Science vs economics of cess on diesel

Published: Saturday 30 June 2007

Science vs economics of cess on diesel

-- (Credit: SHYAMAL)Anumita Roychowdhury's story on fuel efficiency ('The great guzzle's puzzle', Down To Earth, February 15, 2007) had Sacchidananda Mukherjee, Madras School of Economics, arguing against tax brakes on diesel cars. The author replied.

Sacchidananda mukherjee The Lovelace Respiratory Research Institute, Albuquerque, New Mexico, usa, has taken up relative toxicity studies of exhaust emissions from petrol and diesel cars. The results indicate petrol and diesel emissions demonstrate varying levels of potency for different levels of toxicity. For a unit of mass emitted by new-generation, normal-emitter petrol and diesel vehicles, the toxicity of particulate matter (pm) and semi-volatile organic fractions (svoc) of petrol and diesel vehicle exhaust emissions was similar.The toxicity of pm+svoc emissions from both petrol and diesel high-emitters is greater than the toxicity of emissions from normal-emitters. It is difficult to conclude that diesel emissions are more toxic than petrol emissions or vice versa.

Anumita roychowdhury The established facts about diesel toxicity and evidence of the acute cancer-causing potential of diesel pollutants are not in the realm of conjecture any more. International and national health agencies have reviewed relevant data on diesel exhaust and classified either the exhaust mixture or the particulate component as probable human carcinogen. Diesel exhaust contains 40 different toxic compounds that cause cancer, reproductive abnormalities and other toxic impacts. While direct emissions of particulates and nox from diesel vehicles are very high, they also contribute significantly towards the build-up of secondary particulates and ozone. More than 90 per cent of diesel pm is in the range of ultrafines that go very deep into our lungs. California has a special diesel toxic risk reduction programme. It is time we take cognisance of this science and act on it.

sm It is being said that according to the current standards diesel cars emit more pm and nox (gm/km), compared to petrol cars in the same segment. On the other hand, petrol cars emit more co, and poly-cyclic aromatic hydrocarbons (pahs) and benzene, compared to diesel cars in the same segment. In gm/km, standard overall emission (pollution) load from diesel cars is lower compared to petrol cars. Now the question is whether pm and nox are more toxic as compared to co, pahs and benzene.

ar It is wrong to simplify the issue by saying that diesel vehicles emit more pm and nox and petrol vehicles emit more co, pahc and benzene, and therefore we do not need any special action on diesel. Other governments are framing fuel-neutral emissions regulations to address this issue. The us emissions standards, for instance, do not differentiate between petrol and diesel vehicles any more. But the outdated European emissions regulations that we follow allow diesel cars to emit more pm and nox, and petrol cars are allowed to emit more co and pahc. But this is a flaw in our standards that should not be used to legitimise the position you hold. Down to Earth

We need additional strategies to counter the effects of the emissions standards that allow diesel vehicles to emit more pm and nox compared to petrol cars. In fact, emissions levels in petrol vehicles have improved more significantly over time. In Delhi, for instance, the ambient co levels (that come predominantly from petrol vehicles) have declined despite high rate of motorisation. Comparatively, the emissions from diesel vehicles have been slower to improve especially as simultaneous lowering of pm and nox presents an engineering challenge. This also explains why diesel cars are finding it difficult to penetrate the us market where both pm and nox limits are the tightest.

Besides, it is unscientific to treat different generations of diesel and petrol technologies so generically while comparing, and confuse the risks that different generations of diesel technologies pose as opposed to their petrol counterparts. You are counterposing conventional and advanced technologies without noting the varying levels of toxicity of different generations of technologies.

Toxicity of both diesel and petrol emissions have improved since the Euro i levels but at each stage diesel emissions remain several times more toxic than the petrol counterparts (see graph Toxic profile). Only when new diesel vehicles are fitted with advanced emissions control technologies--particulate traps--and run on fuel with less than 10 ppm sulphur fuels, do toxicity levels of the emissions drop and come close to petrol emissions levels. This should guide our regulations.

sm Researchers have shown that petrol cars also emit pm, mostly in the nuclei mode range (with aerodynamic diameter of 3 to 30 nanometre). The nuclei mode contains 1 per cent of the particle mass, but often more than 90 per cent of the particle number. For petrol particulates, the ratio of concentration of particles in the nuclei mode to that in the accumulation mode (size range of roughly 30-500 nanometre) is higher than for diesel cars. Though mass of petrol-particulate emission from petrol-fuelled cars is considerably lower, compared to diesel-fuelled ones, both have similar size and number distribution of pm. It was assumed that health damage associated with pm is related to the mass of particulates only ('Regulation faces a hurdle', Down To Earth, September 15, 2003 and 'Micro is big. Talk nano', Down To Earth, February 15, 2000). However, toxicity of pm is related to the size, number, surface-area as well as particle bound chemicals and their composition (and corresponding toxicity). Ultrafine and nanoparticles are concerns for human health. Therefore the mass-based emission standard for particulate matter is not the proper measure for potential health hazards. New particle emission standards should be developed on the basis of number, size, and surface area of the particles. If we use these criteria, exhaust emissions from diesel cars may not be more toxic than emissions from petrol cars.

ar We disagree with your misplaced use of the science of nanoparticles to undermine the initiative to clean up the poor diesel technology that is swamping Indian cities today. Nanos--one billionth of a metre that defies measurement--is a challenge of a different paradigm of technologies that we have not even begun to see in India. You are confusing what level of vehicle technologies warrant particle count-based regulations as opposed to mass-based regulations.Research also shows when diesel engines are poor and the exhaust is full of carbon particles, they act like sponges and soak up vocs and sulphur compounds and prevent them from forming particles of their own. But when engines improve and quantity of carbon particles is reduced, vocs and sulphur compounds form extremely tiny particles of their own, including very harmful sulphate particles. As a result, ultrafines may not decrease with reduction in mass emissions in new generation engines but may actually increase.

But emissions-control technologies for diesel vehicles have evolved globally to address the problem of mass and numbers. Only, we have not developed policies to enable their use. We cannot remain locked in technologies represented by Euro ii, and iii that have higher levels of ultrafine compared to petrol vehicles and harmful sulphate particles (especially if high-sulphur fuel is used). We need to move to technologies that need diesel with 10 ppm sulphur diesel and advanced particulate traps that can almost eliminate mass and number of particles.

SM It may be preferable not to recommend an environmental cess on diesel cars.

AR We need policies to get advanced and clean diesel technologies to reduce the mass of pm to be able to begin counting their numbers. Once we reach that generation of technologies, even India should enforce particle number-based standards. Already the us Tier 2 bin 5 norms and the forthcoming Euro v norms have set an extremely low mass pm limit of 5 to 6 miligramme per kilometre. This is 90 per cent lower than the Euro iii levels of 50 miligramme per kilometre currently in force in a few Indian cities. To further counter the next challenge of ultrafine particles the Euro v norms have proposed pm-count based emissions standards to ensure the use of effective aftertreatment devices that can control extremely low mass and high numbers of particles. Advanced technologies may have the problem of near-invisible particles in future. That does not mean we must refrain from pushing to improve the current level of dirty diesel technology through fiscal instruments.

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