Scientists solve mystery of spontaneous smog formation

How Smog is formed ?
– The atmospheric pollutants or gases that form smog are released in the air when fuels are burnt.When sunlight and its heat react with these gases and fine particles in the atmosphere, smog is formed. It is purely caused by air pollution.

The cause of this spontaneous generation of pollution particles has been a mystery, but now scientists from Caltech, working in conjunction with researchers from Carnegie Mellon University and the European Organization for Nuclear Research ( CERN ), have uncovered the mechanism they belive is responsible.

The researchers found that two gases that are normally an invisible component of air pollution can, under the right conditions, chemically react with each other to create tiny particles that act like seeds that can grow to significant sizes. These particles are partly responsible for visible air pollution and some cloud formation. A paper about the work appears in May 13 issue of the journal NATURE.

Nearly one half of all Americans, an estimated 150 million, live in areas that don’t meet federal air quality standards. Passenger vehicles and heavy-duty trucks are a major source of this pollution, which includes ozone, particulate matter and other smog-forming emissions.

The health risks of air pollution are extremely serious. Poor air quality increases respiratory ailments like asthma and bronchitis, heightens the risk of life-threatening conditions like cancer, and burdens our health care system with substantial medical costs. Particulate matter is singlehandedly responsible for up to 30,000 premature deaths each year.

Passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. In 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.

Stephanie Kong (MS’18), a chemical engineering graduate student in the lab for Richard Flagan, Caltech’s Irma and Ross McCollum-William H. Corcoran Professor of Chemical Engineering and Environmental Science and Engineering, says the discovery came while she was investigating the source of atmospheric haze.

Haze is the visible air pollution we see mostly in cities, like the brown smog in L.A,, but also the dark gray fog we see in Asian cities,” she says. “ Haze matters because it’s made of particles that deposit in our respiratory system when we breathe. One of the big causes of asthma is the deposition of particulate matter in the lungs.”

The mechanism Kong and her fellow researchers discovered begins with tiny droplets of sulfuric acid formed from gases sent floating into the atmosphere by the burning of sulfur-containing fossil fuels, such as coal. These droplets react with atmospheric ammonia to form tiny crystals of ammonium sulfate. Most of the time, these crystals will grow very slowly as they clump together with other ammonium sulfate crystals and other compounds in the air. However, under the right conditions—when the air is very cold, around 5 degrees Celsius (41 Fahrenheit)—the sulfate crystals will begin gathering the nitric acid formed from automobile and factory emissions of nitrogen oxides and ammonia present in the atmosphere. Both nitric acid and ammonia are gases, but when they come into contact with each other on the surface of an ammonium sulfate crystal, they react to form ammonium nitrate. Under even colder conditions, around -15 degrees C, the nitric acid and ammonia can condense together without the aid of the ammonium sulfate, and start gathering into ammonium nitrate particles on their own.

Clean vehicle and fuel technologies provide us with an affordable, available means of reducing transportation-related air pollution and climate change emissions. These include fuel-efficient vehicles that use less oil; cleaner fuels that produce fewer emissions; and electric cars and trucks that can entirely remove tailpipe emissions.

Strong federal and state policies also help. Vehicle emission standards have helped cut pollution from cars and trucks by about 90 percent since 1998, with further improvements coming from the Tier 3 standards. Future emissions reductions from trucks and other freight sources are essential for meeting air quality standards and protecting the health of those who live and work close to ports, rail yards, and freight corridors.

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