Reconstructing urban CO₂ emissions utilising the radiocarbon composition of tree rings from the Wellington Region, New Zealand

This study demonstrates the utility of tree ring radiocarbon analysis to quantify a temporal record of recently-added fossil fuel-derived carbon dioxide (CO₂ff) in the urban atmosphere, to retrospectively measure emissions and potentially validate local emissions inventories. Currently, there is no internationally recognised method to test emissions inventories against direct atmospheric estimations of CO₂ff. With the increasing interest in emissions control legislation, independent and objective research to validate emissions reported by governments and industries is needed.  As CO₂ff emissions are completely depleted in radiocarbon (¹⁴C), an observed decrease in the ¹⁴C content of the atmosphere is mostly due to additions of CO₂ff. As trees incorporate CO₂ from the local atmosphere into annual growth rings, it was hypothesised that an urban located tree would reflect emission rates of its local surroundings. Measurements of the ¹⁴C content of cellulose were made from the annual tree rings of a Kauri tree (Agathis australis), located in the downtown area of the Wellington suburb of Lower Hutt (KNG52). This record was compared with tree rings from two Kauri at a nearby coastal site (NIK19 and NIK23) and the long-term clean air ¹⁴CO₂ record from Baring Head. The clean air Kauri trees, NIK19 and NIK23, demonstrated excellent agreement with the Baring Head atmospheric record, indicating that the trees were accurately sampling the atmosphere. The KNG52 tree, demonstrated good agreement with the clean air record in the early part of the record (with some variability), however, exhibited significantly lower Δ¹⁴CO₂ values from the 1980s onward. Calculation of the influence of the terrestrial biosphere on the ¹⁴CO₂ record showed very little impact, determining that the variability seen was due to local additions of CO₂ff.  Historic CO₂ff emissions were calculated using the Δ¹⁴CO₂ measurements from the KNG52 ¹⁴CO₂ record for the period 1972 – 2012. Biosphere correction calculations showed that the biosphere was the dominant influence on the record in the early part of the record (1972 – 1980), with fossil fuel emissions dominating the record from 1980s onward. The observations were compared qualitatively with meteorological data and urban development in the area to assess variability in CO₂ff. A minor trend towards lower wind speeds associated with higher levels of CO₂ff was identified, indicating that local meteorology may be responsible for 10% change seen in the record. The influence of local development demonstrated some possible relation but a correlation was not significant. The KNG52 CO₂ff record was compared with national-level reported liquid (road traffic) emissions from the Carbon Dioxide Information Analysis Centre (CDIAC). The observed KNG52 CO₂ff in the tree ring record appeared to increase in tandem with road traffic emissions.