BEIJING (REUTERS) - The Chinese capital was on the second-highest orange smog alert in the depth of winter on Tuesday (Jan 3) as city officials said the air quality was improving overall, citing data for the whole of last year.

Over the new year holiday, hundreds of flights were cancelled and highways closed across northern China as average concentrations of small breathable particles known as PM2.5 soared above 500 micrograms per cubic metre in Beijing and surrounding regions.

Pollution alerts are common in northern China, especially during bitterly cold winters when energy demand, much of it met by coal, soars.

But the Beijing Municipal Environmental Protection Bureau told state media that PM2.5 concentrations dropped 9.9 per cent on the year to an average of 73 micrograms per cubic metre in the Chinese capital in 2016.

The total number of "blue sky days" reached 198 in 2016, up 12 from the previous year. However, the average PM2.5 measure still exceeded national air quality standards by 109 per cent, the bureau said.

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Despite a brief respite on Monday, smog returned to the Chinese capital on Tuesday, with PM2.5 readings again at "hazardous" levels. The city environment bureau said the orange alert was expected to last until Wednesday.

China is in the third year of a "war on pollution" aimed at reversing the damage done to its skies, soil and water after decades of untrammelled economic growth.

It has created emergency response systems that restrict traffic and shut down factories and construction sites during periods of heavy smog, and it has also vowed to punish local officials and enterprises that break rules.

During a bout of smog in December, inspectors identified 21 enterprises that had violated regulations by failing to close operations on time, and 10 more inspection teams were dispatched to cities across the region over the new year.

But government officials have expressed frustration that persistently heavy winter pollution, brought about by unfavourable weather and the use of coal-fired urban heating systems, has overshadowed the genuine progress China has made to reduce smog.