small in size, big problem
Nanoparticles, as tiny as a billionth of a metre, pose a challenge to scientists and regulatory authorities alike. It may also be a huge health problem, says CHANDRACHUR GHOSE
The Western world has come a long way in its drive to reduce air pollution from vehicles. Just over a decade back, the focus was on total suspended particulate matter (which includes natural dust too) and the black smoke vehicles emitted. Epidemiological evidence made governments shift the focus to respirable particles (particles smaller than 10 micron). With increasing evidence that vehicular emissions were a major health menace, these countries scaled up vehicle emission standards: Euro norms in the European Union and Tier I norms in the US. These norms reduced emissions from vehicles, but regulation kept up the demand for more stringency in the future. The automotive industry had no option but to find technological solutions -- which have moved from the engine to the tailpipe -- to make cleaner cars. The fuel industry, too, had to produce cleaner fuels, cleaning up other toxic components at the same time. (Thanks to the regulations, the task is now to go for almost sulphur-less fuel.) Yet, large regions in the US and in Europe still exceed the air quality standards for respirable particles. Responding to the scientific evidence that smaller particles do more harm, the US has also established air quality standard for smaller PM2.5. The problem, however, is that evolving science now shows that even these particles aren't the predominant kind of particles that actually swamp the air and are emitted in greater numbers by technologically sophisticated vehicles.
Paradoxically, while vehicular emission norms have taken care of reduction in mass of particles, many scientists point out that the number of particles emitted by vehicles has gone up. Measured in the scale of nanometre (a billionth of a metre), these particles are called nanoparticles and almost defy geometrical definition. The nanoparticles comprise only 1-20 per cent of the total particulate mass emitted from a diesel vehicle, but may constitute more than 90 per of the total number of the emitted particles. What is worrying the scientific and regulatory community is that it is still not easy to measure these extremely tiny particles, with diameter less than 50 nanometre (nm), let alone control their emissions.
Diesel engines have always been highest emitters of toxic particles. However, results of studies across the world on the impact of cleaner fuels and engines on nanoparticle emissions are showing mixed results. Scientists are still not clear on the role these play in nanoparticle emissions. Mayer points out, " Conventional gasoline engines emit much less nanoparticles than diesel engines when running at part load. But at full load they can emit as much or more -- particularly the older ones. Direct injection gasoline engines are as bad as diesel engines except in their homogenous operation range." Even Compressed Natural Gas (CNG) engines have been found to emit nanoparticles." Lennart, however throws a spanner of caution on CNG. "Most of the particle emissions from CNG engines are results of bad maintenance of the engine and bad lubrication oil," he says. According to K Boulouchos of the Swiss Federal Institute of Technology, Zurich, "optimised premixed combustion, particularly with CNG as fuel helps achieve particle number concentrations in the engine exhaust comparable to laboratory air quality."
Maintenance of the vehicle also has a role to play. Points out Wexler, "Generally, diesel engine is worse than a gasoline engine, but it depends much on how well the vehicle is maintained."
"Available evidence indicates that diesel engines are a far greater source of nanoparticle emissions than gasoline engines. Gasoline engines can emit large quantities when running rich under load. When running close to stoichiometric as they do most of the time, emissions from gasoline engines are very modest," says Roy Harrison, Professor at the University of Birmingham, UK.
When the vehicle particulate emissions club (VPEC) -- a partnership between the UK government and the automotive industry -- studied on-road emissions from petrol and diesel cars, it found that though new petrol cars with three-way catalytic converters have low particle emissions, old petrol cars have high nanoparticle emissions and the emission rate increased with increase in speed. The study found that diesel cars with indirect injection had very high emissions of both ultrafine and nanoparticles, while those with direct injection were found to be high emitters of ultrafine particles. However, ultrafine and nanoparticle emissions from diesel cars were found to be much higher than petrol cars.
In yet another study, the Switzerland-based University of Applied Sciences tested two-stroke and four-stroke two-wheelers for particulate emissions and found that two-stroke two-wheelers emit much more nanoparticles than the four-stroke ones. While the four-stroke engines emitted more solid particles, the two-stroke ones emitted mostly soluble particles. Even the use of a catalytic converter did not improve matters. The converter lowered the number of nanoparticles emitted from the four-stroke two-wheeler, but increased emission of solid particles slightly. Besides, it increased nanoparticle emissions in the two-stroke two-wheeler to as high as in a diesel engine, with particle size smaller than typical diesel particles (50-60 nm as compared to 90-100 nm of diesel particles).
