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Following is a question by the Hon Cheng Kai-nam and a reply by the Secretary for Planning, Environment and Lands, Mr Gordon Siu, in the Legislative Council today (Wednesday):

Question :

It is reported that 63 per cent of the areas in Guangdong Province has been marked by the relevant Mainland authorities as acid-rain control area. Besides, according to the results of tests conducted on the Mainland, acid rain accounts for more than 50 per cent of the total rainfall in the Pearl River Delta Region. In other words, out of every 100 rainfalls, over 50 are acid rain. In this connection, will the Government inform this Council whether:

(a) it knows if Hong Kong has been included in the acid-rain control area and the percentage of acid rain in the total rainfall in Hong Kong in each quarter of the past three years;

(b) it has conducted any studies on the main causes of acid rain in Hong Kong and the reasons for the increasing frequency of acid rain; and

(c) it has any plans and measures to deal with the problem of acid rain?

Reply :

Madam President,

(a) Acidity of rainfall is expressed in pH numbers. Acidic water has a pH value less than 7 and alkaline water has a pH value larger than 7. Because of the presence of carbon dioxide and other gases from natural sources such as volcanic activities, natural rainfall is slightly acidic and has a pH value of around 5.6. It is therefore a common practice to consider rainfall having a pH value below 5.6 as acid rain. Due to emissions from power generation and all kinds of fuel burning activities, rainfall is usually more acidic in populated areas, commonly with pH values between 4.5 and 5.6. The normal international practice is to report acid rain levels in terms of their acidity on an annual average basis. The annual average acidity of rainfall in Hong Kong was pH 4.7 in 1996, pH 4.7 in 1997 and pH 4.3 in 1998. In the first quarter of 1999, the average acidity of rainfall was pH 5.0. The average acidity of rainfall collected in each quarter of the past three years is given at Annex.

(b) sulphur dioxide, and to lesser extent oxides of nitrogen, are the main contributors to acid rain. They are emitted from power plants, industrial activities as well as motor vehicles, marine vessels and aircraft. Formation of rainfall involves water vapour travelling often over hundreds or even thousands of kilometres in the atmosphere. Hence the acidity of rainfall in Hong Kong is affected by human activities over large areas in the region. Our acid rain phenomenon is closely associated with the development of economic activities such as power generation, industrial emissions and use of motor vehicles in Hong Kong and cities in the Mainland.

(c) A major means in reducing acidity in the rain is through a reduction in the emissions of sulphur dioxide and nitrogen oxides. Between 1992 and 1997, we have adopted a wide range of measures which helped to reduce the emissions of these pollutants. These measures include for example banning the use of high sulphur fuels, using natural gas for power generation, and installing gas de-sulphurisation system and low nitrogen oxides technology on existing coal power units. As a result, the total amount of sulphur dioxide emitted was reduced by 54 per cent from 175,000 tonnes in 1992 to 80,500 tonnes in 1997, and nitrogen oxides emitted was reduced by 44 per cent from 221,000 tonnes in 1992 to 123,000 tonnes in 1997.

We are also working with Guangdong on a joint study looking into the sources of air pollution in the Pearl River Delta Region. Acid rain is one of the issues to be addressed. We intend to work out joint action plans to effectively control and reduce the regional air pollution problem. This will help reduce acidity in our rain water.

End/Wednesday, June 23, 1999

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