Resolving Auckland’s CO2 budget: urban biosphere, diurnal cycle and constraints from isotopes

Carbon dioxide (CO₂) is the single largest contributor to anthropogenic radiative forcing, with 70% of global CO₂ emissions originating from urban areas. New Zealand has set ambitious greenhouse gas emission reduction targets, and its largest city, Auckland, will play a key role in achieving those reductions as it houses over 25% of the national population. To meet reduction targets, it is vital to understand current emissions and monitor the impact of implemented policies (e.g., planting trees). For these reasons, we are developing the first observation-constrained, urban-scale emission estimation framework for Auckland. This work is part of the New Zealand CarbonWatch-NZ project that also includes emission estimation at the national scale.

A new and developing atmospheric observation network is operated in and around Auckland to measure CO₂, ¹⁴CO₂, CO, CH₄, and COS. The combination of trace gases is useful in distinguishing between source sectors, especially biosphere from anthropogenic fluxes. This is important for Auckland: a green city with a year-round growing season. High-resolution bottom-up emission estimates have been developed specifically for anthropogenic (Mahuika-Auckland) and biospheric (UrbanVPRM) CO₂ fluxes in Auckland. We combine bottom-up estimates and atmospheric CO₂ observations in an inverse emission estimation framework that includes atmospheric transport simulations with the Lagrangian NAME-III model, driven by meteorological data from the 333-m horizontal resolution Auckland Numerical Weather Prediction model. Use of such high-resolution meteorological data is unique and helps interpret atmospheric measurements in the heterogeneous landscape of Auckland, especially when combined with our high-resolution bottom-up estimates. Finally, we explore the value and difficulties of including the full diurnal cycle of CO₂ data. The resulting emission product will be a policy-relevant instrument that can help evaluate and meet New Zealand’s emission reduction targets.