Bad air in Hong Kong is likely to persist until the weekend as weak winds fail to disperse pollutants.

“Very high” concentrations of air pollution engulfed Causeway Bay and Central this afternoon, while “high” levels were recorded in several other districts.

A dry continental airstream affecting the southern China coast is to blame for the deterioration in air quality as weak winds made it harder to disperse pollutants, leading to a build up in choking air in dense urban areas, according to the Environmental Protection Department.

The bad air is likely to persist until the weekend when winds switch from moderate northeasterly to easterly winds, the department said, citing forecasts from the Observatory.

The black 10 reading was recorded at the Causeway Bay roadside monitoring station at noon, while the station at Central hit 9 and Mong Kok reached 8 on the 11-tier Air Quality Health Index.

The Environmental Protection Department said it is expected that the AQHI at the roadside may reach the “serious” level – a reading of “10 +” and the highest in the index – later today, as winds remain light to moderate.

It also said a higher than normal level of pollution is expected to linger until wind would increase in the next two days.

The department said higher than normal levels of particulates and nitrogen dioxide have been recorded in the territory since Tuesday evening.

“In addition, light winds hinder effective dispersion of air pollutants and lead to the accumulation of air pollutants in part of the urban areas and at the roadside,” it said.

According to department data, the hourly concentration of roadside-dominant nitrogen dioxide at about 6pm yesterday was at 347 micrograms per cubic metre of air.

Concentrations of fine suspended particulates (PM2.5) – hazardous particulate matter that can enter the lungs – hit 122 micrograms per cubic metre.

The World Health Organisation air quality guidelines set 200 micrograms per cubic metre as the maximum hourly concentration for NO2 and 25 micrograms per cubic metre as the maximum 24-hour average concentration for PM2.5.

A reading of “very high” or above on the index prompts a warning by the department to children, the elderly and people with existing heart or respiratory illnesses to reduce physical exertion and outdoor activities. The general public is also advised to reduce time outdoors, especially in areas with heavy traffic.

Yesterday, the Education Bureau urged all schools to safeguard students’ health and take note of changes in the index.

Hong Kong witnessed its worst air pollution of the year on January 21, with half of the 15 monitoring stations recording “serious” levels.

Visibility in Victoria Harbour fell to around 3,000 metres under the influence of haze this morning, the Observatory said.

Today’s weather is likely to be fine, mild and dry with a maximum temperature of 20 degrees and light moderate northeasterly winds. Tomorrow is forecast to be cloudier and relatively humid with the trend continuing into the weekend.

via ‘Very high’ Hong Kong air pollution levels set to continue until the weekend | South China Morning Post.