ABSTRACT

The problems encountered in conducting air quality monitoreng (AAQM) are many. Both physico-chemical and biological methods of AAQM are prone to errors. A critical evaluation of the practice, problems and prospects of the use of bioindicators for air quality monitoring with particular relevence to Indian conditions is attempted. The experimental work done along these lines during 1976 to 1983 in the cities of Bombay and Delhi revealed that the biological indices derived from the responses of the indicator plants can be supplimentary to the AAQM using physico-chemical method which then could provide more comprehensive information of the air quality. The need for the formulation of a national programme on the use of bioindicators for air quality monitoring is also strongly felt.

INTRODUCTION

Industrialization and extensive use of fossil fuels has caused the increase in the general levels of air pollutants in ambient air. The presence of certain air pollutants can have chronic and acute health effects on human, or it may damage the valuable vegetation and property directly or indirectly and thereby adversely effecting the general welfare of the human beings in short term or long term depending upon the situation. To protect these interests it is imperative that the ambient air quality monitoring (AAQM) be done. The objectives of AAQM (W.H.O., 1977) are:

1. Assess the spatial temporal trends.
2. Evaluate control strategies.
3. Activate episode controls.
4. Evaluate risks to human health.
5. Evaluate risks to environmental damage.
6. Data base fore land use planning.
7. Test dispersion models.
8. Investigate complaints.
9. Initial assessment.

The methodological approaches to AAQM are (i) physicochemical methods, (ii) biological methods and (iii) sophisticated continuous monitoring system but the same is not covered in the scope of this paper because the use of automatic sophisticated instrumentation in AAQM is yet to come of age in this country due to various reasons.

The problems encountered in the AAQM are many and both physico-chemical and biological methods in them have inherent short-comings. Efforts are made in this paper to critically evaluate both the methods and to evolve a multidisciplinary approach for the AAQM on the basis of work done in this country.

Biological Indicators of Air Quality

The two aspects of biological indicators have to be borne in mind when evaluating them for AAQM ; (i) biological indices may not be as precise or narrowly accurate as the sophisticated instruments would be, and (ii) sessile, rooted plants have been selected more often as indicators. A population of single species is preferable to an individual for the same purpose. This becomes necessary as the organisms are diverse in their behaviour, their response to and stress being dictated by their limits of tolerence which are as varied as their genetic, physical and physiological makeup. There have been numerous studies on the use of plants for indication of air quality and on the basis of plant response air quality indices have been suggested. In European countries Scientists have found high correlation between visible lichen transplant damage and sulphur dioxide levels, and SO2 pollution gradients were described by means of lichen transplantation method.

The pollutant-sulphur dioxide used as an index for the study, since it was the most common phytotoxic industrial emission in Bombay. Identical sets of plants were grown in the laboratory and distributed them in 10 different locations in the city. After 4-week's exposure to the ambient air, the plants were brought back to the laboratory and analysed for simple parameters like shoot length and shoot dry weight. The plants from the known-cleanest area were treated as the control and reductions in the observed values at other places were calculated as percentages. Average of the addition of percentage losses in shoot heights and weights were used to arrive at Air Pollution Indices (API). Using the API values as a radius and the dot indicating the area of exposure as the centre, circles drawn were placed on the map of Bombay city ; bigger circles indicated proportionately higher pollution levels (Chaphekar et al. 1989).

Limitations in the Use of Bioindicators

(i) A major limitation in the use of accumulator indicator plant species lies in the fact that, under natural conditions many plant species are capable of evolving tolerance by genetic recombinations over generations or through mutations (Chaphekar, 1978), (ii) Many damage symptoms are identical, limiting the use of plants as indicators ; e.g. chlorosis of interveinal areas of leaves takes place due to reasons as varied as SO2-pollution of air, 2-4-D sprays, Magnesium deficiency, etc. Reductions of net primary production of plants takes place under continued stress of practically any type of chronic level. Under such conditions, reading one symptom may give an idea of some anomaly in the situation, (iii) Since capacity of an organism to respond to a particular stress is affected by other environmental factors, especially nutritional status, water stress and temperature, the plants have to be studied and read carefully for the indiction that they give (Chaphekar, 1978).

Ambient Air Quality Monitoring Using Physicochemical Methods

The AAQM done by the most of the regulatory agencies and other environmental organisations is by using physico-chemical methods. The National Environmental Engineering Research Institute, Nagpur, has published ambient air quality data for ten selected cities in India (NEERI, 1981) where AAQM was done by using physico-chemical methods.

The problems of air quality monitoring are many, such as :

1. Size of Sample
It is very important to determine the size of sample which will be representative of the area. This will vary from place to place depending upon physiography of the location.

2. Rate of Sampling
After deciding upon the size of sample one has to fix up the rate at which the sample would be drawn so that the absorbing solution would not be saturated before the end of sampling period. It is also essential that the flow rate be maintained, through out the sampling period, which is influenced by fluctuations in the voltage, pressure drop at the filter, etc.