Daytime CO 2 urban surface fluxes from airborne measurements , eddy - covariance observations and 1 emissions inventory in Greater London 2

11 Airborne measurements within the urban mixing layer (360 m) over Greater London are used to quantify CO2 12 emissions at the meso-scale. Daytime CO2 fluxes, calculated by the Integrative Mass Boundary Layer (IMBL) 13 method, ranged from 46 to 104 μmol CO2 m -2 s -1 for four days in October 2011. The day-to-day variability of 14 IMBL fluxes is at the same order of magnitude as for surface eddy-covariance fluxes observed in central 15 London. Compared to fluxes derived from emissions inventory, the IMBL method gives both lower (by -37%) 16 and higher (by 19%) estimates. The sources of uncertainty of applying the IMBL method in urban areas are 17 discussed and guidance for future studies is given. 18 Capsule: CO2 airborne-derived fluxes by Boundary Layer Mass balance are an independent measure of meso19 scale urban fluxes complementing urban eddy-covariance fluxes and emissions inventory 20