There's a fixed limit to photosynthetic CO2 sequestration without genetic modification: about 11% energy efficiency (based on Photo-synthetically Active Radiation, Resonance Energy Transfer, and the Z-scheme https://goo.gl/RH8udA). Given an annual average insolation of around 250 W/m2, it results in a maximum of 40 kg/m2 per year. This is with 11% efficiency; most plants don't exceed 1%, maybe 2%. If the CityTree is 3.5 m2, it's max sequestration rate is ~140 t/yr. Oh but wait, it's orientated vertically...