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Sulphur dioxide
Suspended particulate matter
Carbon monoxide
Oxides of nitrogen


Ambient air quality in Delhi has been monitored by the National Environmental Engineering Research Institute (NEERI) as part of the National Air Quality Monitoring Network (NAQMN). Sulphur dioxide (SO2), SPM and nitrogen dioxide (NO2) have been measured at three sites

(Figure 1) representing commercial (Town Hall), industrial (Najafgarh) and residential (Netaji Nagar) areas between 1978 and 1987. Monitoring was suspended in 1988 before recommencing in 1990. Results were published in a series of Air Quality in Selected Cities reports. Annual mean airborne lead (Pb) concentrations were determined at the three NEERI sites in 1990. The Central Pollution Control Board (CPCB) commenced air quality monitoring at five stations in 1987; however, these results are not directly comparable with those from the NEERI sites because of differences in siting criteria and methodologies. NEERI have not undertaken any monitoring of carbon monoxide (CO) or ozone (O3) in Delhi. Reference has been made to research studies on these pollutants.

Sulphur dioxide

Emissions Figures 3 and 4 demonstrate that industrial sources, and power stations in particular, are responsible for the majority Of SO2 emissions in Delhi. It is estimated that the two power stations, Baderpur and Indraprastha, produce around 25,550 tonnes of SO2 per annum. Indraprastha alone produces 30 per cent of total SO2 emissions. Estimates produced by the CPCB (NEERI, 1991a) put total anthropogenic SO2 emissions at approximately 59,000 tonnes per annum in 1987. NEERI estimates are lower; their 1990 estimate is approximately 45,000 tonnes per annum with a projected increase to 49,000 tonnes per annum by 2000. There are major differences in the two inventories. NEERI suggest that domestic emissions have remained relatively stable throughout the 1980s through changes in fuel use. However, in the inventory produced by the CPCB, domestic and commercial emissions estimates are higher and increase significantly from 8,395 tonnes per annum in 1981 to 12,400 tonnes per annum

in 1987, a period of just six years. Ambient air quality data displayed in Figures 5 and 6

support NEERI's suggestion that industrial emissions have increased during the 1980s. The CPCB inventory (Figure 3) also points to an increase in most industrial SO2 emissions, including power stations, between 1981 and 1987.

It is agreed that SO2, emissions from Transport sources have increased and will continue to increase due to the increasing motor vehicle population. The number of diesel-driven vehicles (the main vehicular source Of SO2) increased from 16,658 in 1971 to 75,709 in 1987. Delhi is a major goods distribution centre and therefore it is likely that many public and private carriers will be based in Delhi. Many lorries will also travel into Delhi every day; however, it is unlikely that the number of lorries registered in Delhi are ever all operational in the city on any single day.

Ambient Concentrations Figure 5 shows increasing SO2 concentrations at the three GEMS/NEERI stations. Concentrations are consistently higher at the commercial and industrial sites. Annual mean concentrations at the commercial and industrial sites have exceeded the WHO guideline range in each year since 1984 until monitoring was discontinued in i988. However, data for 1990 suggest that concentrations have fallen below the WHO annual guideline range at all three sites. Figure 6 shows a similar trend in 98 percentile concentrations. At the residential site 98 percentile levels were also increasing and approaching the WHO daily guideline (NEERI, 1983, 1988, 1990, 1991b).

Sulphur dioxide concentrations peak in winter (November to February), probably as a result of lower temperatures, especially at night. These cause demand for space heating and therefore the burning of coal. More importantly, winter calms and ground-based temperature inversions are likely to hinder dispersion of emissions.

Suspended particulate matter

Emissions Estimated emissions of SPM follow a very similar pattern to SO2 emissions. It is estimated by NEERI that total SPM emissions were around 115,700 tonnes per annum in 1990 and will increase to 122,600 tonnes per annum in 2000 (NEERI, 1991a). NEERI attribute increasing emissions throughout the 1980s mainly to industry (Figure 4). However, inventories conducted on behalf of CPCB indicate that industrial and domestic SPM emissions decreased between 1981 and 1987 and that the overall increase in SPM was attributable to Transport. Figure 3 shows that the glass and ceramic (bricks) industries are the major sources of SPM after power stations. It is estimated that the two power stations, Baderpur and Indraprastha, produce 25,500 tonnes of fly ash per annum, causing a major soot problem (settled particulates) around these places. Electrostatic precipitators at the plants are old and poorly maintained, although a drop in SPM emissions from this source between 1981 and 1987 would suggest that their efficiency is improving.

