“…Therefore, we also use the simultaneous assimilation framework; the ensemble Kalman filter (EnKF) was used to constrain CO 2 concentrations and the ensemble Kalman smoother (EnKS) was used to optimize surface CO 2 fluxes. Since the regional chemical transport models, compared to global models, have some advantages in reproducing the effects of meso-microscale transport on atmospheric CO 2 distributions (Ahmadov et al, 2009;Pillai et al, 2011;Kretschmer et al, 2012), we choose a regional model, Regional Atmospheric Modeling System and Community Multi-scale Air Quality (RAMS-CMAQ) (Zhang et al, 2002(Zhang et al, , 2003(Zhang et al, , 2007Kou et al, 2013;Liu et al, 2013;Huang et al, 2014), to develop this inversion system. For simplicity, this system is referred to as CFI-CMAQ (Carbon Flux Inversion system and Community Multi-scale Air Quality).…”