Spatial and temporal variability of urban fluxes of methane, carbon monoxide and carbon dioxide above London, UK.

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. [ABSTRACT FROM AUTHOR]