The data of NO2 were collected by diffusion tubes which contain a chemical reagent Triethanolamine (TEA). Since TEA can absorb NO2 so the concentration of NO2 can be calculated by laboratories.

Given its smaller size, NO2 diffusion tube can be installed in locations at street level, such as bus stops and parks, providing a more accurate measurement of quality of air we normally breathe in. In fact, NO2 diffusion tubes have been used in London by local government or even concern groups to measure air qualities in communities for decades, providing sufficient data to support implementation of different policies to improve air qualities. Moreover, the tubes are distributed to citizens for encouraging participation in citizen science projects, which allows citizens to set up tubes at desired locations and provide enormous amount of data for visualizing air pollution in forms of interactive maps or figures.

NO2 diffusion tubes have been one of the less costly options for passive air quality measuring tool, while being convenient to use and flexible to install, and these tubes can generate valuable data for concern groups to raise public awareness through publication of results or urging local governments to take up a more active role in alleviating air pollution. By engaging the public through citizen science projects, the public will be more active in taking actions to change their lifestyles to reduce emission of air pollutants, or to reduce their exposure to polluted air.



The NO2 diffusion tubes used in this air measurement activity do not need calibration, and reflects the air quality in 50 spots in Tuen Mun. They do not reflect the air quality and air pollution problem in the whole Tuen Mun District.


Spot selection

Tuen Mun Road, as a major expressway connecting Tuen Mun and Tsuen Wan, is known for its frequent traffic jams and road accidents. According to the Annual Average Daily Traffic (AADT) by the Transport Department, most of the road segments of Tuen Mun Road have a daily traffic of more than 100,000 in 2019. Moreover, many schools are built near Tuen Mun Road, reflecting that the air pollution is seriously affecting students’ health. Therefore, we installed NO2 tubes in school zones near Tuen Mun Road, to measure the NO2 concentration and visualize the seriousness of air pollution. On the other hand, some of the schools are located far away from the main roads and MTR station. These schools should have a lower NO2 concentration, and hence create less impacts to children’s health. Therefore, we also installed NO2 tubes in school zones such as Leung King, to compare the air pollution level. In this project, we have chosen 15 school zones as our scope, reflecting the air pollution of more than 30 schools, since many schools are located nearby and have similar air pollution levels.

Parks are usually filled with greenings, should have lower NO2 concentration and impose less impact on children’s health. Moreover, most of the parks in Tuen Mun District are located far away from main roads. This further improves the air quality of parks and protects children’s health. On the other hand, children spent more time on physical activities than adults. Therefore, by measuring the NO2 concentration of parks, we hope to showcase that parks are cleaner and healthier for children to stay, and advise children to spend more time in parks.

A bus station for many bus routes may have several buses idling at bus stations at the same time, emitting a worrying amount of NO2. Furthermore, bus stations at busy roads may mean that buses are unable to drive away from the stations under traffic congestion, idling for a longer time at the stations. Since children’s airways are still developing, they are more vulnerable to breathing in polluted air than adults. They breathe more rapidly than adults, and thus car exhaust will impose a higher risk to health. In this campaign, we installed NO2 tubes on bus terminus and bus stops with many bus routes, to showcase that these bus stops have serious air pollution and advise students to wear masks when waiting for a bus at bus stations or choose bus stops with better air quality.


1. Annual Average Daily Traffic (AADT)

During the spot selection, we chose some spots with high traffic and compare the NO2 concentration with spots with low traffic. By the data provided by AADT, we are able to compare the traffic in different years (2018 and 2019) and understand the road traffic of different road segments for spot selection

Image 1:

Road traffic in different road segments in Tuen Mun


PRAISE, a mobile application developed by HKUST, can predict and analyze the air pollution level in different spots, and show the concentration of air pollutants such as NO2 and PM2.5. From the image below, we can observe that the NO2 concentration in the afternoon remains high, especially in Tuen Mun Town. By the mobile application PRAISE, we are able to know the air pollution level of every spot, to help us in selecting appropriate spots for installing NO2 tubes.


Image 2: the interface of PRAISE (Tuen Mun District)

 the interface of PRAISE (Tuen Mun District)


3. ArcGIS

After selecting the spots, by the GIS system, the road distance between main road (Tuen Mun road) and the selected spots can be calculated. We can analyze the relationship between road distance and NO2 concentration and provide us more evidence in advising schools to move main entrances away from main roads.


ArcGIS system

Story posted on
25th Mar, 2021


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