Domestic emissions have remained, and will continue to remain, relatively stable mainly due to rapid urban population growth which counters the reductions achieved by changes in fuel use. Delhi has seen a doubling of kerosene and liquid petroleum gas (LPG) use between 1981 and 1987 and this trend is likely to continue. The use of soft coke, firewood and charcoal reduced substantially over the same period, as did that of dung cakes (NEERI, 1991a).

Anthropogenic emissions are not the only source of SPM in Delhi. Natural dust is blown in from the surrounding arid areas, such as the Great Indian Desert (Thar Desert). The Andhi dust storms are a regular climatic feature in June, preceding the monsoon rains and depositing large amounts of dust and SPM into the atmosphere. The monthly mean SPM concentration in June is over 600 micro g m-3, the WHO 24hour guideline is 150-230 micro g m-3. This natural dustfall will remain in circulation for long periods. Settled particulates and those 'washed out' of the atmosphere by the monsoon rains are likely to be resuspended during drier conditions. Wind speeds also increase during the summer leading to further resuspension.

Some estimates Attribute one-third of all anthropogenic SPM in Delhi's atmosphere to road Transport. A survey of over 50 roadside sites in 1987 on the influence of motor vehicles on the spatial distribution of SPM and other pollutants indicated that there is a strong correlation between motor vehicle densities (i.e., at junctions) and SPM concentrations (Mathur, 1988). Besides direct motor vehicle emissions, resuspension of natural dust and settled particulate is also likely to be an important factor. Spot SPM samples taken at Connaught Place (the commercial centre of New Delhi) were found to exceed those around the Indraprastha power plant which has a widely recognised smoke and soot problem. Transport SPM are probably of greatest concern in terms of health effects due to the carcinogenic nature of certain diesel smoke constituents (e.g., polynuclear aromatic hydrocarbons (PAHs)). Buses in particular are a major source of diesel smoke. Measurement of smoke emissions from three bus companies showed that 82 per cent of buses have a smoke density higher than the permissible limit of 65 Hartridge Smoke Units (HSU). Around 46 per cent gave a smoke density of 90 HSU and above which is extremely high and indicative of jet black smoke (Maximum 100 HSU). Trucks and 'Tempos' are also particularly bad emitters. It has been suggested that buses and public goods carriers should be inspected by an independent organisation for smoke levels. Such an organisation would require legal powers to enforce emissions limits on vehicles. Maintenance schedules and guidelines for operators would be required.

Ambient Concentrations Analysis of annual mean ambient SPM concentrations shows no significant trend. However, 98 percentile concentrations at all stations (Figure 7) have increased and at the residential and commercial sites concentrations have more than doubled between 1982 and 1990 (NEERI, 1983, 1988, 1990, 1991b). These concentrations are far in excess of all the recognised WHO guidelines and also exceed the Indian air quality standards. Even taking into account the huge natural component of SPM in Delhi it is clear that these levels constitute a substantial risk to human health.


Emissions The Pb content of petrol distributed from the Mathura refinery which serves Delhi is relatively high at 1.8 g/l. Given this Pb content and the number of petrol-driven motor vehicles registered in Delhi it is estimated that Pb emissions from this source are of the order of 600 tonnes per annum.

Ambient Concentrations Annual Pb concentrations in Delhi in 1990 at the three GEMS/NEERI sites ranged from 0.37 micro g m-3 at the residential site to 0.67 micro g m-3 at the industrial site (NEERI, 1991c). Results of spot samples taken at over 50 traffic junctions in 1987 show a wide range from 0.129 micro g m-3 at Janak Cinema to 4.6 micro g m-3 at Palika Bazar (Mathur, 1988). Concentrations at the vast majority of junctions were below 1 micro g m-3, the WHO annual guideline.

Carbon monoxide

Emissions Emissions inventories for CO in Delhi suggest that the major source, as in most other cities of the world, is Transport and in particular motor vehicles. It is estimated by NEERI (1991a) that emissions have increased from 140,000 tonnes per annum in 1980 to 265,000 tonnes per annum in 1990 (Figure 4). Total CO emissions are projected to reach over 400,000 tonnes per annum by the year 2000. Domestic emissions were around 42,000 tonnes per annum in 1990, similar to those estimated for 1970 and the projected figure for 2000. As with other pollutants, changes in fuel use account for the reduction in per capita domestic emissions.

The increase in motor vehicles (Figure 2) throughout the 1980s accounts for the huge increase in CO emission. By the year 2000 it is estimated that CO emissions from two-wheelers will be approximately equal to those for all motor vehicles in 1991-92.

Modelling studies have been conducted on the concentrations of traffic CO in Delhi. As expected, maximum CO concentrations are reached at peak traffic hours. These are 0900 hours to 1100 hours and 1700 hours and 1900 hours (Singh et al., 1990).

