BigFrank is absolutely right, in that air pollution in the LHR area has long been seen as the most intractable barrier to the airport's expansion, even if the air pollution is much more due to non airport-related road traffic on the M4 and M25.
However, the recent Supreme Court judgement might actually favour LHR expansion, ironically. It could force the UK Government to address the air pollution issue irrespective of whether R3 goes ahead. And once that problem is solved, it conveniently removes the obstacle to expanding LHR.