Finchley Road is probably the most polluted (and polluting) street in North West London between Regent’s Park and Brent Cross. Traffic levels in Swiss Cottage are dangerous.
The pollutants monitored include nitrogen oxides (NOx) and fine particulates or particulate matter (two sizes, PM10 and PM2.5).
The diffusers capture nitrogen dioxide, NO2. The tubes are not precise but they are convenient and cheap to monitor long term patterns of pollution. I don’t know if Swiss Cottage site measures sulphur dioxide (SO2), ozone (O3) and carbon monoxide (CO).
Surprisingly, the kerbside site in Swiss Cottage is not a meteorological monitoring. The nearest one is located at the Camley Street Nature Reserve, by the King’s Cross Gasholders, a few miles from Swiss Cottage.