Most of the M25 is outside the GLA area and therefore not shown on the map. It forms the boundary in the Waltham Cross and Heathrow areas, and cuts across the easternmost extremity of Greater London, and slightly elevated levels can be seen there.
In any case motorways, despite having higher volumes of traffic than the radial routes and the North Circular, probably produce no more particulates as (most of the time!) traffic is moving smoothly, with engines turning at their most fuel-efficient speeds and little use of the brakes.