Monitoring

Calcutta Metropolitan District map shows the location of the three GEMS/Air monitoring sites operated by the National Environmental Engineering Research Institute (NEERI). Monitoring of suspended particulate matter (SPM) dates back to 1972 at the Dalhousie (Commercial) and Cossipore (Industrial) stations and to 1973 at the Bhowanipore (Residential) site. Monitoring of gaseous pollutants sulphur dioxide (SO₂) and nitrogen dioxide (NO₂) began in 1978 at all three sites. Monitoring was discontinued at the NEERI sites in 1988. Monitoring has since been carried out by the Central Pollution Control Board (CPCB) - West Bengal. However, the results are not comparable with the NEERI data and no indication of methodology, site location or sampling frequency has been provided and therefore these data are not presented here. NEERI recommenced monitoring at the three GEMS sites in April 1990 and these data have been included.

Sulphur dioxide

Emissions:
Emissions estimates calculated by NEERI indicate that industry and power generation are the main sources of SO₂ in Calcutta. Figure 2 shows that the effects of significant reductions in domestic emissions brought about by a decline in domestic and commercial coal use have, to a large extent, been cancelled by increasing industrial and transport emissions. This trend is projected to continue at least until the year 2000. Overall, it is estimated that SO₂ emissions have remained stable since 1980 at approximately 25,000 tonnes per annum and will remain constant until 2000.

A breakdown of industrial SO₂ emissions conducted by NEERI and CMDA in 1977-78 ( Table 2) reveals that thermal power plants accounted for 34 percent. The chemical and engineering industries are responsible for 11 percent each. Table 1 ranks industries (excluding power generation) in terms of coal consumption. In general, the engineering industries consume large quantities of coal and this is reflected in SO₂ emissions. The chemical industry ranks sixth in terms of coal consumption, suggesting that SO₂ emissions may be attributable to actual industrial processes.

The increase in Transport SO₂ emissions is attributed to the increase in diesel-driven vehicles. The bus and truck population of Calcutta and Howrah grew by 78 percent between 1980 and 1989.

Ambient Concentrations:
Figure 3 shows annual mean SO₂ concentrations at the three GEMS/NEERI monitoring sites. The graph shows that concentrations approximately doubled at all sites between 1980 and 1981; a maximum annual mean concentration of 104 micro g m^-3 was observed at the Dalhousie commercial station. It is important to note that this apparent doubling is partly due to missing data for the monsoon months (when concentrations are at a minimum); therefore, the annual values for 1981 cannot be considered representative. However, it is also interesting to note that concentrations in the following year, 1982, were of a similar order when sampling remained consistent throughout the year. Annual 98 percentile concentrations were higher in 1982 than in 1981. Following 1981 and 1982, annual levels fell steadily and by 1985 annual average concentrations at all sites were within or below the WHO guideline range. Concentrations at the Bhowanipore residential station never exceeded the upper limit of the WHO annual guideline range (60 micro g m^-3) between 1978 and 1987. Monitoring recommenced at the GEMS/NEERI sites in April 1990. The annual arithmetic mean concentrations in 1990 (April to December) were still within the WHO guideline range (NEERI, 1991b).

Ambient SO₂ concentrations peak during the winter (November to February inclusive) with the monthly maximum occurring in November (Figure 4). Climatic factors, such as the high percentage of calms and ground-based temperature inversions, are of great importance during the winter. In winter diurnal concentrations are generally higher at night between 2000 hours and 0400 hours due to temperature inversions. Minimum SO₂ concentrations occur in May before the onset of the monsoon season (June to October inclusive). Levels remain relatively low throughout the monsoon.

Suspended particulate matter

Emissions:
Calcutta has a very severe SPM problem. Estimated anthropogenic SPM emissions were approximately 200,000 tonnes per annum in 1990, a similar value to 1980, and it is projected that emissions will remain fairly stable until 2000 (Figure 2). Industrial sources account for 98 percent of the 1990 total. The high emissions and ambient concentrations of SPM (Figure 5) result from the high level of coal use by Calcutta's industry, particularly at thermal power plants. Table 2 shows that in 1977-78 Calcutta's two thermal power plants accounted for 44 percent of industrial SPM emissions, followed by the chemical industries which emitted a further 15 percent. The coal burned in the industrial boilers has a relatively low sulphur content (0.3 percent) but is high in ash content (24-36 percent). In 1989-90, 1,451 out of 2,218 registered factories burnt approximately one million tonnes of coal. The influence of the natural dust component of the particulate fraction is not clear as no analysis has been presented.

