The city has not seen any policy intervention with the power of producing "overnight" improvements in air quality for more than 25 years, according to a think tank researcher. But an upcoming mandate ordering ships to use cleaner fuel while stopping by in Hong Kong was likely to be the "second time" this would happen since 1990, said Simon Ng Ka-wing, chief research officer at Civic Exchange. The new law, expected to come into effect in July, requires all ocean-going vessels to switch to cleaner marine diesel of a maximum 0.5 per cent sulphur content while berthing in the city. Ng said the first policy intervention to proved highly effective was in 1990, when a cap was imposed on the sulphur content of industrial fuel in factories. "The [air pollution index] dropped almost overnight. This is the second time, I think, such a policy will produce such a drastic effect," he said. "Sulphur emissions will be cut by almost 80 per cent almost immediately." Ng called for standards of low-sulphur fuel to be tightened even more to bring the city in line with international levels, given the government's plan to continue subsidising shipowners for another three years. That incentive scheme, meant to encourage the use of cleaner fuel by halving berthing fees, would be extended to March 2018, Financial Secretary John Tsang Chun-wah announced in his budget speech last month. "Our position is that if they are to continue their subsidy, then the standards should even be stricter and perhaps closer to the international standard of 0.1 per cent [sulphur content]," Ng said. "Shipping companies must be given an incentive to go an extra mile." Undersecretary for the environment Christine Loh Kung-wai, who founded the think tank, has stated that a clean fuel emissions control area (ECA) for the Pearl River Delta would be "the ultimate goal" for Hong Kong - and the region. Such an area can be formed only if Beijing applies to the International Maritime Organisation, which in January updated the requirement on low-sulphur fuel under an IMO convention to 0.1 per cent in designated ECAs. Three ECAs are in place internationally, in North America, the Baltic Sea and the North Sea. "If an ECA is the objective, then it makes sense for these ships to start switching to 0.1 earlier," Ng said. But a regional ECA would probably not be set up until at least 2020. "Even if [Beijing] applies now, it won't be ready till 2018. Hong Kong has taken the first step and Shenzhen is not far behind, but many of Guangdong's port cities need to do more." The Environmental Protection Department said there was no plan to further incentivise the use of fuel that had less than 0.5 per cent of sulphur content. But ocean-going ships berthing in Pearl River Delta ports other than Hong Kong might also be forced to switch fuel. The department said it was exploring the feasibility of working with Guangdong authorities to mandate the change.