Sensitivity of optimal extension of CO₂ observation networks to model transport

Optimal extensions of the surface CO₂ observation network have been determined using 15 global transport models and a time-independent inverse model. The regional average CO₂ flux estimate uncertainty is minimized based on the TransCom-3 (level 1) framework. An ensemble model calculation shows that the regional average CO₂ flux uncertainties could be reduced to about 0.36, 0.32, 0.28 or 0.26 Gt C yr⁻¹ per region, from about 0.53 Gt C yr⁻¹ per region corresponding to the basic network, after adding 5, 10, 15 or 20 optimally located stations, respectively. The additional station locations are mostly found in continental South America and Africa. The distribution of the efficiency in estimation of flux uncertainty reduction per station tends to become more uniform with the extension of the network. We show that the multimodel approach to network design converges if a large enough extension is considered; about 20 stations in this inverse model framework. The reduction in the flux uncertainty for the first few stations depends on the model of atmospheric transport, and is nearly proportional to the simulated signal from local emissions in the surface layer. In addition, it is seen that the simulated spatial and temporal variability of CO₂ concentration has significant influence on the distribution of the additional stations as well as determining the regional flux estimate uncertainty.