Ambient Concentrations:
Historical emissions estimates indicate a 66 percent increase in SPM emissions between 1970 and 1980. However, monitoring data do not support these estimates, as can be seen in Figure 5. Regression of the annual mean SPM concentration between 1972 and 1980 shows no significant overall trend during the 1970s. In fact, a negative trend is observed at the industrial monitoring site at Cossipore for the period 1972-1980. Between 1972 and 1987 all sites exhibited a positive annual mean trend (although not statistically significant). lt is possible that SPM from construction activities and the entertainment of street dust has declined whereas industrial emissions have increased (Aggarwal, 1991).

Annual mean and 98 percentile concentrations (Figure 5 and 6) at all stations greatly
exceed both WHO guidelines and Indian Air Quality Standards. The overall average concentration in 1987 was 557 micro g m^-3, over six times the maximum WHO annual guideline (60-90 micro g m^-3). The annual 98 percentile concentration of the Cossipore industrial monitoring site reached 1,680 micro g m^-3 in 1987, 14 times the WHO daily guideline and the second highest ever in Calcutta, indicating that episodes of short duration also constitute a problem. There is no significant difference in concentrations between the various sites indicating that high concentrations occur throughout the city. Annual arithmetic mean concentrations in 1990 (April to December) were lower than in 1987 at all three sites (268-453 micro g m^-3), but were still well above the WHO annual guidelines. Annual 98 percentile concentrations are also extremely high (1,014-1,145 micro g m^-3)

The contribution of natural dust to overall SPM concentrations is not obvious. However, it is likely to be lower than for other 'drier' Indian cities such as Delhi. As Figure 4 shows, the monsoon ( June to October inclusive) has a pronounced washout effect; concentrations during this period are half those of the winter. Concentrations reach a peak in December and are likely to be influenced by temperature inversions and low wind speeds.

Lead

Ambient Concentrations:
The lead content of petrol from the Halidia refinery, which supplies Calcutta, is lower than that in Delhi or Bombay at 0.1 g l^-1. Despite the relatively low lead content of petrol, annual airborne lead levels, monitored at the three GEMS/NEERI sites, are the highest in India (NEERI, 1991c). Annual concentrations were found to be highest at the residential and commercial sites (0.73 micro g m^-3, but were below the WHO annual guideline of 1 micro g m^-3.

Carbon monoxide

Emissions:
Figure 2 shows estimated and projected carbon monoxide (CO) emissions in Calcutta between 1970 and 2000. lt is estimated that in 1990, CO emissions totalled approximately 177,000 tonnes per annum. Transport was the greatest source of CO, accounting for 48 percent of the total, followed by industry at 34 percent and the remainder classified as domestic emissions - 18 percent. Figure 2 shows the decline of domestic emissions due to changes in fuel use while at the same time motor vehicle emissions have increased by over ten times between 1970 and 1990. Industrial emissions increased from 33,000 tonnes per annum in 1970 to 60,000 tonnes per annum in 1980, but have remained relatively stable since then and are projected to remain at present levels to 2000.

Ambient Concentrations:
lt is not possible to comment on the reliability of emissions estimates as there is no monitoring of ambient CO in the city. lt has also not been possible to locate any recent studies referring to CO in Calcutta. lt is understood that a detailed CO modelling study is under way in Calcutta (Aggarwal, 1991). Emissions estimates are on a par with those of Bombay, which experiences similar meteorology. However, the contributions of the various sources are very different (industrial and domestic sources are much more important in Calcutta) and it is likely that street-level exposure is low in Calcutta due to lower motor vehicle numbers and emissions and to a relatively 'open' urban topography.

