HK: NO2 Country for Clean Air – Our polluted city

Researchers from the Chinese University of Hong Kong (CUHK) have found that not only are Hong Kong’s NO2 emissions are not declining as planned – they are increasing. They place the blame squarely on the increasing number of private cars and call for policies such as an electronic toll system in an effort to get the situation under control.

“Overall, the amount of cross-border air pollutants is decreasing over the years but local NO2 emission is on the rise,” says Prof Steve Yim Hung-lam, assistant professor at the Department of Geography and Resource Management at CUHK.

Prof Yim and his team track the concentration of air pollutants, namely PM10, PM2.5, SO2 (sulfur dioxide) and NO2, on a monthly basis in Hong Kong. They collect data to reflect the long-term trend of air pollutant emission.

In Hong Kong, the four major air pollutants are PM10, PM2.5, SO2 and NO2. The study finds that over 80 percent of PM10, nearly 80 percent of PM2.5 and 50 percent of SO2 in Hong Kong come from mainland China.

However, NO2 from mainland China only makes up 30 percent of the concentration. The rest is emitted locally.

No No NO2! – Noxious NO2

From 2002 to 2016, NO2 emissions rose by 15 percent in summer in Hong Kong. In 2015 alone, the concentration level exceeded the limit of 40 μg/m3 set by the World Health Organization.

NO2 primarily gets in the air from the burning of fuel. The gas comes from emissions from cars, trucks and buses, power plants and off-road equipment. Breathing air with a high concentration of NO2 can worsen respiratory diseases, especially asthma.

“It could be related to the increasing number of cars in Hong Kong. The total registration number is growing every year,” Prof Yim says.

“Even though the authorities are promoting the Euro V and VI emission standards, it doesn’t help address the problem if there are more cars on the road,” says Prof Yim.

He adds that there are measures targeting commercial vehicles but restrictions on private vehicles are insufficient.

“More cars on the road give a higher chance of traffic congestion. And the longer the cars stay on the road, the higher NO2 concentration” he adds.

Toll me clean

Prof Yim calls for policies that aim to tackle air pollution in the city.

“There has to be long-term planning, such as electronic toll system to raise costs to discourage people from driving into the city, as well as better road design to enhance ventilation,” he suggests.

Meanwhile, Prof Yim also points out that the concentration of these air pollutants can vary from season to season due to climatic factors.

La Niña weather patterns worsened air pollution in Hong Kong as the northern wind brought air pollutants from China and the lower wind speed facilitated the accumulation of pollutants in the city.

On the other hand, when El Niño hit Hong Kong, the concentration of PM10, PM2.5 and SO2 was lower than that of other years.

“These climatic factors can make it difficult to control the air pollution level,” Prof Yim says.

Currently, the Air Pollution Control Ordinance sets out Air Quality Objectives (AQOs) and provides for the periodic review of the AQOs at least once every five years with an aim to promote the conservation and best use of air. The AQOs are measured in concentration limit and the allowable number of times when an air pollutant exceeds the concentration limit. 

“To make Air Quality Objectives more reasonable, the government should tighten the concentration limit but allow more exceedances,” he explains.

Just as he suggests, the government is reportedly considering lowering the concentration limit for PM2.5 from 75 μg/m3 to 50 μg/m3 per 24 hours but increasing the number of exceedances allowed from 9 to 35.