SAN FRANCISCO -- Could a network of cell phones improve air pollution monitoring?

That's the hope of Nithya Ramanathan, an assistant research professor of computer science at the University of California, Los Angeles. She's part of a team that has developed a system that relies on cell phones to report concentrations of black carbon, a particle produced by burning fossil fuels and biofuels like wood and dung.

Tiny particles of black carbon are potent warmers. They absorb heat from sunlight, warming surrounding air and, when they fall from the atmosphere onto ice or snow, hasten melting. One recent study estimated that black carbon emissions caused half the total warming in the Arctic between 1890 and 2007.

Because black carbon doesn't linger in the atmosphere -- just one to four weeks, compared to centuries to millennia for carbon dioxide -- some scientists argue cutting black carbon pollution could have a near-immediate cooling effect. It would also carry health benefits, especially in developing nations where wood- and dung-powered cookstoves produce severe indoor air pollution.

Ramanathan said she hopes her cell phone-powered monitoring network will lead to cheaper, more widespread measurements of black carbon.*

"This system will work with any cell phone that has a camera," she said, and requires no special software. "There are over 4 billion cell phones in use globally. Almost all being manufactured today come with a camera."

'Why don't you do something useful?'
The plan begins with a small sampling machine that collects air samples. Black carbon in those samples is deposited on a filter. At a pre-determined interval, a user removes the filter, photographs it with a cell phone and uses the phone to e-mail the time-stamped image to scientists for processing.

Scientists can be assured the measurements are accurate because each filter is photographed next to a card that depicts a range of filter colors produced by different concentrations of black carbon. That means both the reference card and the filter sample are exposed to the same lighting, removing any errors caused by variations in photo quality.

Ramanathan, who says each sampler costs $500 to $600 and each filter about $1, has tested her system in India, where she used it to measure black carbon emissions from cookstoves, and California, where she monitored outdoor air quality. NASA's Jet Propulsion Laboratory is also interested in testing the system, she said.

One of Ramanathan's most enthusiastic collaborators is her father, V. "Ram" Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography who has conducted pioneering research on black carbon's climate effects and started a charity to distribute solar-powered cookstoves to reduce black carbon pollution in his native India.

"Nithya's my daughter," he said. "Three years ago, at Thanksgiving, she was telling me all the things she was doing with computers and cell phones -- so I said, 'Why don't you do something useful?'" he said, chuckling at the memory.

California cuts its black carbon emissions by half
Ram Ramanathan was also presenting new work at the American Geophysical Union meeting -- a study, published yesterday in the journal Atmospheric Environment, that finds California's efforts to reduce air pollution have cut the concentration of black carbon in the state's air by half over the past two decades.

That carries both climate and health benefits, Ram Ramanathan said. In addition to being a potent warming agent, black carbon can exacerbate heart and lung problems, including asthma.

The new California study is based on data collected by a statewide monitoring network. It was funded by California's Air Resources Board.

Ram Ramanathan said he and his co-authors believe the reduced amount of black carbon in the state's air is a direct result of California's effort to cut air pollution produced by diesel engines, starting with the state's 1987 rule capping particulate emissions from heavy trucks.

Black carbon emissions from diesel engines in the state have also dropped by half during the past 20 years.

"For scientists, it's a spectacular controlled experiment, because we can see the results of our policies," he said.

*Correction (12/16/10): An earlier version of this story made an incorrect statement about the uncertainty inherent in black carbon measurements.