Daytime CO2 Urban-Regional Scale Surface Fluxes from Airborne Measurements, Eddy-Covariance Observations and Emissions Inventories in Greater London

Abstract:

As the global population becomes increasingly urbanized, spatially concentrated centres of anthropogenic CO₂ and other greenhouse gases (GHG) arise. While mitigation measures exist at national and international scales, their implementation will be more effective if linked to the urban-scale of the sources. Routine top-down approaches that quantify emissions of GHG from cities and megacities are needed to understand the dynamics of the urban carbon cycle to eventually define relevant policy decisions. London is the biggest urban conurbation in Western Europe with more than 8 million inhabitants. It emitted roughly 45000 ktn CO₂ in 2010¹. To understand the carbon dynamics and quantify anthropogenic emissions from London, airborne surveys of atmospheric CO₂, O₃, particles and meteorological variables were carried out over the city, onboard the NERC-ARSF Dornier-228 UK research aircraft. We applied an Integrative Mass Boundary Layer method (IMBL) using airborne CO₂ observations obtained in horizontal transects crossing London at 360 m at different times of the day and by sampling upwind-downwind profiles. IMBL CO₂ fluxes were compared to an emissions inventory and neighbourhood-scale eddy-covariance fluxes in central London.

Daytime fluxes in October 2011 from the IMBL calculations ranged from 46 to 104 µmolCO₂ m^-2 s^-1 and covered 30-70% of the urban region. The IMBL CO₂ fluxes were the same order of magnitude as observed eddy-covariance fluxes and were statistically comparable to the emission inventory for the same footprint area.

A sensitivity analysis suggested that horizontal variability of the CO₂ field in the urban mixing layer is the most critical factor affecting IMBL fluxes. The determination of the boundary height and vertical wind speed had more impact on fluxes calculated from upwind-downwind profiles.

Furthermore, low-altitude airborne measurements of CO₂ provide the advantage of direct observation of the CO₂ urban dome of a megacity and relate the spatial distribution of concentrations of CO₂ to the spatial distribution of emissions and to meteorological conditions.

References

¹ DECC (2012), http://www.decc.gov.uk/en/content/cms/statistics/indicators/ni186/ni186.aspx