Could CNG be a clean solution for megacities with mixed-traffic conditions?

Vehicular pollution is considered to be a major source of air pollution in mega-cities like Delhi. A continuous increase in the population of Delhi from ∼13 to ∼17 million during the first decade of 21st century has led to an explosion in the vehicular density in the city, which increased from ∼3.6 million to ∼7 million. Exhaust from vehicles is a complex mixture of gases and particulates that significantly influence the health of residents. To improve urban air quality, entire public transportation system involving taxis, three-wheelers and buses has been switched from conventional liquid fuels to compressed natural gas (CNG). Many researchers have speculated that while CNG looks cleaner, it would cause far more severe health effects because of large number of finer nano-particles it emits. Unfortunately, there is lack of awareness in the society/policy makers as to which fuel and engine types offer the lowest risk to the public health.

Prof. Avinash Kumar Agarwal, Prof. Tarun Gupta and Prof. Bushra Ateeq led a very large inter-disciplinary team to resolve this complicated issue. The researchers at the Engine Research Laboratory of IIT Kanpur have done a study to figure out the toxicity of particulates emitted by engines using different emission compliance, different fuels such as gasoline, diesel, CNG etc. with an objective to realistically find the toxicity and mutagenicity of these emissions in the Indian context. Human cell lines were exposed to the particulates emanating from different engines. The three year-long study is first such study from India and it conclusively proves that CNG is indeed the cleanest fuel among all fuels used in India. This study is published in one of the top environmental engineering journals Environmental Pollution.

The fleet considered for this study was similar to the current fleet running on Delhi roads. Different engines (BS-II, BS-III and BS-IV) using different fuels (gasoline, diesel and CNG) were tested and tail-pipe emissions especially the particulate matter was collected and subjected to detail physical, morphological, chemical, toxicological analysis using a suit of advance instruments. The results revealed CNG as a significantly cleaner fuel compared to diesel and gasoline. Diesel engines through their tailpipes emitted about 30-50 times higher number of fine particles than CNG, which could penetrate to the deepest parts of human lungs. Higher surface area of smaller particles provided more active sites for surface adsorption of volatile species and PAHs, which increased the diesel particulate toxicity. Particulates emitted by CNG driven engine exhaust had significantly lower surface area compared to diesel and gasoline exhaust particulates, thus lower toxicity and mutagenicity. In addition, CNG engine exhaust particulates adsorbed lower number of PAHs (carcinogenic organics present in engine exhaust emissions) compared to the other fuels. The amount of PAHs reduced as we moved from BS-II to BS-IV engines, even with diesel fuel. However, CNG and gasoline exhaust had nearly 2-3 times higher amounts of formaldehyde (which can cause skin and eye irritation) compared to other test fuels. Diesel engines emitted significantly higher levels of toxic transition trace metals than CNG. Ames test for in-vitro mutagenic toxicity was performed in Salmonella strains TA98 and TA100, with and without metabolic activation by rat liver enzyme (S9). Known mutagenic agents were used as positive controls. The particulates from BS-IV CRDI diesel showed much lower mutagenicity relative to BS-II IDI diesel engine.

Overall, diesel and gasoline exhaust particulates showed higher mutagenicity than CNG exhaust particulates. Hopefully, this study will help the government of India to take correct decisions at choosing the right fuels for mega cities and clean cities of the country, which without any doubt is CNG.

Qualitative analysis of particulates emitted from different engines having different emission compliances.


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