Oxides of nitrogen

Emissions:
Figure 2 shows the estimated and projected increase in oxides of nitrogen (NO_X as NO₂) between 1970 and 2000. Transport is now the dominant source of NO_X in Calcutta through the growth in motor vehicle traffic in recent years. lt is estimated that in 1970 industry was the major source of NO_X (69 percent) and that emissions from industrial plant had increased to over 11,000 tonnes per annum by 1980. Estimated industrial emissions have since stabilized and are not projected to increase significantly before 2000. Transport emissions have risen from an estimated 1,825 tonnes per annum in 1970 to 25,550 tonnes per annum in 1990. The main vehicular sources of NO_X are diesel-driven trucks and buses. Although diesel-driven vehicles only account for approximately 10 percent of Calcutta's motor vehicle population, it is estimated that they are responsible for approximately 90 percent of motor vehicle NO_X emissions.

Ambient Concentrations:
Monitoring Of NO₂ at the three GEMS/NEERI sites since 1978 has revealed a significant positive trend in annual mean concentrations. Figure 7 shows that ambient annual 98 percentile concentrations at all three sites peaked in 1985. In 1986 and 1987 ambient levels decreased significantly, but generally were still above accepted guidelines. Data from 1990 (April to December) suggest that concentrations have fallen further and are now well below accepted guidelines (NEERI, 1991b). No explanation can be given for the decrease in urban concentrations; emissions are believed to be increasing because of increasing traffic. lt is possible that meteorological factors such as insolation, the frequency of calms and the frequency of ground-based temperature inversions have influenced annual statistics throughout the 1980s. lt is also possible that improved traffic circulation in recent years has helped to reduce NO₂ concentrations in the short term. Maximum concentrations are generally recorded at the Dalhousie commercial site, probably owing to the high traffic densities and congestion in this area.

Figure 4 indicates that there is no clear seasonal influence upon monthly NO₂ concentrations. The monthly peak in December is influenced by high insolation and ground-based temperature inversions.

Ozone

Ambient Concentrations:
Ozone (O₃) is not currently monitored in Calcutta on a regular basis. Monitoring of tropospheric O₃ should be initiated in all major Indian cities to identify whether photochemical smog constitutes a problem. Given the recent rapid increase, and the projected future increase, in precursor emissions in these cities it is likely that O₃ and other photochemical oxidants will increase in importance. Monitoring is needed to quantify the scale of the problem so that remedial measures can be identified.

Conclusions

It is estimated by NEERI that 60 percent of Calcutta's residents suffer from some kind of respiratory disease due to air pollution. The burning of coal as an industrial and domestic fuel accounts for a significant proportion of pollutant emissions, especially SPM. Suspended particulate matter from coal combustion is clearly a major problem throughout Calcutta and should be the main focus of immediate control efforts. Surprisingly, SO₂, concentrations are relatively low (within WHO guidelines) which is due to the low sulphur content (0.3 percent) of the local coal.

lt would appear from the data that industrial emissions have, to a large extent, stabilized and in some cases declined. lt is not clear what the reasons for these changes are, but it is likely that planning measures restricting industrial development have played an important role. Changes in domestic and commercial fuel use, principally a reduction in coal use, and improvements in burning efficiency will have also helped to reduce emissions.

Carbon monoxide and NO_X, emissions from motor vehicles are of increasing concern and probably present the greatest long-term threat to Calcutta's air quality. Calcutta's motor vehicle population doubles every six years, a trend which is likely to continue at least up to 2000. With this rate of growth it is unlikely that even the introduction of the most stringent control measures would reduce overall emissions and ambient concentrations from this source.

The data presented here give only a very limited picture of the air quality situation in Calcutta. A survey of air pollution levels and emissions throughout the Calcutta metropolitan district is required for proper air quality management.

References

Aggarwal, A. L. 1991 Personal Communication, NEERI, Nagpur.

Faiz, A., Sinha, K., Walsh, M. and Varma, A. 1990 Automotive Air Pollution: Issues and Options for Developing Countries, World Bank Policy and Research

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NEERI 1988 Air quality status in ten cities: India 1982-1985, National Enviromnental Engineering Research Institute, Nagpur.

NEERI 1990 Air quality status in ten cities: [ndia 1986-1987, National Environmental Engineering Research Institute, Nagpur. (Unpublished report.)

NEERI 1991a Air pollution aspects of three Indian megacities, Volume III: Calcutta, National Environmental Engineering Research Institute,Nagpur.

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