When researchers from the Queensland University of Technology tested particulate emissions from vehicles run on leaded and unleaded petrol (ULP), CNG and Liquefied Petroleum Gas (LPG) they found that converting a ULP vehicle, which are already low particulate emitters, to CNG produced no significant change in the emission of nanoparticles. However, they found a significant reduction in polycyclic aromatic hydrocarbons (PAHs) and formaldehyde emissions when the vehicle was operated on CNG, along with a reduction in its global warming potential. Similar results were found in case of dedicated LPG vehicles.
A test conducted by the California Air Resources Board showed surprising results: It predictably revealed that average idle mode ultrafine total particle emissions from a CNG bus were 40-180 times lower than a diesel bus operating on ultra-low sulphur fuel. But it showed that at 55 miles per hour (mph) steady state driving, the CNG vehicle particle number emissions were 20 times higher. This did not change even when an oxidation catalyst was added to the CNG vehicle. The researchers however point out that they have not been able to identify the source of these particles. They speculate that lube oil composition and its consumption had a role to play in this.
According to researchers, similar emission levels of nanoparticles seen for same type of engine running on different fuels (ULP, CNG and LPG) indicate that the fuel type does not play a crucial role in nanoparticle formation, as long as the engine is properly tuned. They opine other factors like vehicle management system and the consumption of lubricating oil could play major roles in the particulate formation mechanisms. Thus the majority of the formed particles are either volatiles formed during the dilution process or solid ash particles from the combustion of lubricating oils, they say.
According to Mridul Gautam of West Virginia University, USA, reducing fuel sulphur levels alone may not be an answer to reducing nanoparticle concentrations in diesel exhaust emissions. Lube oil sulphur levels and additive packages too greatly influence size distributions and concentrations of particulate emissions, he says. He also points out that nanoparticles are generated as a result of highly complex interactions between fuel and lube oil hydrocarbon chemistry and sulphur content, engine operating modes, the exhaust dilution system itself (in addition to dilution conditions).
Recent scientific studies have thrown up a highly interesting and relevant debate regarding monitoring of ambient particles. It is being argued that measurement of particulates in terms of mass only does not bring out the risk from exposure to the number of extremely tiny particles. Therefore what is more important is to monitor the number of particles in different size categories. A recent joint report by Lidia Morawska of the Centre for Medical and Health Physics at Queensland University of Technology, and Dietrich Schwela of World Health Organisation says that most of the mass of particles in the ambient air is due to the presence of a very small number of large particles. But, when considered in terms of number, ultrafine particles constitute over 80 per cent of particulate matter, although their total mass is usually insignificant in comparison with the mass of few large particles. The report argues that the highest level of concentration of toxic compounds is related to these ultrafine particles, and therefore throws up the question that whether physical, chemical and biological characteristics should also be measured in addition to numbers.
In a study conducted to find out the particle number concentration in urban environment, researchers from the National Environment Research Institute, Denmark measured particles with a range of diameter from 10nm to 700nm. They found that the average traffic particle emissions between 6 am and 6 pm showed a size distribution with the maximum around 25-30nm. According to the researchers, this indicates that petrol and diesel vehicles emit particles in a similar size range. However, they found that from 12 pm to 5 am, when the dominating traffic was that of diesel taxis equipped with an oxidising catalytic converter, the maximum in the emitted particle size distribution was shifted to smaller sizes of about 15-18nm.
While scientists are trying to grapple the import of the new findings, results of another study by the University of Birmingham in 1999 implicate road transport for emitting such ultrafine particle emissions. The study shows that at busy roadside, more than 44 per cent of the measured particles are below 10nm, while the corresponding figure at an urban background can be 30 per cent. This has led researchers to demand that number of particles in the ambient air be used as a yardstick to evaluate air quality. At present, air quality standards of all countries in the world measure the concentration of particles only in terms of mass.
In sum, the regulatory mechanism is still not prepared to build on the emerging science of nanoparticle emissions. Even as it continues to set more and more stringent norms -- both for emissions and ambient air quality -- it now faces an unexpected challenge. Once more the emergent knowledge on nanoparticles, still somewhat nebulous but decidedly indicative, demands the scientific community and the regulatory authorities to put their heads and capacities together.
Of primary concern is the fact that finer particles penetrate more readily into cells and through tissue barriers, that they have greater surface area per unit mass and a large number of toxic reactions occur at the surface. Years of research have validated the adverse effects of fine and ultrafine particles. Yet not much definitive information is available on how nanoparticles affect human health. Greenbaum points out, "To date, there is evidence that nanoparticles can cause adverse reactions in animals and humans, but epidemiology studies have not yet determined whether those reactions are worse than those caused by other parts of the particle mixture." Wexler, however, is of the opinion that conclusions about adverse health effects of nanoparticles is more of a logical than direct: the minuter the particle, the greater its propensity to adversely affect health. Hence PM1 would shows a better correlation with adverse health than PM2.5, which in turn shows a better correlation than PM10. "As you move towards smaller particles, their number is substantial," he says. Therefore, they can be critical to public health, he says.
