Bigshea13 I think others have explored most of this elsewhere in other threads, but if you have a real desire to get some data into you look here for reliable reseasrch and info on the very topic you ask about
Pacific Southwest Research Station
Also here
Urban Forests Effects on Environmental Quality
This site is excellent too
Sacramento Tree Foundation: Greenprint Program: Overview
Bottom line for me is that single trees even groups of trees like any other single element of our ecosystem cannot fix what we have broken and seem determined to smash up even further....However I do believe very strongly that trees are a
major part of the answer more in the sense that by generating a real understanding and appreciation of what trees mean for the whole system us included we will very gradually change the way enough people feel and think about their own environment to make their own small changes in the way
they live...we're Arborists so thats the part of the puzzle we deal with, thats the areas we focus on....yes there's a whole lot more to it than just the trees but the trees are what we deal with...so we should be able to help others focus more clearly on how trees and proper progressive long term tree care can assist in improving with the mess we have created.