Understanding Smog

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There’s a lot of talk about smog lately, especially with the increased North American and European  focus on environmental air quality. Local governments and commercial fleet operators are looking for ways to reduce smog-generating emissions. We all know smog as that ugly haze that looms over many urban and industrial areas, making it hard to breathe, but what is it? What causes it? How can we help stop it?

Smog is a white or brown haze or fog which envelopes many cities, especially those with heavy vehicular traffic and a large industrial sector. Local weather conditions and topography can also contribute to smog: cities which are ringed by mountains, and which are subject to temperature inversions are likely to see smog more frequently. Examples are many Chinese cities, New Delhi, Denver, Mexico City and Los Angeles. (And, apparently, Paris.)


Probably the primary harmful component of smog is ground-level ozone. Ozone, which is a variant of oxygen gas, became famous about 30 years ago due to the appearance of a “hole in the ozone layer” in the upper atmosphere. At that altitude, ozone is a beneficial because it protects the ground from harmful ultra-violet radiation from the sun. However, ozone at ground level is a different matter, as it’s harmful to health, especially as an irritant to the human respiratory system. Short term exposure can irritate eyes and throat and trigger asthma attacks in sufferers. Long term exposure may cause permanent damage to lung tissue.

Ground-level ozone is generated by the sunlight-powered interaction of two classes of pollutants: nitrogen oxides (NOX) and volatile organic compounds (VOC) such as benzene and its derivatives. NOX is a by-product of the combustion of fossil fuels, with gasoline and diesel engines being the primary source in the United States. Volatile organic compounds come from solvents, consumer products and – you guessed it – petroleum fuels. Motor vehicles are about level with industrial processes as a source of VOCs in the USA. However you look at it, fossil fuel vehicles are a major contributor to ground-level ozone.


Another major harmful component of smog is “particulate matter” (PM). Characterized by particle size in microns, PM10 (10 micron) and PM2.5 (2.5 micron) concentrations are major pollutants. PM2.5’s effects on health are related to the body’s respiratory and cardiovascular systems and have been shown to lead to increased mortality, with hospitalizations and deaths spiking during particularly heavy smog incidences. Long term exposure is also detrimental to health. PM2.5 is typically generated by the burning of coal and in industrial processes such as steel manufacture; but it is also a by-product of automotive diesel use. Diesel vehicles in North America and Europe are equipped with particulate filters which dramatically reduce PM2.5 emissions; but many or most diesel trucks in India and China lack filters, in which case truck-generated PM2.5 becomes a major contributor to smog.EPA PM in smog

Sulfur dioxide, another component of smog, is the result of burning fossil fuels with a high sulfur content. Although a component of smog, sulfur dioxide’s effects can be felt further afield, with results such as acid rain and atmospheric aerosols, which can contribute to climate change. Locally, sulfur dioxide is a harmful irritant to the respiratory system. The adoption of ultra-low sulfur diesel fuel in developed countries has reduced the incidence of SO2 pollution, however it remains severe in China and India where low-grade coal and high sulfur content diesel are burnt.

Thankfully, lead has been removed from gasoline and is no longer a significant automotive pollutant. However, this article makes thought-provoking reading.

Smog’s primary detrimental impact is on human health, though it also contributes to environmental degradation. Respiratory irritation can become serious for people already suffering from respiratory conditions, and long-term exposure can produce permanent damage to lungs, heart and other organs. An ongoing air-pollution study by the University of Southern California correlated exposure to smog to stunted lung development in children, and it found that children growing up in polluted areas are more likely to develop asthma. Not exactly any surprises there, but recent analysis in the study found that children’s lung development improved as air quality in the study area got better in the past 20 years.

Smog is a major problem around the world, but the good news is that it, and the damage it causes to human health are reversible. Increased awareness and a great effort to decrease smog are underway. And what, you may ask, can Lightning Hybrids do to help?smog bus

Internal combustion engines pollute most heavily during acceleration. Not only is this phase inefficient (high engine load, low revs, and low distance traveled), but it is the phase during which diesel engines generate most PM2.5 particles, and NOX gas too. The beauty of Lightning Hybrids’ hydraulic hybrid system is that vehicles accelerate using hydraulic energy stored during the previous braking event, which has no fume exhaust whatsoever. This releases the vehicle’s engine from the task of propelling the vehicle during this most inefficient and dirtiest phase. Indeed, diesel trucks which can be expected to emit a visible cloud of (PM) smoke when accelerating typically do not do this when launched hydraulically. This is even true for the dirtiest, filter-lacking vehicles one finds in, say, Africa.

As you may have read in another recent post (here), Lightning Hybrids has worked with a third party emissions testing facility for accurate characterization of the beneficial effect on emissions of the hydraulic hybrid, and it is typical to see a 50% reduction in NOX emissions when comparing non-converted with converted vehicles. Of course, these (and other) benefits are seen most strongly for trucks and buses which exhibit a heavy stop-start drive cycle.

The recent smog event in Paris showed that even with modern pollution mitigation there is still a long way to go before we have reliably clean air. Until then, people will suffer health effects and compromised lifestyles. We think that doctors should start prescribing Lightning Hybrids conversions right away.