I'm hoping one of these may help you, or be a jumping-off point toward the info you need...
https://www.quora.com/Which-plant-or-tree-purifies-air-the-most
several trees are mentioned, though the brief list seems like it'd be climate limited
I’ve had good results when seeking many answers to tree-related questions on this site
:
http://forums2.gardenweb.com/forums/trees
These people definitely know the tree topic — and since they’re active both in Africa and in the UK, they likely would have knowledge for a zone with conditions like yours
http://internationaltreefoundation.org/
BTW, I'm in zone 6, with the forest quite near, but still near our house we've planted some trees (such as a hard-maple cousin to sugar maple), a weeping willow, and some spruces that I'm glad we have for shade and for the influence on the air.
