Research Project

Air Quality Impact Assessment by Changeover to CNG Buses in Delhi

Dinesh Mohan, S.R. Kale, and Sanjeev Sanghi

Project Details


CNG buses have been made mandatory in Delhi from 1st April 2001. Due to inability to effect this changeover and various operational difficulties, this deadline has been extended by 6 months to 30th September 2001. The motivation of this changeover is to reduce emission from buses because C.N.G. fuelled buses are expected to be less polluting than diesel fuelled buses, which in turn will improve the ambient air quality. The changeover increases the cost of the bus, a cost that will be eventually borne by the bus users. With about 40% of the chartered bus users and 20% of stage carriage (DTC, blue line, etc.) bus users owning 2-wheelers, this increase in bus fares could make 2-wheelers economical for some of them. Consequently, some of these bus users are likely to give-up bus transport and travel by their personal 2-wheelers or cars, most of which are 2-stroke and 4 stroke petrol engines. As a result, the gains in emission reduction from buses are likely to be offset by increased use of 2-wheelers and some cars. Yet another option would be to convert part or the entire fleet to Euro II standard diesel engines. This change-over, entails lower costs than conversion to CNG and results in some reduction in emissions. In the analysis presented below, change in emissions under these different scenarios has been described. These vehicular emission data are well correlated to road-side ambient air quality. It cannot, however, be correlated with the ambient air quality that is influenced by all other emitting sources, such as, stationary diesel generators, portable generators, factories, wood burning stoves, and kerosene cooking stoves, amongst others. Even when such an exercise is carried out, the impact of changeover to CNG is likely to be overshadowed by emissions from all these other sources. Since data on these sources is unavailable, the impact on ambient air quality because of changeover to CNG has not been estimated quantitatively.


This study can be summarized as under:

Conversion to CNG reduces NOx and PM emissions but increases CO and HC emissions from buses. The total cumulative health effect of increase in CO and HC and decrease in NOx and Suspended Particulate Matter (SPM) has not been evaluated in this study. No decision should be taken without such an evaluation.

There are no major differences in total emission from vehicles based on present traffic mix whether all buses use CNG, all buses use Euro-II engines or the mix is half and half between these two technologies.

As bus passengers shift from bus to 2-wheelers as a result of fare increases, the emission of all pollutants increases. Reductions in vehicular PM emission are realized only if the shift is less than 15%. For even a 5% shift of passengers from buses to 2-wheelers CO emission increases by 10% to 18% and HC emission increases by 15% to25%. These numbers rise to 32% to 53% and 40% to 69% for CO and HC respectively for a 15% shift. Even the PM emission shows an increase of about 1.5% for a 15% shift. The relative decrease in NOx is 22% to 26% for this case. These estimates are conservative as all the shifts have been assumed to be from buses to 2-wheelers and not to cars. Further, the increased congestion due to additional number of vehicles is likely to increase the emission levels of the pollutants.

Two-wheelers and buses are comparable contributors to PM emissions and, hence, any strategy for reducing PM emissions must target both these modes.

The per person per kilometer emission of every pollutant is lowest for bus travel and highest for cars and 2-wheelers, in that order.

Trends between 1994 and this study show that there has been a significant shift to cars and two wheelers. Unless this shift is controlled it would be difficult to control increase in emissions. This can only be done if bus transport is improved and fares not increased. A comprehensive policy regarding this has to be put in place before major technological changes are mandated.

A vast majority of people continue to travel by buses and their interests must be given the highest priority.

This study has not considered the pollution caused by SPM less than PM10 as no data are available regarding the same. It is known that smaller particles go deeper in the lungs and have adverse health effects. It is also known that CNG using engines produce a higher proportion of these particles.

Ambient air quality estimates have not been made in this study as data for other sources is not available. However, since there may be thousands of petrol, kerosene and diesel generators in Delhi, their contribtion to SPM and NOx would be substantial. Therefore, the actual reduction in NOx and SPM levels in ambient air would be much less than that calculated for vehicular emission alone. The limited benefit of complete CNG conversion with no passenger shift to personal modes would be significantly reduced. With shifts to personal vehicles the benefit may be offset at much lower levels than 15% as calculated earlier.


Emission standards should not specify particular fuels or technologies as such measures encourage monopolies and discourage technology innovation and competition in industry. They should only lay down regulations regarding emissions from engines and associated fuel quality.

The results of this study show that the benefits of converting the full bus fleet are not clear as there is not much improvement in overall air quality if the calculations are based on the actual traffic conditions and modal shares present on Delhi roads. The increase in fares as a result of more expensive buses is likely to shift bus passengers to cars and two wheelers and thus increase total pollution. Therefore, the requirement that all buses convert to CNG should be reassessed and put on hold until more definitive data are available.

It is not advisable for a whole bus fleet of a city to be based on the same technology of the same age. This can result in major disasters if something goes wrong. This will also preclude induction of new technologies as they become available in the future. A plan for a phased technology change should be developed and instituted.

The benefits of better technologies will be defeated if bus passengers shift to personal modes of travel due to fare increases. Therefore, the government must put in place a comprehensive policy of financing of public transport including cross subsidies. This could be done by invoking the "polluter pay principle". The data in this study show that cars and two wheelers have the highest pollution per passenger transported. Therefore, owners of cars and two-wheelers must be made to pay a pollution tax, the proceeds from which could be used for financing more efficient bus transport.

The problem of shift to 2-wheelers and cars from public transport has to be addressed irrespective of the fuel used by buses. Therefore, public transport has to be made more convenient, safe and efficient. The safety and efficiency of bus transport, and its attractiveness for users could be increased substantially if modern low floor buses are inducted in the Delhi fleet. This cannot be done if gas cylinders are fitted below the bus floor (present technology). Therefore, all future CNG buses should be required to have gas cylinders integrated in the roof of the bus.

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