Hong Kong Meteorological Society

Hong Kong Secondary Schools
Acid Rain Survey 2002

Guidelines for Teachers

prepared by

Dr. Bill Kyle & Dr. Mervyn Peart
Department of Geography
The University of Hong Kong

October 2001

WHAT IS ACID RAIN?

Acid rain is a popular term related to a whole range of atmospheric acidic pollution effects associated with precipitation which also include acid fog, acid mist and acid snow.  Indeed, it is not necessary to have wet conditions to have acidic pollution effects.  Some pollutants can be transported directly to the surface as gases or small particles without being absorbed by moisture, a phenomenon known as dry deposition.

WHAT CIRCUMSTANCES GIVE RISE TO ACIDIC POLLUTION?

Acidic pollution can arise from both natural processes and human activities. Volcanic activity and animal and plant decomposition are well known examples of the former.  However, in many urban and industrial areas the dominant source is from human activities.  There are four major pollutants that cause acid air pollution: sulphur dioxide, nitrogen oxides, hydrocarbons and ozone.

Sulphur dioxide (SO₂)

Sulphur occurs widely in nature, in all animal and plants, and in the fossil fuels - coal, oil and natural gas - which were formed from ancient plant and animal remains.  Oil and natural gas generally have relatively low sulpur contents, usually less than 1 percent.  However, some coals can have as much as 14 percent sulphur.  When fossil fuels are burned to generate energy the sulphur combines with the oxygen in the atmosphere to form sulphur dioxide (SO₂).  Globally, about 50 percent of the SO₂ levels in the atmosphere  areestimated to be of natural origin.  However, in many urban and industrial areas human activity may account for around 90 percent of the SO₂ in the air.

Nitrogen oxides (NO_x)

A number of different kinds of nitrogen oxides exist in the atmosphere. Collectively they are given the name NO_x.  Like SO₂, about 50 percent of the NO_x in the air on a global basis is estimated to be from natural sources.  However, in industrial areas only about 10 percent of NO_x are  of  'natural' origin.  The vast majority are produced when fossil fuels are burned.  The nitrogen present in the fuel and in the atmosphere combines with oxygen to produce several different types of NO_x.  Coal-burning power stations and transport are the major sources.

Hydrocarbons and ozone

Unburnt hydrocarbons emitted as a result of improper combustion of fossil fuels constitute a major air pollutant in many urban and industrial areas.  These hydrocarbons can react with NO_x in the presence of sunlight to produce ozone which can reach levels 10 times higher than the natural background level which is generally low.  This ozone then reacts with other pollutants to form acids.

WHAT ARE THE ACID PROCESSES TAKING PLACE IN THE ATMOSPHERE?

The SO₂ and NO_X are transformed into acids in the atmosphere either by dry deposition or by wet deposition.  During dry deposition, which usually occurs close to the source of the pollution, the pollutants are deposited on to surfaces where they then turn into acids.  With wet deposition, which can occur over 1,000 km from the source, the gases become acids after coming into contact with water droplets in the air.  Generally the longer that the pollutant stays in the air the more of it will be converted into acid.  The diagram summarizes these processes.

HOW ACID IS ACID RAIN?

Acidity is measured on the pH (potential hydrogen) scale which runs from    pH 0, the most acid, to pH 14, the most alkaline.  The midpoint of the scale, pH 7, is termed neutral.  The scale is logarithmic so that each point is ten times greater or less than the point above or below it respectively.  For example a pH of 5 is ten times more acidic than pH 6, and is 100 times more acidic than pH 7.  The scale below shows the acidity of some everyday objects on the pH scale.

Normally, rainwater from an unpolluted atmosphere will have a pH of about 5.6 or 5.7, that is, it is naturally acidic, being about 25 times more acidic than neutral distilled water which has a pH of 7.  This is because the carbon dioxide (CO₂) in the air reacts with water to form a weak carbonic acid.  In addition, during thunderstorms, some nitrogen oxides are formed, which when dissolved in water produces dilute nitric acid.  However, when acidic pollutants are present the additional sulphuric and nitric acids formed as a result of the 'acid' processes in the atmosphere greatly reduce the pH value.  Badly polluted rainwater can have a pH as low as 2.4, about the same as lemon juice.  In severe situations pH may be as low as 1.5, about the same as the acid in a car battery, and some 10,000 times more acidic than unpolluted rainwater.  On average, rainwater in Hong Kong probably varies between pH 4 and pH 5 though it probably varies considerably both in space and in time.

WHAT DAMAGE DOES ACID RAIN DO?

