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City Enjoys Rare Winter Days Of Clear Blue Skies – Northern Monsoon Sweeps Pollution Out To Sea, Experts Say

Joyce Ng and Phyllis Tsang, SCMP – Dec 02, 2008

Hong Kong saw clear blue skies rarely seen during the winter over the weekend, but data shows that the overall picture was no better than last year and roadside pollution remains a serious problem.

Visibility recorded by the Observatory in Central came close to 20km last Thursday, and continued to improve over the weekend.

Sunday morning saw readings as high as 30km, and the same reading was found at the Chek Lap Kok station. A view from the peak of Tai Mo Shan would afford a clear view of the city.

Visibility, affected mainly by the concentration of particulate matter in the air and humidity, is said by the Observatory to be “reduced” by haze if it reads below 8km and there is no fog, mist or rain.

The Observatory and other atmospheric scientists attributed the clear sky to a strong northeast monsoon, which blew at an average speed of 48km/h at Waglan Island last Thursday, stronger than the monthly average of 30km/h.

The monsoon was strong enough to push pollutants from the north into the South China Sea, dispersing particulate matter, senior Observatory scientific officer Yeung King-kay said.

However, the Observatory headquarters in Tsim Sha Tsui recorded 20 more hours of reduced visibility last month than the same month a year ago.

Alexis Lau Kai-hon, an atmospheric scientist at the Hong Kong University of Science and Technology, said visibility readings of 20km to 30km at the airport were “very rare” in winter, but pointed out that this did not mean the air was any cleaner.

All last Thursday the air quality monitoring station in Central recorded indices over 100, meaning “very high” pollution levels. The pollution levels there remained “high” or “very high” through the weekend.

The index continued to be over 100 yesterday, with Central and the other two roadside stations all exceeding 100 in the afternoon and evening. “No matter how strong the winds are, you see the roadside pollution indices remain high, showing the pollutants are all nitrogen dioxide,” Professor Lau said.

The major source of the pollution was vehicles. Heavy vehicular emissions were still a serious problem, Professor Lau said, and the government should step up efforts to combat it.

Meanwhile, factory closures in the Pearl River Delta triggered by the financial meltdown have not meant increased visibility in the city.

Professor Lau said continued monitoring was needed, though he would not rule out that closures could help contribute to clearer skies.

Wang Tao, a specialist on atmospheric chemistry at Polytechnic University, expected factory closures would bring improved visibility.

He said closures would mean less electricity consumption, meaning power plants would generate less sulfur dioxide.

Through a chemical reaction, sulfur dioxide will become sulfate, a particulate matter lowering visibility.

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