Estimating CO2 sinks and sources in New Zealand from atmospheric measurements and Lagrangian modeling

New Zealand's total greenhouse gas emissions are estimated to have reached 20 Mt C yr-1 in 2011. However, its net emissions are lower, about 16 Mt C yr-1, in particular due to CO2 uptake across the land biosphere. This uptake, therefore, plays a crucial role in the country's greenhouse gas budget, yet its magnitude as well as regional distribution are not well-known. Numbers such as above usually represent bottom-up estimates, i.e., are based on local measurements of changes in the various biospheric biomass pools and while they are extensive and very detailed, they are also difficult to upscale to regional and national levels. We have developed a top-down approach to estimate regional sinks and sources of CO2 in New Zealand during 2011-13, which will serve as a complement to the bottom-up estimates. The approach uses a statistical method, Bayesian inversion, to combine CO2 observations from three sites across both Islands with source/sink regions at the surface through modeled atmospheric transport. Preliminary results are presented along with a footprint analysis of the three sites (i.e., the sensitivity of the observed CO2 concentration to potential source and sink areas across the country), a description of the observations and a priori information on oceanic and terrestrial sinks and sources.