Improving ∆14CO2 measurements to estimate anthropogenic CO2 emissions in the UK

  • Katherine Pugsley

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract


Anthropogenic fossil fuel CO2 (f f CO2) emissions are an important topic of research as they increase atmosphericCO2 which affects the global greenhouse gas budget and radiative forcing of the Earth’s climate. The challenge with investigation of atmospheric CO2 mole fraction variation is the difficulty of separating biogenic and anthropogenic sources. Radiocarbon dioxide
(14CO2) observations provide a method for separating fossil fuel emissions from other sources. This is because fossil fuels, having been buried under ground for millennia, do not contain 14C; which has a half life of 5730 years. Therefore, 14C can be used as a tracer of anthropogenic activity. When combined with models of atmospheric transport, 14CO2 measurements could be used to determine
the magnitude of the UK’s fossil fuel CO2 emissions. Previous 14CO2 measurements in the UK, as part of the Greenhouse GAs UK and Global Emissions (GAUGE) project, have shown that it is difficult to use 14CO2 to quantify UK f f CO2 emissions. The reasons for this were: (1) Only a limited number
of samples under low f f CO2 mole fraction conditions were taken; (2) The abundance of nuclear power stations in the UK that release 14CO2 emissions means a correction must be applied to the measurements.

Flask samples are most commonly taken over the period of minutes providing only a snap shot of the atmospheric gas composition, these are then compared to models which are on the time frame of hours. To make the samples collected more representative and comparable to the models an integrated sampling system was designed and tested that is capable of collecting samples
representative of hours. The next stage in the sample analysis is the extraction of the CO2 from the whole air sample. Traditionally, this has been achieved cryogenically using liquid nitrogen. An extraction line similar to those utilised at other laboratories was constructed and tested. In addition, an alternative extraction system that utilises a commercially available graphitisation system was
also investigated. The precision obtained from both methods was 2 h which is within the World Meteorological Organisation (WMO) guidelines.

The final step is the calculation of f f CO2 mole fraction including the calculation of the corrections for other influences on 14C. Data from the GAUGE campaign was used to calculate the required corrections and f f CO2 mole fractions. Consideration of the spatial and temporal 14C emissions from nuclear power stations is required. There is high temporal variation in the emissions from the
nuclear power stations that was considered in this thesis by using daily emission data. Sporadic high emissions from the nuclear power stations occur. However, if samples are taken to avoid these, the nuclear correction is of similar magnitude to the correction required for biospheric disequilibrium. The correction required for biospheric disequilibrium was also investigated as part of the work in
this thesis using three different biospheric models.

Overall, this thesis has contributed to the use of ∆ 14CO2 to quantify f f CO2 emissions in the UK. It has built on work from a previous project and the improvements made as part of this thesis will be used in a future project to use ∆
14CO2 measurements to improve the verification of f f CO2 emissions using atmospheric measurements and modelling.
Date of Award23 Jun 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorIan D Bull (Supervisor) & Simon O'Doherty (Supervisor)

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