The Hong Kong public was caught completely off guard on the early morning of 22 March when they woke up to find the city shrouded in thick smog. A sandstorm from northern China had descended upon the city and pushed its Air Pollution Index readings to unprecedented“severe” level. In its wake, the storm also prompts a public debate on whether Hong Kong should put in place a better sandstorm forecasting system.

At PolyU, a team of researchers from the Department of Land Surveying and Geo-Informatics (LSGI) has been working for years to help regional authorities build up more accurate air quality models and pinpoint the major pollution sources affecting Hong Kong.

In this issue, Professor Janet Nichol, the project’s Site Principal Investigator, shares her views on the need of a forecasting system in Hong Kong and introduces the Aerosol Robotic Network (AERONET) stations set up by PolyU. Professor Nichol is a geographer specializing in the applications of remote sensing to environmental problems. Research projects she has previously worked on include the impact of smoke haze due to forest fires in Singapore and South East Asia.

Q : There are arguments for and against setting up a better forecasting system in Hong Kong. What is your opinion on that?

Nichol : I do think there is a need to set up a general air quality warning system in Hong Kong so that regional pollution events coming here can be known up to 24 hours in advance. Sandstorms would be just a part of this system since most pollution events comprise urban-industrial pollutants from the Greater Pearl River Delta region. A system for sandstorms alone would be inefficient.

Q: Could you brief us about your research on sandstorms?

Nichol : Our group (the Remote Sensing Research Group) in LSGI has set up two AERONET stations in 2005 and 2007 for monitoring air pollution around Hong Kong. These two stations, one on the roof of the PolyU library and one at our university’s climate station at Hok Tsui, are complementary. The former represents both urban and regional emission sources whereas the latter measures regional pollution. The data we collect can tell us the size and characteristics of particulates in the atmosphere. For example, we find that concentrations of large particles – as in desert dust – are rare in Hong Kong, as most pollutants are very fine emissions from vehicles and factories.

AERONET is a collaborative project with the US National Aeronautics and Space Administration (NASA). We send the instruments to NASA every year for calibration and the data we collect is uploaded automatically to their website. NASA also does some higher level processing of our data, and information about the particles, such as their shapes and size distribution, is made available to us. At present, we are the only institution in southern China with an AERONET station.

Q : What is the geographical significance of installing these stations in Hong Kong?

Nichol : Observations of dust pollution in humid tropical southern China – including Hong Kong – are rare, and dust events are often unrecognized by environmental authorities and the media. In recent years, due to the severe industrial pollution from the nearby Pearl River Delta, a dust event may be categorized as local urban-industrial pollution. We can use the AERONET data in conjunction with satellite images to show the source and coverage areas of the dust, as well as its progress over several days.

Q : Could you tell us some interesting observations from this project?

Nichol : Between January 2006 and May 2009, twenty dust events were observed by the AERONET stations. By combining data analysis with satellite images, we have confirmed that several dust storms in northern China have reached Hong Kong with the biggest ones occurred in April 2006 and April 2009. During these storms, we observed the dominance of particle sizes of 1-10 microns in diameter, whereas the norm for Hong Kong is below 1 micron. Counting the sandstorm which hit Hong Kong last week, we have now had two consecutive years with significant dust storms.

Q: This latest sandstorm seems to be the most severe to have impacted upon Hong Kong. What are the characteristics of the pollutants?

Nichol : It is mainly fine dust originating in arid and desertified regions of northwest China. But since the storm passed through industrial regions on its way to Hong Kong, there may be other substances adsorbed - trace metals, dioxins which are carcinogenic, sulphur dioxide, black carbon and pathogens such as bacteria and fungi.

Q: Can these AERONET data be used to predict future sandstorms?

Nichol : Monitoring can be done with satellite images and a dust storm may be detected even before it hits Beijing. Then we can predict if it will come to Hong Kong with the help of the National Oceanic and Atmospheric Administration’s HYSPLIT back-trajectories, which are wind and precipitation models in near-real time.

Q: Can they help in alleviating the pollution problem?

Nichol : Our data cannot alleviate the pollution problem directly but we should try to find out more about the relationship between dust events in Hong Kong and public health. This has been done in countries where there is better access to medical records than in Hong Kong.

Q : How can the government make better use of the data in future?

Nichol : Some government departments have approached us for AERONET data related to this dust event. I suppose they have not made use of these data before because dust events have been relatively rare up to now. Hopefully we can contribute to setting up a monitoring and forecasting system of air pollution and dust events in Hong Kong and the PRD region using satellite and AERONET data. Actually, this has been the subject of a recent application for Research Grants Council funding. The project intends to work with the Hong Kong Observatory as well as researchers in the USA who have been involved in the planning of the next phase of global monitoring satellites.