- MegaParis project, France
Résumé
On average, atmospheric CO2 increases in the atmosphere at a rate of about 2 parts per million (ppm) per year, due to the accumulation of about half of the anthropogenic CO2 emissions in the atmosphere (mostly from the combustion of fossil fuels), while the other half is being re-absorbed by the ocean and the continental biosphere. today, more than 70% of global fossil-fuel CO2 emissions come from punctual sources such as megacities. paris is the third megacity in europe and it emits about 15% of the total French emissions, while it covers only less than 2% of the national territory. Currently, most of the estimates of urban CO2 emissions are given by bottom-up CO2 emissions inventories, which rely on activity proxies and benchmarked emission factors. the associated uncertainties can be as high as several tenths of percents, especially when it comes to discriminate the CO2 urban emissions by emission sectors. therefore, there is an urgent need for developing new methods to better Monitoring, reporting and Veryfying (MrV) CO2 emissions from megacities, dedicated to provide robust results to policy makers for taking efficient decisions and actions in matter of controling CO2 anthropogenic emissions and mitigating climate change. since 2009, the CO2-Megaparis project aims to quantify CO2 emissions from paris using top-down approaches based on a synergy between atmospheric observations and modeling. For the first time, a mini-network of 3 greenhouse gases (GHG) monitoring stations was developed by lsCe in paris agglomeration within the infrastructure of the regional air quality monitoring agency, airpariF, completing 2 other GHG stations from the iCOs european greenhouse monitoring network. One of our urban station was located on top of the eiffel tower above paris megacity. the analysis of one year of data showed that paris CO2 emissions lead to a mean increase of the atmospheric CO2 concentration in the mid-afternoon of 2 to 3 ppm, and is strongly season, windspeed and wind direction dependent: the CO2 urban plume is characterized by a very large spatio-temporal variability and can reach about 60 ppm at low windspeeds on top of the eiffel tower. in addition, analysis of correlations between CO2, CO and 14C02 were carried out from field measurements and allowed an independent assessment of the inventories emission sectors. Furthermore, direct modeling of CO2 at a very fine resolution (2x2 km2, 1h) was performed and matched well with the observations. last but not least, inverse modeling efforts at the same resolution allowed a significant improvment of the regional inventory from airparif. Finally, a campaign conducted during springtime and based on lidar facilities revealed that due to the effect of the urban heat island, the boundary layer height (that can be seen on the first degree as the man dilution factor of CO2 emissions in the atmosphere), is 10 to 40% time higher in Paris than in surrounding rural areas: this is an important result that supports the implementation of urban canopy models in future fine scale urban CO2 modeling framework. a synthesis of the different results will be presented, as well as an attempt of defining the strengths and weaknesses of the atmospheric approach to quantify urban CO2 emissions. Contributions from sister studies (MultiCO2 - ipsl, le CO2 parisien - Ville de paris 2030, CarboCount-City - KiC Climat...) will also be mentionned.