China's fight against pollution faces new threats from rising levels of harmful ozone gas despite an "impressive" reduction in other airborne particles, according to a report released Thursday.
The country cut its national average level of airborne PM2.5 -- tiny particles that can penetrate the lungs and enter the bloodstream -- by 27 percent between 2015 and 2019, according to the Finland-based Centre for Research on Energy and Clean Air (CREA).
China also reduced its average sulphur dioxide levels by 55 percent during this period, which CREA said was "very impressive progress".
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However, levels of ozone gas, which is linked to drops in cognitive performance and educational outcomes, rose 11 percent over this period despite national efforts to fight pollution, the report said.
Ozone, along with nitrogen dioxide, "could be the next frontiers in China’s war on pollution", CREA said.
China's air quality gains were mostly achieved through "end of pipe" measures that filter out pollutants right before they enter the environment, CREA lead analyst and the report's author Lauri Myllyvirta said.
In the past few years, "most of the existing coal-fired power plants have been retrofitted to comply with new emissions standards, and currently the focus is on implementing similar retrofits in the iron and steel industry," Myllyvirta told AFP.
But "once these retrofits are completed, if coal consumption keeps rising, it will be harder and harder to make progress on air quality".
Ozone and nitrogen dioxide are also "harder to control with filters", according to the report, which said exposure to the two gases caused hundreds of thousands of premature deaths in China every year.
China built enough new coal-fired plants between January 2018 and June 2019 -- nearly 43 gigawatts worth of capacity -- to cancel out the decrease in the rest of the world, a report in November by US-based Global Energy Monitor found.
The country also plans to add an additional 147.7 gigawatts of coal plants, nearly as many as the European Union's entire gigawatts of existing capacity, the report said.
In September, Swiss air purification technology company IQAir said Beijing was expected to drop out of the list of the world's 200 most polluted cities in 2019.
But PM2.5 levels in the city are still four times higher than those recommended by the World Health Organization.
Image copyrightReutersImage caption Although it is the first time a red alert has been issued, pollution has previously reached far worse levels in the city
Schools in Beijing are closed and outdoor construction halted as the Chinese capital's first ever pollution "red alert" came into effect.
The alert, the highest possible warning level, was issued late Monday and will last until midday on Thursday.
Limits have been placed on car use and some factories have been ordered to stop operations.
It comes as China, the world's worst polluter, takes part in talks on carbon emissions in Paris.
It is the first time China has declared a red alert under the four-tier alert system, which was adopted a little over two years ago, although pollution levels were far from the city's worst.
At 07:00 local time on Tuesday (23:00 GMT on Monday), when the alert came into effect, the US Embassy's air pollution monitor in Beijing reported that the intensity of the tiny particles known as PM 2.5 was at 291 micrograms per cubic metre.
By 11:00 it had dropped very slightly to 250 - still a level it described as "very unhealthy". Levels of the poisonous particles in the suburbs were reported at several times that number.
The World Health Organization recommends 25 micrograms per cubic metre as the maximum safe level.
John Sudworth, BBC News, Beijing: "Why red now?"
As I cycled into the office this morning, smog mask clamped firmly in place, the traffic was certainly lighter. Beijing's first red alert means half of all cars must stay off the roads; odd numbered license plates today, even numbered ones tomorrow.
But although the air is indeed an unpleasant, filthy grey, the pollution index is actually a good deal lower than it was this time last week, when the quantity of dangerous particulate matter (PM 2.5) surged to around 40 times the World Health Organisation's maximum guideline. Today, it is a mere 10 times that limit.
So why red now? Well, the lack of any previous red alerts has been met with increasingly loud howls of derision. What would it take, people wondered last week - as their children felt their way to the still open schools through the poisonous gloom - for the government to act?
Perhaps it is the growing public pressure that has finally made the difference this time round.
Coal-powered industries and heating systems, as well as vehicle emissions and dust from construction sites, all contribute to the smog which has been exacerbated by humidity and a lack of wind.
The order will last until 12:00 on Thursday, when a cold front is expected to arrive and clear the smog.
As well as limits on construction and schools - which were advised to close if they did not have good air filtration systems - cars are only permitted to drive on alternate days, with the day depending on whether a car's number plate ends in an odd or even number. Officials promised additional public transport to cope with demand. In pictures: Smog's effect on skyline The smog film taking China by storm 'Air dark from pollution' Image copyrightSamImage caption Near the East 4th Ring Road, facing west towards Beijing on 1 December... Image copyright SamImage caption ... and the same location a day later Analysis: Matt McGrath, BBC environment correspondent, Paris
China's air quality is a key factor in its push for a new global deal on climate change.
Its negotiators here point to their continued investment in renewable sources of energy, in an effort to cut down on coal consumption, particularly in urban areas. Around 58% of the increase in the country's primary energy consumption in 2013-14 came from non-fossil fuel sources.
These efforts to go green may not be having an immediate effect on the air in Beijing but they have had an impact on global output of carbon dioxide. This year's figures, just published, show carbon levels have stalled or declined slightly even while the world economy has expanded.
A strong agreement here in Paris won't immediately solve China's air woes, but if it ultimately pushes down the price of renewables even further, it could play a part in solving the issue long term.
"You have to do whatever you can to protect yourself,'' Beijing resident Li Huiwen told AP news agency. "Even when wearing the mask, I feel uncomfortable and don't have any energy.''
While the smog's effects have been worsened by weather conditions and the city's geography - bordered to the south and east by industrial areas that generate pollution and to the north and west by mountains that trap it - it has prompted increasing concern that China has prioritised economic growth at too high an environmental cost.
"It is a sharp warning to us that we may have too much development at the price of environment and it is time for us to seriously deal with air pollution,'' said Beijing worker Fan Jinglong.
The scale of the health impact is vast. There have been 1.4 million premature deaths in China because of air pollution, according to a study led by Jos Lelieveld of Germany's Max Planck Institute and published this year in Nature magazine.
What are PM 2.5 particles?
Particulate matter, or PM, 2.5 is a type of pollution involving fine particles less than 2.5 microns (0.0025mm) in diameter
A second type, PM 10, is of coarser particles with a diameter of up to 10 microns
Some occur naturally - e.g. from dust storms and forest fires, others from human industrial processes
They often consist of fragments that are small enough to reach the lungs or, in the smallest cases, to cross into the bloodstream as well
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