Cheung Chi-fai – SCMP
If you are looking for the place in Hong Kong where you can breathe most easily, Tai Po has the cleanest air – on the face of it.
It had the fewest hours of unhealthy air in the first half of the year – 1,188 hours, equivalent to 50 days.
But take a closer look and you will see a different picture.
A South China Morning Post (SEHK: 0583, announcements, news) analysis of Environmental Protection Department data from the 11 general air-quality monitoring stations citywide shows Tai Po was one of only three to record an increase in unhealthy air between the first half of 2005 and the first half of this year.
The increase in “high” and “very high” pollution readings in Tai Po between 2000 and 2008 was also the third-biggest across the city.
Sha Tin – like Tai Po, a New Territories new town – and Eastern district are the next cleanest, both recording fewer than 1,300 hours of unhealthy air in the first six months of the year.
Sham Shui Po and Kwun Tong were the most polluted built-up areas, with more than 1,500 hours of high or very high pollution in the first half of the year. These areas all experience heavy traffic on their roads.
Under the city’s 22-year-old air quality guidelines – which critics say are outdated – an air pollution index reading of 51 to 100 is classified as high and a reading above 100 as very high.
The government says persistent exposure to high levels of air pollution may have long-term effects and that when pollution is very high, people with heart or respiratory illnesses should reduce outdoor activity.
The general monitoring stations measure concentrations of sulphur dioxide, nitrogen dioxide, ozone and respirable suspended particles.
The number of hours of high and very high air-pollution readings in Tai Po in a full year rose by 48 per cent, from 2,309 in 2000 to 3,422 in 2008. Only remote Tap Mun Island, with a 110 per cent increase, and the Sha Tin and Eastern districts, where the hours of unhealthy air rose 50 per cent, did worse.
The figures came as a big surprise to Yau Wing-kwong, a green activist and Tai Po district councillor.
“I have all along believed Tai Po is the greenest part of the city. I had no idea the air quality had worsened at such a pace,” he said.
“It is clearly very alarming and the district council should follow this up immediately. We should get involved with experts from universities to look into the problem.”
Mr Yau suspects a wave of property development, and reclamation of Tolo Harbour – the sea inlet on which Tai Po new towns sits – have played a role in changing the district’s microclimate.
Chan Chak-keung, a chemical engineering professor at the University of Science and Technology, said ozone pollution – caused by the reactions between volatile organic compounds and nitrogen oxides under strong sunlight – could be a big factor in Tai Po’s air pollution.
While the district had one of the lowest annual average concentrations of three of the four major pollutants last year, it had the second-highest average ozone pollution.
Only Tap Mun in Mirs Bay, into which Tolo Harbour runs, had worse ozone pollution. Government officials see the high levels there as a indicative of regional pollution levels, given the island’s proximity to the border.
Professor Chan said ozone pollution involved a complex process and its impacts in urban areas and the countryside were different. Ozone pollution tended to be less acute in urban areas since the gases could be “consumed” by nitrogen oxides emitted by motor vehicles, he said. But this offsetting process might be weaker in Tai Po.
“The district is relatively open and large, making dispersion of pollutants easier, while emissions from transport sources in the district are lower than in the urban areas.”
Frank Lee Shun-cheng of Polytechnic University said prevailing winds were a key factor in variation in air quality between districts.