I was just in Paris last October and at that time it was no smoggier than any major city.
If your roof is covered with wet soil it will retain water. So, you need to have an epoxy coated rebar reinforced concrete roof that is poured in two slabs, each with a water retention membrane applied and the structural steel to support it -- that ain't cheap. The bearing weight load would be phenomenal for 100% coverage -- we are talking road bridge construction techniques.
Planter boxes over bearing walls would be a lot cheaper and feasible -- I don't know what the new regulations call for -- 10% coverage with green areas?
I built in this method; bearing wall planters on a steel beamed concrete deck, an architectural nightmare on a multi-million dollar waterfront house. I was a building contractor in my past life. You also have to design for earth retention and the necessary weep (drainage).
If you want to stop smog just have everyone walk or ride a horse
Electric cars for short city commuting make sense and they don't produce smog. But retrofitting the "fleet" will take time and planning.