According to a team of scientists a chemical reaction that takes place in the atmosphere above major cities results in urban ozone, which is the main compotent of smog.
This discovery by Amitabha Sinha, a professor of chemistry and biochemistry at UC San Diego, defies the long held assumption that this reaction is unimportant in urban air pollution.
Sinha stated that the discovery could prove to be of help to air quality experts in developing better strategies to reduce ozone in many countries worldwide.
"This study provides us with additional insight into the chemistry of urban ozone production. It shows us that the chemistry of urban ozone is even more complicated than we initially assumed. With improved knowledge of how ozone is produced, we should be in a better position to control the air quality of large urban areas across the United States as well as around the world," said Sinha.
In large cities, the levels of urban ozone shoots up in the afternoon after they are generated through a complex series of chemical reactions involving the interaction of sunlight with hydrocarbons and nitrogen oxides from automobile exhaust. Ozone production is initiated when hydroxyl radicals, OH, are produced from water vapour.
It's been long assumed that the major part of the OH involved in urban ozone production is generated when ultraviolet radiation with wavelengths less than 320 nanometers dissociates ambient ozone to form excited oxygen atoms, which, in turn, react with water vapour to produce hydroxyl radicals. These OH radicals later attack hydrocarbons and the resulting products combine through a series of chemical reactions with nitric oxide, NO, to produce nitrogen dioxide, NO2, and eventually ozone, O3.
The researchers conducted laboratory experiments and found that another chemical reaction also plays a significant role in urban OH radical production, which is comparable to that from the reaction of excited oxygen atoms with water vapour under certain conditions.
This novel mechanism consists of reactions between water vapour and NO2 in electronically "excited states," produced when NO2 absorbs visible light between the wavelengths of 450 to 650 nanometers.
This method producing OH radicals was first proposed by German scientists in 1997. But, their measurements failed to detect any OH radicals being formed and, thus it was suggested that the reaction would play a fairly insignificant role in the atmosphere.
However, the latest measurements by the UC San Diego team indicate that this method of OH radical production occurs at a rate that is ten times faster than previously estimated.
Now due to the fact that radiation in the 450 to 650 nanometers wavelength range is not filtered out as effectively in the lowest portion of the atmosphere as the ultraviolet radiation in the vicinity of 320 nanometers that generate OH radicals from water vapor and ozone, atmospheric scientists believe it's likely to have a major role in the formation of smog.
"Identifying the sources of atmospheric OH radical production is important to understanding how to control the ozone problem, since it is the reaction of OH radicals with hydrocarbons that ultimately leads to urban ozone. The chemistry of urban ozone production is complicated and it just got bit more complicated with the addition of this new source of OH radicals," said Sinha.
Sinha's team was able to make the most precise measurement to date of the rate of this reaction with an innovative laser technique that allowed the team to directly monitor the OH radicals with significantly higher sensitivity then previously used to study this reaction.
The details of this study are published in the latest issue of the journal Science.