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

This project aims to reconstruct historic fossil fuel derived CO₂ (CO₂ff) emissions from two closely located point sources in Taranaki, New Zealand. The Vector gas processing plant and the Ballance agri-nutrients ammonia urea plant have combined emissions of ~0.16 TgC yr⁻¹ since 1970 and 1982 respectively. Previous work found 2–5 ppm CO₂ff in short term integrated samples collected 600m downwind of the Vector plant. This study extends the dataset back 30 years using radiocarbon measurements in tree rings. Trees incorporate CO₂ from the local atmosphere into their annual growth rings. Measurements of ¹⁴C in polluted and clean air trees were compared to the Baring Head Δ¹⁴CO₂ atmospheric record. As CO₂ff emissions are devoid of ¹⁴C addition of CO₂ff will cause a decrease in ¹⁴C directly related to the amount of CO₂ff present. Trees growing immediately downwind of the Vector plant and from clean air locations in Taranaki and Baring Head Wellington, were cored and cut into one year growth increments. Two cellulose preparation methods were tested to confirm effectiveness at removing mobile extractive components and lignin. Radiocarbon and stable isotope results showed that the ANSTO method was more effective than the Rafter method. The clean air trees compare well with the Baring Head atmospheric record whereas trees growing downwind of the Vector plant demonstrate lower ¹⁴C content consistent with CO₂ff addition. Historic CO₂ff emissions were reconstructed for the polluted trees, with 1–3ppm of CO₂ff in the Luscombe chestnut tree and 4–7 ppm CO₂ff in the Vector pine tree. CO₂ff observations were compared with reported emissions from the Vector and Ballance plants. Observed CO₂ff increased by 10% in the Vector pine tree for the period 1994–2012 relative to pre-1994 levels, whereas combined CO₂ff emissions increased by 64%. No increase was observed in the Luscombe chestnut tree for the same time period. Meteorological analysis was performed to assess the relative contribution of CO₂ff from the sources to the trees. It is proposed that the trend observed in the Vector pine is due to the dominance of emissions from the Ballance plant and a relatively minor contribution from the Vector plant.