Ambient Concentrations Carbon monoxide is not routinely monitored by NEERI or by CPCB in Delhi. However, spot surveys found that concentrations around heavy traffic junctions such as Delhi Gate, Seelampur, Okhla and Vikas Minar were all around 5 mg m' over eight hours; the WHO eight-hourly guideline value is 10 micro g m-3 (Mathur, 1988).

Oxides of nitrogen

Emissions Total emissions of oxides of nitrogen (NOx) in 1990 were around 73,000 tonnes per annum and follow a similar trend to CO emissions (Figure 4). Industrial emissions accounted for around 15,000 tonnes per annum during the 1980s and it is estimated that Indraprastha power station produces 50 per cent of industrial NOx alone. Over 90 per cent of industrial NOx emissions are attributable to power stations. Transport NOx emissions have increased significantly over the past 20 years and projections for the rest of the century suggest that NOx emissions will continue to rise due to increasing motor vehicle traffic. Diesel-driven goods vehicles and buses are by far the most important source of Transport NO. It is estimated that goods vehicles and buses, which constitute 7 per cent of vehicle numbers, contribute 85.5 per cent of vehicular NOx emissions (NEERI, 1991a). It is estimated that by the year 2000 diesel-driven vehicles will account for 96 per cent of vehicular NOx emissions (excluding two and three-wheelers).

Ambient Concentrations Figure 8 shows the increasing trend in average NO2 concentrations through the 1980s, although annual concentrations in 1990 were below those of 1987. Levels at the commercial Town Hall site are greatest owing to high motor vehicle densities in this area. Levels at the Najafgarh industrial site are higher than those at the residential station. Ninety-eight percentile values at all sites were still well below the WHO 24hour guideline of 150 micro g m-3. However, the increasing trend in NO2 concentration gives cause for concern because of the forecast increase in motor vehicle numbers and NOx emissions (NEERI, 1983, 1988, 1990, 1991 b).

A survey of busy traffic junctions revealed eight-hourly concentrations of NOx in excess of 500 micro g m-3. lt is not clear whether measurements represent nitric oxide (NO) or NO2 concentrations or both. Even if these results are for NO it can be assumed that, given Delhi's meteorology, NO2 levels will also be high (Mathur, 1988). Such concentrations cannot be considered representative of Delhi as a whole. However, they are important when considering personal exposure of individuals at these locations (i.e., drivers, traffic police, street vendors, etc.).

Levels of NO2 peak in November following the monsoon. Maximal insolation occurs in October (288 hours) and November (285 hours) immediately after the monsoon. Levels remain relatively constant for the rest of the year. The lowest concentrations occur during the monsoons when precipitation is at a maximum and insolation is at a minimum.


Ambient concentrations Ozone is not routinely monitored by NEERI or by CPCB in Delhi. No reference can be found with regard to any long-term monitoring of O3 in Delhi. Several reports refer to an increasing incidence of smog in New Delhi over the years (CSE, 1985). However, it is not clear whether these are referring to photochemical smog or simply a reduction in visibility brought about by the increase in SPM and other pollutants. As with other Indian cities, Delhi's climate is favourable in terms of ozone formation, especially during the winter. Oxides of nitrogen and hydrocarbon emissions are forecast to increase dramatically by the year 2000 and it is possible that O3 will become a serious air quality problem over the next ten years. Continuous long-term monitoring of O3 should be regarded as a major priority in Delhi in order to identify if and when O3 levels pose a significant risk to health in the city.


Air Pollution Situation The trend in air pollution in Delhi is upward, through increasing urbanisation and associated motorization and industrialisation.

The suspension and resuspension of natural dust is one of the main air quality problems in Delhi. Reducing exposure, for example through education, would probably be the most cost-effective way of dealing with this problem. Continuing urbanisation and increases in motor vehicle traffic mean that suspended dust is likely to increase even if steps are taken to reduce manmade SPM emissions.

Although Indian crude oil and coal are low in sulphur, SO2 levels repeatedly exceed WHO guidelines in Delhi. The increase in motor vehicle SO2 and SPM emissions is particularly disturbing because of the potential numbers of people exposed to this source. Some form of emission test for both petrol and diesel-driven vehicles should be introduced. Delhi still has a relatively small motor vehicle population, especially cars, when compared with cities in high income countries. This is borne out by NO2 concentrations which are still within accepted guidelines.

The degree of pollution attributed to Delhi's power stations gives cause for concern. It is reported that there is a high incidence of tuberculosis in workers inside the Indrapastha site and the high levels of SPM are believed to be an important cofactor (NEERI, 1991a). This plant is associated with high SPM, SO2 and NOx concentrations not only in the immediate vicinity of the plant but throughout Delhi. Pollution control measures should be introduced to all urban power Generation facilities. Alliteratively these plants should be replaced with modem power stations located away from the city to the south-east.


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