Reconstruction of historic fossil CO2 emissions using radiocarbon measurements from tree rings

Abstract:

This project aims to reconstruct historic fossil CO₂ emissions from a point source. As a test case we use the Vector gas processing plant in Taranaki New Zealand which has emitted 0.1Tg C yr-¹ (as CO₂) since 1970. Previous work using air samples found 2-5 ppm mole fraction CO₂ff 600m downwind of the plant; this study extends the data set back 30 years using radiocarbon measurements in tree rings.

Trees incorporate CO₂ from the local atmosphere into their cellulose which is laid down in annual growth rings during photosynthesis. To relate this to the fossil CO₂ content of the air we measure ¹⁴C in annual tree rings at a local clean air site and compare this to measurements of ¹⁴C in the annual ring for the same year at our test site. Fossil CO₂ is devoid of ¹⁴C so addition of CO₂ff will cause an observed decrease in¹⁴C in samples directly related to the amount of CO₂ff present.

Trees growing immediately downwind of the Vector plant and from clean air locations in Taranaki and Wellington were cored. Annual rings were counted and cut into one year growth increments. Testing was performed on two cellulose extraction methods to confirm removal of contaminating material before the cellulose component was chemically isolated, combusted, graphitised and ¹⁴C measured by Accelerator Mass Spectrometry.

We will present initial results of the data; showing that Wellington tree and Taranaki clean air trees compare well with the Wellington atmospheric record whereas trees growing downwind of the Vector plant demonstrate lower ¹⁴C content consistent with fossil CO₂ addition. We compare historic CO₂ff emissions as sampled by the trees with reported emissions from the Vector plant to quantify and evaluate the ability of the technique to monitor changes in fossil CO₂ emissions. We demonstrate how this technique could be applied alongside complimentary methods to evaluate fossil CO₂ emissions at point sources worldwide to determine compliance of CO₂ emitters with emission reduction targets