The Air Quality Index (AQI) is a useful tool as it helps in rating the overall air quality based on the observed values of different air pollutants. Such an index is always associated with specific descriptor category. It may be noted that AQI is a complementary tool and should not replace precise scientific information’s.
A number of pollutants are known to occur in air but for the purpose of assessment of the air quality of a particular place, the following five pollutants are considered to be important.
These are:
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(i) Suspended Particulate Matter (SPM)
(ii) Nitrogen oxides (NO and NO2)
(iii) Carbon monoxide (CO)
(iv) Sulphur oxides (SO2, SO3)
(v) Hydrocarbons (HC)
Recently lead has also been added to this list in view of its increasing content in the atmosphere and corresponding health effects. The maximum permissible limits or standards of these major air pollutants are presented in the following Table 13.16.
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The standards are in two parts:
(i) Primary standards which apply exclusively to human health and
(ii) Secondary standards which apply to property damage and general welfare.
The differences observed among the primary permissible limits of these pollutants can be attributed to their toxicities. The combined adverse effects of two or more pollutants should therefore be considered carefully while adopting these standards. A comparison of individual air pollutants against the maximum permissible limits, no doubt, gives some clues on the quality of air; the cumulative combined effect of all the pollutants provides real information on the quality of air, whether good or bad.
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Air Quality Index (AQI) is a tool to reduce the vast amount of data to the simplest form rating all the essential information. The Air Quality Index (AQI) is a useful tool as it helps in rating the overall air quality based on the observed values of different air pollutants. Such an index is always associated with specific descriptor category. It may be noted that AQI is a complementary tool and should not replace precise scientific information’s.
At present a variety of Air Quality indices are adopted by different organization. Here AQI developed by Thomas and co-workers (1971) at the Oak Ridge National Laboratory, USA is discussed. It is called ORAQI (Oak Ridge Air Quality Index). This is based on 24 hr. average concentrations of five major air pollutants (CO, NO2, SPM, SO2, Hydrocarbons).
AQI has been modified for three pollutants (SO2, NO2 and SPM) for the same 24 hrs. standards as follows:
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ORAQI: 39.02 ∑3 (X1 / Xs) 0.967
Where Xi and Xs denote the pollutant concentration and the sum total of respective permissible limits of the major criteria of air pollutants.
For 5 critical Air pollutants ORAQI is represented mathematically by the following equation:
Where Ci = concentration of pollutant.
Si = sum of the standards for 5 criteria of air pollutants. ORAQI system use nomograph, a scale representing concentrations for various pollutants and their relation to the established standard value. Thus the pollutants concentration in a fraction of the appropriate standard (Ci/Si) coefficient (5.7) and the exponent 1.37 in the expression are used to scale the ORAQI in a such way that non-polluted background level attains Index value of 10 and the polluted air attains an index value between 10 and 100. ORAQI categories are as follows (Table 13.17).
Central Pollution Control Board (CPCB) in 1994 used SPM, SO2, and NO2 as Air Quality data for calculating Air Quality Index for important cities of India. Because there is no clear cut demarcation between industrial and non-industrial areas, the annual average values for residential areas were used in calculation of AQI. There are 14 cities which are in bad category. Seven cities are in good category, namely Tuticorin, Coimbatore, Daman, Silvassa, Vasco, Kozhikode and Guwahati. The rest of the cities come in either fair or poor category.
Central Pollution Control Board has published data for the purpose of analyzing the Air Quality Index for the period from 1987 to 1996 of the following cities:
1. Top ten polluted cities:
On the basis of 1991 census data, top ten polluted cities are: Mumbai, Kolkata, Delhi, Chennai, Hyderabad, Bangalore, Ahmedabad, Pune, Kanpur and Nagpur,
2. Next other smaller cities:
These are Chandigarh, Dehradun, Faridabad, Howrah, Vadodara, Mysore, Cochin, Bhopal, Jaipur, Shimla, and Agra. The most polluted cities in terms of suspended particulate matter (SPM) are Dehradun, Gajroula, Surat, Kanpur, Delhi and Kolkata. In most of the cities the average value of suspended particulate matter (SPM) in open area may be double or triple to that of residential areas.
Among top 10 polluted cities, the worst in terms of SPM are Kanpur, Delhi, Kolkata, where the average concentrations are nearer to the levels of maximum permissible limit. In terms of NOx (nitrogen oxides) some top polluted cities are Howrah, Kota, Dhanbad, Jharia, Gajroula and Mumbai.
The Central Pollution Control Board has suggested the following classification of Indian cities on the basis of Air Quality Index (Table 13.18.)