With more than 10 million inhabitants (20% of the French population), Great Paris is the 3rd largest megacity in Europe. According to emission inventories, CO2 emissions of the Paris region represent about 15 % of the total French anthropogenic CO2 emissions whereas its surface extends over only 2% of the French territory. However, the assessment of such inventory is quite difficult and uncertainties are poorly constrained. During the incomplete combustion processes of fossil fuels, CO2 is co-emitted with other trace species in ratios that are characteristic of each emission sector and fuel type. Simultaneous measurements of CO2 and co-emitted species can thus allow the quantification of the relative contribution of each emission sector to the total amount of CO2 emitted. These ratios can be obtained from inventories. However, they are given for benchmark conditions that often may not represent well real world conditions. These ratios should thus also be assessed independently. Road traffic is one of the main sources of CO2 emitted by the Paris region : according to the regional French emission inventory (Airparif), 29% of CO2 emissions in the region of Paris are due to traffic which is also responsible for emissions of more than half of the CO (56%) and nearly 20% of the volatile organic compounds (VOCs) . As part of the PRIMEQUAL 'PREQUALIF' project, CO2 to co-emitted species emission factors were determined for the traffic sector. A 12-day measurement campaign was performed in a highway tunnel located in a peri-urban area about 15 km south of the Paris center. Several atmospheric compounds were monitored simultaneously with CO2, including CO, black carbon, NO-NO2 and VOCs. We will present here preliminary results from this campaign, based on the study of correlations between the concentrations of CO2 and co-emitted species, but also using information such as velocity, traffic rate, vehicle type and day of the week. Depending on these parameters, a classification of the different CO2 to co-species concentration ratios will be proposed. A first comparison will be provided with available inventories.