The Mayor said he would “micro-target” hotspots in the outer suburbs beyond the North and South Circular roads and said “nothing is off the table” in terms of future restrictions on vehicle use.
Mr Khan has resisted demands from environmental campaigners for the Ulez to cover all 33 boroughs.
But in an interview with the Standard, he made clear he wanted to deliver cleaner air for all Londoners, which he sees as a matter of “social justice”.
His comments came as he was on Wednesday confirmed as the new chairman of the C40 network of almost 100 world cities committed to action on climate change.
Mr Khan said: “If it’s the case, a few months after we have done the expansion, there are still parts of London outside the North Circular/South Circular within Greater London that require additional help, we will micro-target those areas.
“We think we can have a 30 per cent reduction across London in nitrogen oxide emissions [as a result of] the expansion, but nothing is off the table in relation to future progress being made.
“If we don’t get to zero carbon by 2030 we won’t have fewer people with permanently stunted lungs, fewer people dying prematurely and fewer people with health issues from asthma, to cancer, to heart disease.”
The Ulez widens to an area 18 times bigger than the existing central London zone on Monday, with an estimated 135,000 motorists a day facing a £12.50 levy - generating about £2m a day in levies and fines for Transport for London.
“Micro-targeting” could involve only allowing zero-emission vehicles to use certain roads at peak times, as has been introduced by the City of London Corporation at Barbican and Hackney council in Shoreditch.
Mr Khan also hopes the Ulez - which will operate 24/7, 364 days a year - will cut traffic, which in parts of the capital now exceeds pre-pandemic levels. “Absolutely - I don’t apologise for that,” he said.
In central London, the number of vehicles fell by about 24,000 a day when the Ulez was launched. Mr Khan said: “You imagine if we can replicate that inside the North Circular and South Circular. It would be amazing in relation to the reduction in emissions from vehicles.”
He said it would be “nonsensical” for Londoners to retain a non-compliant vehicle to use only in outer London.
His aim is to have 80 per cent of journeys made by sustainable means by 2041. Last year this fell from 63 per cent to 57 per cent as a consequence of the pandemic as Londoners got back in their cars.
“What we want to do, particularly for the shorter journeys, is to get people to walk, cycle, use public transport,” he said.
“There is now an incentive - not just cleaner air, but active travel makes you fit but also you are saving £12.50 as well.”
He anticipates the launch, which has been timed to coincide with the lower traffic levels in the school half-term holidays, will be a “smooth landing” due to years of preparations by Transport for London.
The new boundary was chosen because it was “practical” in terms of camera infrastructure, and because Mr Khan was not willing to delay his plans until it was possible to cover all of Greater London. He didn’t want to “wait for perfection”, he said.
More than 80 per cent of vehicles that circulate within the zone are already Ulez-compliant. All motorists spotted in the area should have received letters warning of the launch, alongside a TV and advertising publicity blitz. “Nobody can complain that they don’t know about it,” he said.
The Mayor said he was more interested in making the Ulez expansion a success than rushing to replace it and the congestion charge with a form of “smart” road pricing, which would vary according to time and distance travelled.
“My point is: you’ve got to make sure you land the schemes you have got,” he said. “If this goes wrong, it makes our chances of being bold going forward really hard.
“Smart road-user charging: We are keeping it under review. We have got TfL people talking to many people who are doing this. My point was: it’s not ready, in relation to the technology I have seen. Forget rolling it out.”