I wonder how much carbon dioxide does grass sequester, compared to trees. Moreover, is grass still efficient in absorbing pollution? It seems to me that grass would be more efficient in storing carbon in soil than trees.
I found this article regarding lawns. It suggests that lawns could be regarded as carbon sinks only if they are not (frequently) mowered, and do not use pesticedes.
An acre of established temperate forest can hold from 2,000 lbs. up to 6,000 or more lbs. of carbon per year, depending on the age of the trees and other conditions. Mature grasslands sequester 2,400-3,600 lbs. per acre each year.
Two things you need to consider:
Storing more CO2 in the soil means you want to have more life in there. This points to the avoidance of fertilizer and pesticides in your lawn.
Peat has a huge capacity to store CO2, so you may consider turning your lawn into wetland if possible ;)
I believe its all about mass, biomass is effectively sequestered CO2.
In year one a lawn probably has a greater mass than a one year old tree, and so more CO2 sequestered. But in year twenty the tree has a greater mass about 1 tonne which is 3 tonnes of CO2, and the lawn, if its been regularly mowed still only has the same biomass as it did in year one.
Of course by year twenty there is a twenty years worth of lawn clippings somewhere which has either returned to the air or broken down into soil somewhere, its in some other lifecycle.
That said the tree might lose its leaves every year which would join the same lifecycle as the lawn clippings.
In conclusion, the lawn is more efficient for carbon sequestration in the first year or so, but over the long term the tree is more efficient in locking up the Carbon in a more permanent manner.