Reconstruction of historic fossil CO2 emissions using radiocarbonmeasurements from tree rings

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