Helfter, C., Tremper, A.H., Halios, C.H., Kotthaus, S., Bjorkegren, A., Grimmond, C.S.B., Barlow, J.F., Nemitz, E. (2016):
Spatial and temporal variability of urban fluxes of methane, carbon monoxide and carbon dioxide above London, UK.

Atmospheric Chemistry and Physics Discussions 1–31. doi:10.5194/acp-2016-216

Abstract

We report on more than three years of measurements of fluxes of methane (CH₄), carbon monoxide (CO) and carbon dioxide (CO₂) taken by eddy-covariance in central London, UK. Inter-annual variability in the period 2012–2014 ranged from 36.3 to 40.7 ktons km⁻² y⁻¹ for CO₂, and from 69 to 75 tons km⁻² y⁻¹ for CH₄. Mean annual emissions of CO₂ (39.1 ± 2.4 ktons km⁻² y⁻¹) and CO (89 ± 16 tons km⁻² y⁻¹) were consistent (within 1 % and 5 % respectively) with values from the London Atmospheric Emissions Inventory, but measured CH₄ (72 ± 3 tons km⁻² y⁻¹) was over two-fold larger than the inventory value. Seasonal variability was large for CO with a winter to summer reduction of 69 %. Monthly fluxes of CO were strongly anti-correlated with mean air temperature, and the winter emissions accounted for 45 % of the annual budget. The winter increment in CO emissions was attributed mainly to vehicle cold starts and reduced fuel combustion efficiency. CO₂ fluxes were 33 % higher in winter and anti-correlated with mean air temperature, albeit to a lesser extent than for CO. This was attributed to an increased demand for natural gas for heating during the winter. Seasonality in CH₄ fluxes was moderate (21 % larger in winter) and linear correlation with air temperature was only statistically significant for certain wind sectors (N, NE, E and W), which was also the case for CO₂. Differences in resident population within the flux footprint explained ca. 90 % variability by wind direction in annual CO₂ fluxes and 99 % for CH₄ (wind sectors excluded from linear regressions: S for CO₂; S, SE and E for CH₄). Seasonality and proportionality of emissions with respect to population in the outlying wind sectors (S, SE and E) might be masked by constant sources of CO₂ and CH₄, perhaps of industrial or biogenic origin. To our knowledge, this study is unique given the long-term, continuous dataset of urban CH₄ fluxes analysed.

Full text: Atmospheric Chemistry and Physics Discussions (Open Access, CC-BY 3.0).

Received: 11 Mar 2016 – Accepted: 29 Mar 2016 – Published: 30 Mar 2016

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