However, a recent study presents scary results. Researchers from the US-based West Virginia University found that though the use of synthetic diesel decreased PM mass emission rates, it increased the number of particles deposited in the alveolar region. Moreover, even as the use of oxidation catalytic converter and catalysed particulate trap decreased particle mass emissions and subsequently reduced mass deposition deep inside the lungs, the number of particles deposited in the alveoli -- the lung's breathting sacs -- increased. Their natural suggestion was to do count-based analysis of PM emissions in addition to mass-based emissions.
Though earlier studies have shown that macrophages -- cells that police the lungs, eating up foriegn particles -- ingest the inhaled particles, new studies show that the smallest particles partially escape detection by the much larger macrophages and are not ingested. According to L Hofer, a Switzerland-based occupational health specialist, respiratory pulsation, by which the alveoli dilate and contract, expels some of the particles into the bronchiole, possibly after prolonged retention in the alveoli. He points out that these particles could also penetrate, through the narrow interalveolar pores, into the pulmonary interstices (the interstitium). There the lymphatic fluids can capture and transport the particles to the regional lymph nodes. As these particles do not decompose easily, they are retained for several weeks, months or years in the interstitium and in the lymph nodes. Such particle accumulations are mutational and tumor inducing, he says. The particles are also partially dissolved and transported to various organs. Prolonged high concentrations of particles in the respiratory air cause alveolar overload, that is, the particle invasion overwhelms the scavenging mechanisms of the body. The decomposing macrophages then release inflammatory substances that can damage the alveolar walls.
North Gujarat is notorious for its heavy dependence on groundwater. Water tables have dropped to such levels, that in places what is extracted is fossilised water. Flourosis affects an increasing number of villages in the area. In spite of this, the recent move to use Narmada water to recharge the groundwater is fraught with the suspicion that since infrastructure in the CA is inadequate, the authorities don't know what to do with all the water they have. M S Patel, Secretary of the Narmada Water Resources and Water Supply Department (NWRWSD) has a different explanation. "It is an investment for the future," he says. "Since the network will not be ready for another 5-10 years, we can use this water to fill waterbodies and recharge aquifers to get a jump start, where groundwater levels can be brought up," he adds.
M K Makwana, of the Drainage Circle of the department, spoke of a Rs 2,500 crore plan for pumping 1 MAF of water up to North Gujarat. The region is not a part of the CA. Currently, a Rs 178 crore scheme is in progress -- a 100 km long pipeline from the main canal will lift water to Dharoi dam. Three pumping stations along the way with a lake at each site will lift the water up the 100-metre gradient. Water will be used to fill tanks, ponds and recharge tubewells along the way.
North Gujarat uses 42 per cent of Gujarat's electricity, mostly at subsidised rates. The idea behind this is that the expenditure on electricity to pump water from the tubewells will be greatly reduced. Since water is to be pumped to a greater height, Makwana believes that it will also flow down, filling ponds and other waterbodies on the way, which are naturally connected through local canals, the drainage system and catchment areas of streams etc. "Over 12,500 tanks (ranging from village ponds to minor irrigation schemes) have been desilted and about 1000 new tanks constructed," he says. Twelve or thirteen such pipelines have been proposed to lift water towards the 9 dams of the area -- not to fill them but to use the existing canal network to distribute the water. This water is incidentally only to be used for recharge, and not irrigation or other purposes.
Dinesh Kumar of IWMI expressed grave reservations about the plan. "Government work is all about civil engineering -- hydraulics is not taken into account," he says. At a technical level there are issues regarding how successfully recharge will actually happen. " It will only create recharge at the shallow aquifers, they need recharge tubewells sunk into the ponds to take water down to lower levels," is his contention.
Makwana reckoned that the cost of lifting water "would still be cheaper than the money the government spends on subsidising electricity for pumping groundwater in the region". There is as yet no tariff system in the government's plans for this water. And, for all the talk of people's participation, there is no clear thinking on how the recharge ponds would be maintained or desilted in the future. Who would manage these local water bodies, who would play a role in conflict resolution? Who would ensure that the water brought in would not beget further extraction, just when awareness about the gravity of the situation was spreading in North Gujarat? These questions only elicit silence from the authorities.
Shah points that the behaviour and practices that took root during the initial years of the project will not only be difficult to change, they will also influence the practices in the coming years. It is for this that the utmost care should be taken in the implementation of the project. The project has aroused enormous expectations, gobbled much public money -- now the politicians have taken over and they are well on their way to dash people's hopes.
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