Natural gas plants emit a tiny fraction of the smog-causing gases and slightly more than half of the greenhouse gases emitted by their coal-burning counterparts, according to a soon-to-be published study.

The assessment builds upon earlier reports that substituting natural gas for coal has sharply reduced air pollutants from power generation in the United States.

"Since more and more of our electricity is coming from these cleaner power plants, emissions from the power sector are lower by 20, 30, even 40 percent for some gases since 1997," said Joost de Gouw, lead author of the study and an atmospheric scientist with the National Oceanic and Atmospheric Administration's Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder.

The researchers compared readings from stacks across the country since 1997, then calculated emissions per unit of energy produced.

Coal-based plants emitted on average 32 ounces of carbon dioxide for every kilowatt hour of energy, compared with 19 ounces for natural gas plants. 

Gas-fired combined-cycle plants, using two heat engines in tandem, were even cleaner, releasing 15 ounces of CO2 for every kilowatt-hour produced, according to the study, which was accepted Wednesday for publication in the journal Earth's Future.

Fraction of the pollution
Differences in nitrogen oxide and sulfur dioxide emissions between coal- and gas-fired plants were more dramatic.

Gas-fired plants emitted 7 percent the nitrogen oxides and 0.2 percent of the sulfur dioxide of coal burners, the study found, largely because nitrogen oxides can be more efficiently controlled in a gas plant and sulfur content is very low in natural gas.

Nitrogen oxides are a main ingredient of smog. Sulfur dioxide can aggravate respiratory disorders and heart disease.

The United States has seen a dramatic shift away from coal as gas prices have decreased. As recently as 2005, coal provided half the electricity nationwide. In 2013 its share had fallen to 39 percent, according to the federal Energy Information Administration. 

In some regions the trend is more pronounced: The Northeast is shutting its last major coal plant and will soon be virtually coal-free. And it is likely to continue, with the Obama Administration on Wednesday publishing new rules on carbon emissions that make it virtually impossible to build a new coal-fired plant today.

The shift in the energy industry meant that power plants, overall, sent 23 percent less carbon dioxide into the atmosphere last year than they would have had coal been providing the same fraction of electric power as in 1997, de Gouw said.

The switch led to even greater reductions in the power sector's nitrogen oxides and sulfur dioxide emissions, dropping by 40 percent and 44 percent, respectively.

The authors cautioned that their analysis was limited to pollutants emitted during energy production and measured at stacks, so it did not examine pollutants that leak into the atmosphere during fracking or other types of fuel extraction.

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.