Cross-validation of inferred daytime airborne CO₂ urban-regional scale surface fluxes with eddy-covariance observations and emissions inventories in Greater London

Abstract. Data obtained from eleven flight surveys on six days during October 2011 were used to characterize the urban CO₂ dome in Greater London (GL) and to calculate CO₂ fluxes at the city scale. Flights crossed GL along two transects (SW-NE and SSE-NNW) at an altitude of 360 m. Increments as high as 23 ppmv were measured. The maximum CO₂ mixing ratios were localized over GL under low wind speeds, whereas a displacement of the urban plume downwind from the centre of the urban area occurred during high wind speeds. The urban-regional surface CO₂ flux was calculated for four days by the Integrative Mass Boundary Layer (IMBL) method. The diurnal CO₂ flux in GL obtained from the aircraft observations ranged from 46 to 104 μmol CO₂ m⁻² s⁻¹ during the day time. The mean CO₂ fluxes estimated from the IMBL method were statistically similar to those observed by eddy-covariance systems located in central London and a spatially integrated emissions inventory for GL. This study provides an important cross-validation of two independent measurement-based methods to infer the contribution of urban areas to climate change in terms of CO₂ surface fluxes, both of which complement bottom-up emissions inventories. The uncertainties of fluxes estimated by the IMBL method are considered and the limits of implementation of atmospheric methods to infer city-scale fluxes are discussed.