They were talking about CO2 which is what the algae tank is about.

Trees have other benefits around filtering pollutants that affect air quality such as sulphur dioxide and nitrogen dioxide. Also the shading effect reduces ozone accumulation as well as generally helping reduce the urban heat island effect (which in turn reduces the amount of air conditioning needed, even a small amount saves energy and reduces pollution from power stations).

City parks have clean air partly because of tree but also because youre away from roads and buildings so further from car exhausts and chimney stacks. The concentration of pollutants in wide open spaces is lower because the wind can move it around more easily, and there isn't a pollution source directly near by. Tree and grass do help too.