John Vidal has a long piece on air quality in the Guardian. This seems to be getting more of a profile after years in which it was seen as a fairly minor issue. I recall raising it with some London Councils colleagues only for it to be seen as a fringe concern.
Brent actually has quite a reasonable record on improving air quality. There is, of course, the ongoing blight of poor air quality in Neasden Goods Yard. The trouble here is that the Environment Agency is extremely reluctant to take action. Boris Johnson until recently had no interest in air quality at all, and only seems to have woken up to the seriousness of the situation in the last few months.
Aside from that specific area, London Boroughs main contribution to better air quality is by reducing traffic pollution.
This was one aim behind the introduction of emissions based parking charges in Brent. Although most of the publicity behind this concentrated on carbon emissions, those are also a good proxy for other pollutants, such as PM2.5 and NOx.
Councils can also use planning policy to limit car use and encourage other forms of transport as Brent generally does. This is also the justification behind most of the Council's policies on parking charges.
However, I suspect progress on these issues is likely to be stymied by a reluctance of decision makers to accept that political decisions can have a good outcome in one direction but not in another. I get that sense from some of the comments in the Vidal piece. It also seems to happen locally here in Brent, where the Council has tinkered with parking charges in a way that seems to encourage car use at the same time as having other policies to discourage car use. That really doesn't make much sense.