Acid rain creates a wide range of environmental problems. First, it damages plants, crops and trees by attacking the leaves directly, or through the roots via changes in soil acidity and the mobilization of toxic chemicals.  It also impacts on freshwater habitats, either by falling directly into lakes or by causing damaging chemicals to be washed into them from the surrounding land.  The damage to these ecosystems also has knock on effects in other parts of food webs dependent upon them.  Acid rain also affects materials and buildings leading to increased damage and corrosion.  Stone is often cited as most vulnerable but it also adversely  affects plastics, stainless steel, glass, and even the pigments in paint.  Drinking water can also be affected resulting in health problems.  High levels of lead, copper and aluminium in tap water released by acid attack on old pipes can cause illnesses and complaints such as nausea, hair discoloration and even brain damage in young children.

UNDERTAKING AN ACID RAIN SURVEY IN SCHOOLS

A survey of the acidity of rainfall in a particular area can be conducted relatively cheaply using home-made equipment and inexpensive measuring methods.  However, to ensure that the results are reliable a number of basic methodological steps need to be standardized.

Clearly, in any given geographical area, the denser and more spatially representative the sampling network, the more likely it will be that generalizations can be made.  Thus, it is desirable that the participating schools take measurements in as broadly diverse areas of Hong Kong as is possible.  Equally, to obtain a representative picture of temporal variation it is necessary to conduct the survey for an extended period of time so covering as wide a diversity of rainfall situations as possible.

The collection of the samples, the measurement of pH and the recording of its value as well as recording of other relevant information also need to be standardized to ensure intercomparability between sampling locations.  The following section deals with each of these matters in turn.

COLLECTING RAINFALL SAMPLES

It is very important that the collecting device used to collect the samples be of uniform type and exposure.  The recommended construction of the collector is shown in the diagram.

Basically the collector consists of a plastic funnel the radius of which should be 62.5 mm, so that the exposed area is the same as a standard 125 mm diameter raingauge.   This should allow water to flow into a replaceable plastic bag which rests in a cut-off plastic bottle of appropriate size.  This, in turn, is firmly fixed to a broom handle as shown.  It is very important that the collector funnel be cleaned with distilled water every day at a fixed time, whether or not rain occurs, to avoid build up of dry deposition which would then be washed into the sample when rain does occur thus giving misleading results. The gauge also needs to be placed away from obstructions such as buildings or trees and needs to be sufficiently high above the ground (1.5 m) so that water or other contaminants can't splash in from the surrounding area.

The collection of rain should be on a regular basis at set time of day.  The rainwater, if any, collected should be poured into a calibrated flask which has previously be cleaned with distilled water.  This will give you the volume of rainwater that has been collected and so will enable you to calculate the rainfall amount in millimetres using the following calculation.

1000 x (volume of rainfall (ml) / 3.14 x r² ) where r is the radius of the collecting funnel

For example if 250 ml was collected and the radius of the collecting funnel was 62.5 mm then 1000 x (250 / 3.14 x 62.5 x 62.5) = 20.4 mm.  This calculation will enable comparison with the readings from  a standard rain gauge where one is available.

The collected rainwater should then be transferred to a sealable plastic bottle which has also be thoroughly rinsed out with distilled water. It is essential that no contamination occurs during this process. The bottle should be clearly labelled with the date and time of the collected sample. It is very important that  the sample be analyzed soon after collection.  In any case, more than 24 hours should not elapse between collection and analysis.

ANALYSIS OF COLLECTED SAMPLES

A number of ways of measuring pH of the samples is possible.  The simplest and the most readily standardized method is the use of pH sensitive paper strips which are dipped into the sample and which change colour according to the pH of the sample.  In order to ensure inter-comparability of the measurements its is vital that this procedure be carried out in an identical manner in all sampling stations submitting data for analysis.

The recommended method is as follows.  Use a clean small test tube or beaker.  Transfer enough of the sample from the sample bottle to cover the sensitive part of the pH paper. Remove a piece of pH paper from its container using tweezers and place it in the sample WITHOUT touching it. Leave the pH paper in the sample until the colour change is complete and then remove it using tweezers.  To take the reading hold the paper against the pH scale and match the colour of the paper with that on the scale.  Preferably an estimate should be made by each member of the group and a consensus value arrived at which should then be the value recorded. The reading should then be entered at once on the Data Submission Sheet provided for pH paper tests.  The other relevant information asked for on the chart should also be entered at the same time.

Where pH meters and/or conductivity meters are available schools may choose to use this equipment to perform further analyses on the samples.  These analyses should be performed using the standardized procedures applicable to the particular equipment and advice on standardization may be sought from the organizers of the survey. The results of these analyses will help provide supplementary information and should be entered on the Data Submission Sheets provided for these tests. As before, the other relevant information asked for on the chart should also be entered at the same time.

SUBMISSION OF DATA SHEETS

When all the data sheets are completed they should be forwarded to:

Hong Kong Meteorological Society