As a chill wind from the north descended on the city yesterday, roadside air quality in Central plunged to its worst this year, posing a threat to public health.
The air began to deteriorate on Monday and the pollution index rose further yesterday.
It peaked in the hours running up to and after midnight, reaching a level of 173, and hovered at similar high levels until morning.
Even as the index drifted down, the public remained at threat from poor air dominated by a high concentration of nitrogen dioxide and respirable suspended particles.
Nitrogen dioxide exceeded the World Health Organisation's guidelines by 13 to 26 per cent for most of the day yesterday.
The Causeway Bay and Mong Kok recording stations saw peak API readings of 155 and 135 over the past two days.
Under the city's 22-year-old air quality guidelines - which critics say are outdated - an air pollution index reading of 51 to 100 is classified as high and a reading above 100 as very high.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.
As the smog blanketed the city, advocacy group Clean Air Network joined forces with truck operators' associations and transport sector legislator Miriam Lau Kin-yee to call for higher subsidies for operators who switch to cleaner vehicles.
It said the groups' joint demand represented unprecedented co-operation between a green group and the transport trade.
The Environmental Protection Department said light winds along with high background levels of pollutant concentration worsened the air quality while the high reading in Central was related to the district's setting and landscape. 'There will be air recirculation when the north wind interacts with the hills in Central, resulting in a trapping of pollutants,' a spokesman said.
Hong Kong suffered its most polluted November air in a decade this year. Background air pollution - reflecting regional air quality - was high for more than 80 per cent of the month.
On Saturday, the department warned the city might be under the influence of a sandstorm from northern China.
Meanwhile, the Clean Air Network and its new-found allies said the maximum subsidy for the replacement of commercial vehicles should be raised from 18 to 25 per cent of the value of new, less polluting vehicles and low-interest loans should be available to cover the rest of the cost.
The Legislative Council will debate a long-awaited revamp of Hong Kong's outdated air quality objectives today. The Environment Bureau has yet to make a decision on tightening standards, 13 months after a public consultation on the issue ended.
Hong Kong suffered its most polluted November air in a decade this year
In the hours around midnight on Monday, the air pollution index in Central rose to a very high level of: 173