LONDON — It was a powerful image: soaring high in the sky next to Big Ben was a Mary Poppins figure wearing a protective mask.

And for good reason: London exceeded its annual air pollution limits for the whole of 2017 on Thursday night — a whopping five days into the new year.

London is a world leader in nitrogen dioxide (NO2) -- levels are far higher than they are in the U.S. -- because of its reliance on diesel-powered vehicles. Many of the city’s iconic black cabs and double-decker buses are run on diesel, a major source of NO2.

Activists were expecting London to breach its limits early in the year — in 2016, it took eight days — and the Greenpeace environmental organization responded by staging a protest with a giant mask-wearing Mary Poppins balloon.

According to European Union and British law, no single area is allowed to exceed the hourly limits — of 200 micrograms of NO2 per cubic meter of air — more than 18 times in a year. On Thursday night, an air-quality monitoring station on Brixton Road in south London breached the limit 19 times, reaching 347.7 ug/m3 at 9 p.m.

Clean Air in London, an advocacy group, said that Brixton Road reported 20 exceedances over a 24-hour period, which “could be a new world record for an official monitor.”

Its director, Simon Birkett, said that “with diesel vehicles responsible for 90-95 percent of NO2 from vehicle exhaust,” the only way to comply with the law was to “ban diesel from the most polluted places.”

He also urged pedestrians to avoid Brixton Road and urged locals to abandon outdoor dining.

“Emergency measures and action are needed,” he said.

Britain, along with other countries in Europe, bet heavily on diesel cars before they understood that they emitted harmful pollutants. As my colleague Griff Witte reported:

Governments across Europe have aggressively promoted diesel vehicles, reasoning that diesel’s lower carbon-dioxide output makes it gentler on the planet than gasoline. In London, the streets are filled with diesel-powered buses and taxis. Continent-wide, diesel accounts for about half the car market.

But diesel has one glaring disadvantage: It is a major source of NO2, a pollutant that stunts lung growth and has been linked to a range of respiratory and cardiovascular diseases. The diesel push has meant that although air in Europe is far cleaner overall than in many parts of the globe, it still can be — and often is — deadly.

It’s estimated that nearly 10,000 Londoners die prematurely every year as a result of air pollution.

ClientEarth, an environmental law firm that has taken the British government to court for not tackling Britain’s air pollution crisis, called on the London’s new mayor, Sadiq Khan, to deliver on his promises to reduce air pollution.

“This is another shameful reminder of the severity of London’s air pollution,” said Alan Andrews, a lawyer at the firm.

He said that the London mayor “has promised to introduce a bigger ultra-low emission zone in 2019 and to deploy the cleanest buses on the most polluted roads. While these are vital steps in the right direction, we can’t wait another three years for action. We need immediate action to cut pollution in the short-term and protect Londoners’ health during these pollution spikes.”

“If cars coming off the production line had dodgy brakes, you know the government would step in to sort it out,” said Paul Morozzo, clean air campaigner at Greenpeace. “We urgently need to stop the sale of new diesel models until emission testing is truly fit for purpose. Better still, we need car companies to phase out diesel completely and concentrate on hybrid and electric alternatives.”

For his part, Khan announced on Friday the introduction of new “green bus zones,” which will see low-emission buses deployed on some of the capital's most polluted streets.

“London’s filthy air is a killer,” he acknowledged.