Cross-validation of inferred daytime airborne CO 2 urban-regional scale surface fluxes with eddy-covariance observations and emissions inventories in Greater London

Abstract

Abstract. Data obtained from eleven flight surveys on six days during October 2011 were used to characterize the urban CO 2 dome in Greater London (GL) and to calculate CO 2 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 CO 2 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 CO 2 flux was calculated for four days by the Integrative Mass Boundary Layer (IMBL) method. The diurnal CO 2 flux in GL obtained from the aircraft observations ranged from 46 to 104 μmol CO 2 m −2 s −1 during the day time. The mean CO 2 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 CO 2 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.