Abstract
Human activity in urban areas is responsible for the majority of total (direct and indirect) emissions of carbon dioxide. Accurate assessment of these emissions is necessary for understanding the underlying processes and to assess compliance with environmental legislation. Several urban studies have assessed net emissions at the local scale (102−104 m) using the eddy covariance method, however in practice this is often approximated as the vertical turbulent exchange – carbon dioxide stored within the canopy or advected horizontally is neglected. The main objective of this study is to characterise the carbon dioxide (CO2) exchanges and emissions in urban areas at the micro to local scale. This involves assessing exchanges using a profile system and eddy covariance flux systems located at a height representative of the local scale. The profile system, designed as part of the PhD, allows the microscale and local scale influences to be separated. Horizontal movement of CO2 is investigated using both horizontal profile and transect measurements of CO2 made by mobile sensors at multiple spatial scales, and by field measurements of CO2 exchange by vegetation, soil and Thames river water. Taken together these components enable the final chapter of this thesis which compares inventory and EC/profile estimates of net emissions. Sources of error and uncertainty in these estimates are explored.The centre of London is shown to be a major and increasingly large source of CO2. The effects of individual processes or factors on the atmospheric CO2 concentration are evaluated through comparison of CO2 concentrations measured over different time periods. In this way it is found that CO2 concentrations at a site in central London vary primarily with anthropogenic factors such as traffic density and combustion of fuel for space heating, and secondarily with meteorological factors such as mixing layer depth. Evidence for variation of CO2 concentrations due to non-human biological factors such as photosynthetic uptake by vegetation is almost entirely absent.
Methods used to calculate the change in the CO2 in the air volume below an eddy covariance flux system (CO2 storage) are assessed. CO2 storage (ΔCS) calculated from measurements made solely at the eddy covariance flux system height are found to be an inadequate measure of the change in CO2 stored below; measurements of CO2 concentration need to be made at least at one other site, preferably at or below the mean height of the local roughness elements (e.g., buildings, trees). On an annual basis ΔCS is found to contribute a negligible amount to the calculated net emissions and, if net emissions are not required to be calculated on a sub-diurnal timescale, may be neglected.
The effects of the advective terms of the micrometeorological method of estimating net emissions are found to be more substantial, with vertical advection tending to reduce the estimate of net emissions and horizontal advection tending to increase estimates at this site. Good agreement was found between net annual CO2 emissions calculated using the micrometeorological method and those calculated via the inventory method. From the latter it was clear that the majority of CO2 emissions in central London are due to traffic, following by combustion for building heating and human respiration. Natural sources or sinks of CO2, such as soil respiration and photosynthetic uptake of CO2 by vegetation, had a negligible effect.
Despite the complexity of the urban environment this work enables policymakers to identify key targets for CO2 emissions reduction. It also informs researchers of the main components which must be measured for accurate estimation of net emissions and provides suggestions for the most efficient means of doing so. It contributes not only to our understanding of CO2 emissions in the urban environment, a subject of global importance given the impact of urban CO2 emissions on global atmospheric CO2 levels and hence on climate change, but also to our ability to further that understanding with more accurate and cost-effective measurement methods.
| Date of Award | 1 Jul 2016 |
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| Original language | English |
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| Supervisor | Sue Grimmond (Supervisor) & Bruce Malamud (Supervisor) |