Daytime CO2 urban surface fluxes from airborne measurements, eddy-covariance observations and emissions inventory in Greater London
Introduction
Urban areas are responsible for 70% of greenhouse gas (GHG) emissions despite covering only 2% of the world's surface (IEA, 2008). Knowledge of both concentrations and fluxes are needed to understand how urban emissions affect regional carbon exchanges (Duren and Miller, 2012).
Measurements of urban atmospheric CO2 concentrations are becoming a common means to study local GHG emissions and urban carbon cycles (Velasco and Roth, 2010, Christen, 2014). An enhancement of the CO2 concentration of the urban canopy layer (UCL) is consistently observed in cities (e.g. Idso et al., 1998). However, urban CO2 concentrations can show a high degree of spatial and temporal variability due to different local sources, atmospheric stability and observation locations (e.g. Pataki et al., 2006).
Observations of CO2 fluxes by eddy covariance systems in urban areas have been proven to be a reliable tool to assess carbon exchanges at the neighbourhood or local-scale when conducted above the roughness sublayer (RSL) (e.g. Grimmond et al., 2002, Nemitz et al., 2002, Feigenwinter et al., 2012). Urban areas are a net source of CO2 (positive fluxes) due to emissions from road traffic, electricity production and local heating with natural gas, oil or coal. Daytime fluxes can be reduced by uptake from vegetation during the growing season, but the nocturnal respiration source remains (Kordowski and Kuttler, 2010, Crawford et al., 2011; Ward et al., 2013). Where vegetation is scarce in cities, biogenic fluxes contribute little to the total net flux.
Diurnal concentrations of CO2 vary within the boundary layer (BL) as a response to changes in surface emissions, boundary layer growth, entrainment processes and horizontal transport (advection). Taking into account the changing boundary layer (BL) volume and exchanges at its vertical and horizontal ‘boundaries’, meso-scale fluxes (102–104 km2) can be inferred from diurnal changes in CO2 concentrations observed in the BL, using the Integrative Mass Boundary Layer (IMBL) method (McNaughton and Spriggs, 1986, Raupach, 1992, Denmead et al., 1996, Strong et al., 2011, Christen, 2014). The IMBL method has been applied over heterogeneous areas to calculate the mean regional CO2 surface flux across, for example, the Amazonian basin (Lloyd et al., 2001, Lloyd et al., 2007) or an agricultural area in Spain (Font et al., 2010), while urban applications include nocturnal CO2 and CH4 emissions for Krakow (Poland) (Zimnoch et al., 2010) and turbulent sensible and latent heat fluxes in Sacramento (California, USA) (Cleugh and Grimmond, 2001).
The aim of this study is to estimate top-down CO2 emissions at the urban boundary layer (UBL) scale by the IMBL method using airborne observations taken in the UBL of Greater London (GL). This approach assumes that a representative urban CO2 concentration can be calculated from a transect across a large area of the city or downwind of it. Results of the IMBL method are presented for four case study days, with a sensitivity analysis of the influence of different assumptions being made, and then compared to neighbourhood-scale eddy-covariance measurements and bottom-up emission inventory estimates. Conclusions from this study highlight the applicability of such airborne observations to quantify CO2 exchanges of a large city and also highlight the methodological challenges encountered.
Section snippets
Instrumentation and survey design
The NERC-ARSF aircraft provided the BL observations between the 12 and 25 October 2011 over South-East England (Table 1). The plane instrumented with an AIMMS-20 Air Data Probe (Aventech Research Inc.) measured temperature, barometric pressure, three components of wind speed and horizontal wind direction, with an instrument accuracy of 0.05 °C (temperature), 0.1 kPa (pressure), 0.5 m s−1 (horizontal wind) and 0.75 m s−1 (vertical wind) (Beswick et al., 2008). Atmospheric CO2 dry mole fractions
CO2 mixing ratio observations
The spatial variability of CO2 within and beyond the GL UBL during each flight is shown in Fig. 2. For lower wind speed conditions (<8 m s−1 at 360 m), higher CO2 mixing ratios were measured over central London, with peaks at 400.5 ppm (12 October 2011), 421.5 ppm (13 October) and 399.1 ppm (25 October), compared to ∼394–398 ppm outside the GL area (Fig. 2). With higher wind speed conditions (>8 m s−1), the differences in average mixing ratio within and surrounding GL were within the instrument
Discussion and conclusions
Here we have presented four airborne surveys that measured CO2 mixing ratios in the UBL of GL in October 2011 that were used to estimate urban-scale emissions by quantifying boundary layer growth, entrainment processes and horizontal transport. The top-down inverse IMBL method infers temporally and spatially integrated fluxes that can be used to evaluate emissions inventories at policy-relevant scales such as cities, megacities, and oil and gas fields. Previously, this approach has been used to
Acknowledgements
The flights were undertaken as part of the ClearfLo project funded by the Natural Environment Research Council (NERC) and coordinated by the National Centre for Atmospheric Science (NCAS). The flights surveys were supported by the NERC-ARSF grant (GB11-05). We would like to thank the crew and the operations team of the ARSF team. We extend out thanks to Rebecca Fisher at Royal Holloway University of London (RHUL) for the calibration to the NOAA standards of the references used for the surface
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