WORCESTER, Mass. -- It hangs in the daytime sky like a dirty curtain filtering sunlight into a toxic gray haze, choking throats and burning eyes.

But what smog-causing pollution does when the sun sets is still a mystery to scientists, and one they say needs to be solved if pollution regulation is going to work.

"We don't have a good idea about what's going on at night," said Carl Berkowitz, a scientist for the Pacific Northwest National Laboratory. "And if we don't understand what goes on at night, we can't have confidence that we know the best way to fight smog during the day."

So Berkowitz led a team of scientists into the night sky in search of answers. For two weeks, the group flew a laboratory-equipped plane into pockets of pollution hovering over Boston and floating over the Atlantic.

The scientists who boarded the Gulfstream G-1 work for the U.S. Department of Energy's Pacific Northwest and Brookhaven laboratories, which are spending about $1.75 million to study air quality over New England.

Dirty urban air abounds

Air pollution is just one of the issues on the agenda at the earth summit meeting through Wednesday in Johannesburg, South Africa. A United Nations report found air and water pollution are killing millions of people worldwide.

The daytime and nighttime samples were collected in late July and early August.

"If you're studying pollution, Boston is a good place to be," Berkowitz said. "It's dirty urban air, and it makes for great samples."

The plane would leave from Worcester Municipal Airport after sunset and return by 5 a.m., sucking the toxic wind into onboard chambers. Banks of gadgets and computers analyzed the air, and the scientists recorded the findings.

They already knew that ground-level ozone, the main ingredient in smog, is produced when pollutants from automobiles and factories mingle with sunlight. But when sunlight is subtracted from the gassy mix, nobody is sure how the pollutants react with each other in the atmosphere.

That has posed a problem for state and federal regulators trying to limit the amount of ozone and other man-made pollution created every day, Berkowitz said. By studying pollution at night, the scientists hope to find out what kind of reductions are most needed to prevent the formation of ozone pollution during the day.

The information they gathered will be used as the foundation of state and national policies to fight smog.

Finding solutions

"We're trying to get enough data so regulators will know that if they tell an industry to do something like shut down a factory for a month, that will really work to cut down on pollution," said Stephen Springston, a Brookhaven scientist. "Or would it be better to let the factory run and get people to do more car pooling? Everyone wants cost-effective solutions to cutting down on pollution, but you need to know what's causing the pollution to begin with."

Clean air activists say the tests are a good idea.

"Anything that's going to alert us to new or different environmental dangers that cause lung disease is important," said Carlos Alvarez, executive director of the American Lung Association for Massachusetts. "The more that regulators know about what's going on in the air, the easier it is for them to figure out how to control pollution."

This is the second round of nighttime air sampling flights the government has done with the Gulfstream.

The first came about four years ago in Portland, Ore., and what scientists found surprised them: The more particulate pollution that was in the air, the less ground-level ozone there was.

That led to the belief that cutting down on emissions of solids and liquids that make particulate pollution may have a negative effect by increasing gaseous ozone levels.

"It suggests that the content of one pollutant affects the content of another," Berkowitz said. "So you can't say for sure that cutting down on one pollutant will mean you're automatically cutting down on another. The question now is if we'll see the same thing in Boston that we saw in Portland."

And that answer may not come for months.

"It's much too early to tell what we've found," said John Hubbe, one of the Pacific scientists. "It's going to at least take us the rest of the year to process the data and figure out what went on."