Air pollution problems at New Street have been compounded jointly by the move towards a multiple unit operated railway at a station designed for traditional passenger trains with a big locomotive out in the open at the platform end, increased frequencies on many routes and the cessation of changes to electric traction wherever possible when Virgin introduced the Voyagers. At New Street Crosscountry are probably most of at fault on all these points as a result of inheriting Virgin's Voyager fleet, with their big, fuel thirsty engines on every vehicle, and their fall out from Operation Princess, with a high frequency of services in all directions focused on New Street as a central hub.
As such, policy on passenger traction over the last 20 - 30 years is probably more to blame for increasing pollution levels at major stations than the attitude towards electrification.