Chemists at the Univ. of California, San Diego, have discovered that a chemical reaction in the atmosphere above major cities long assumed to be unimportant in urban air pollution is in fact a significant contributor to urban ozone—the main component of smog.
Their finding, detailed recently in the journal Science, should help air quality experts devise better strategies to reduce ozone for the more than 300 counties across the U.S. with ozone levels that exceed new standards announced last week by the Environmental Protection Agency.
It should also benefit cities in the rest of the world such as Mexico City and Beijing that are now grappling with major air quality and urban smog problems. More than 100 million people worldwide currently live in cities that fail to meet international standards for air quality.
“This study provides us with additional insight into the chemistry of urban ozone production,” says Amitabha Sinha, a professor of chemistry and biochemistry at UC San Diego who headed the research team. “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.”
Urban ozone levels peak in the afternoon hours of large cities after being 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 vapor. Atmospheric chemists had long assumed that the lion’s share of the OH involved in urban ozone production is generated when ultraviolet radiation with wavelengths less than 320 nm dissociates ambient ozone to form excited oxygen atoms, which, in turn, react with water vapor to produce hydroxyl radicals. These OH radicals subsequently 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.
Sinha’s team found in laboratory experiments that another chemical reaction also plays a significant role in urban OH radical production—perhaps comparable to that from the reaction of excited oxygen atoms with water vapor under certain conditions. This new mechanism involves reactions between water vapor and NO2 in electronically “excited states,” produced when NO2 absorbs visible light between the wavelengths of 450 to 650 nm.
German scientists first proposed this method of producing OH radicals in 1997. Their measurements, however, did not detect any OH radicals being formed and, as a result, they suggested that the reaction would play a fairly insignificant role in the atmosphere.
The more recent measurements by the UC San Diego team suggest that this method of OH radical production occurs at a rate that is ten times faster than previously estimated. And because radiation in the 450 to 650 nm wavelength range is not filtered out as effectively in the lowest portion of the atmosphere as the ultraviolet radiation in the vicinity of 320 nm that generate OH radicals from water vapor and ozone, Sinha and other 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,” Sinha says. “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.”
Sinha’s team—which included postdoctoral fellow Shuping Li and graduate student Jamie Matthews—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.
“It’s a relatively slow reaction with a rate that is at least a thousand times slower than that for producing OH from the reaction of excited oxygen atoms with water molecules,” says Sinha. “However, there is a lot of solar radiation coming down over the visible wavelength region, so even a slow reaction can become important. The upshot is that atmospheric models have ignored this reaction altogether, assuming that because nothing can be seen using conventional techniques, nothing must be happening.”
The research was supported in part by the Petroleum Research Fund of the American Chemical Society and the National Science Foundation.
Researchers: Don’t underestimate black carbon pollution
March 25, 2008
Black carbon, a form of particulate air pollution most often produced from biomass burning, cooking with solid fuels and diesel exhaust, has a warming effect in the atmosphere three to four times greater than prevailing estimates, according to scientists in an upcoming review article in the journal Nature Geoscience.
Atmospheric scientist V. Ramanathan at the Scripps Institution of Oceanography at the Univ. of California, San Diego, and Univ. of Iowa chemical engineer Greg Carmichael, said that soot and other forms of black carbon could have as much as 60% of the current global warming effect of carbon dioxide, more than that of any greenhouse gas besides CO2. The researchers also noted, however, that mitigation would have immediate societal benefits in addition to the long term effect of reducing greenhouse gas emissions.
The article, “Global and regional climate changes due to black carbon,” is posted at the online version of Nature Geoscience.
“Observationally based studies such as ours are converging on the same large magnitude of black carbon heating as modeling studies from Stanford, Caltech and NASA,” says Ramanathan. “We now have to examine if black carbon is also having a large role in the retreat of arctic sea ice and Himalayan glaciers as suggested by recent studies.”
In the paper, Ramanathan and Carmichael integrated observed data from satellites, aircraft and surface instruments about the warming effect of black carbon and found that its forcing, or warming effect in the atmosphere, is about 0.9 W/m2. That compares to estimates of between 0.2 W/m2 and 0.4 W/m2 squared that were agreed upon as a consensus estimate in a report released last year by the Intergovernmental Panel on Climate Change (IPCC), a U.N.-sponsored agency that periodically synthesizes the body of climate change research.
Ramanathan and Carmichael say the conservative estimates are based on widely used computer model simulations that do not take into account the amplification of black carbon’s warming effect when mixed with other aerosols such as sulfates. The models also do not adequately represent the full range of altitudes at which the warming effect occurs. The most recent observations, in contrast, have found significant black carbon warming effects at altitudes in the range of 2 km (6,500 feet), levels at which black carbon particles absorb not only sunlight but also solar energy reflected by clouds at lower altitudes.
Between 25 and 35% of black carbon in the global atmosphere comes from China and India, emitted from the burning of wood and cow dung in household cooking and through the use of coal to heat homes. Countries in Europe and elsewhere that rely heavily on diesel fuel for transportation also contribute large amounts.
“Per capita emissions of black carbon from the U.S. and some European countries are still comparable to those from south Asia and east Asia,” Ramanathan says.
In south Asia, pollution often forms a prevalent brownish haze that has been termed the “atmospheric brown cloud.” Ramanathan’s previous research has indicated that the warming effects of this smog appear to be accelerating the melt of Himalayan glaciers that provide billions of people throughout Asia with drinking water. In addition, the inhalation of smoke during indoor cooking has been linked to the deaths of an estimated 400,000 women and children in south and east Asia.
Elimination of black carbon, a contributor to global warming and a public health hazard, offers a nearly instant return on investment, the researchers say. Black carbon particles only remain airborne for weeks at most compared to carbon dioxide, which remains in the atmosphere for more than a century. In addition, technology that could substantially reduce black carbon emissions already exists in the form of commercially available products.
Ramanathan says that an observation program for which he is currently seeking corporate sponsorship could dramatically illustrate the benefits. Known as Project Surya, the proposed venture would provide some 20,000 rural Indian households with smoke-free cookers and equipped to transmit data. At the same time, a team of researchers led by Ramanathan would observe air pollution levels in the region to measure the effect of the cookers.
Carmichael says he hopes that the paper’s presentation of the immediacy of the benefits will make it easier to generate political and regulatory momentum toward reduction of black carbon emissions.
“It offers a chance to get better traction for implementing strategies for reducing black carbon,” he says.
The National Science Foundation, the National Oceanic and Atmospheric Administration and the National Aeronautics and Space Administration funded the review.
