A new development may help atmospheric scientists more precisely determine how much ozone, a prominent greenhouse gas in Earth’s atmosphere, is generated by humans and how much comes from natural processes. Using a new technique, a research team has measured the amount of a major chemical precursor to ozone produced in individual lightning flashes across the United States. These results could help efforts to reduce excessive ozone in the atmosphere, a member of the team said.
Scientists have long known that lightning discharges trigger chemical reactions that yield simple nitrogen-oxygen compounds known as nitrogen oxides, including nitrogen dioxide (NO2) and nitric oxide (NO). In the atmosphere, nitrogen oxides can react with oxygen to form ozone, which blocks radiation from leaving Earth, increasing the planet’s global temperature over time. Humans also produce nitrogen oxides during such day-to-day activities as driving cars and running factories.
Figuring out the amount of NO2 a single lightning bolt produces could enable researchers to better pin down how much of this atmospheric ozone precursor is naturally produced and how much comes from human endeavors, Jeff Lapierre, a postdoctoral researcher in atmospheric physics at the University of Virginia in Charlottesville, told Eos.
However, years of attempts to pin down the nitrogen oxide yield per lightning flash have failed to agree on any one number, he noted. Instead, current research results disagree by more than 2 orders of magnitude. Lapierre wanted to know why there was such a large range in the findings: Is there uncertainty in the measurements, or do different kinds of lightning produce different amounts of nitrogen oxides? He presented his results on 15 December at the 2016 Fall Meeting of the American Geophysical Union in San Francisco, Calif.
Clouds Block Out Anthropogenic NO2
Previously, researchers have used mathematical modeling, laboratory-based studies, and information from aircraft or satellite data to study nitrogen oxides produced by lightning. Lapierre and his colleagues chose to use satellite data for their study so that they could have a large data set to work with. In addition, they added a new measurement that distinguished between different types of lightning.
From May through August of 2014 and 2015, the team obtained NO2 measurements from the Ozone Monitoring Instrument on NASA’s Aura satellite. This instrument tracks from space chemicals, like NO2, in Earth’s atmosphere that play a role in ozone-producing reactions every day. Then the team compared that chemical data to lightning data across the United States from Earth Networks’ Total Lightning Network. The lightning network gave them information about when lightning flashes happened and whether the lightning was of the cloud-to-cloud or cloud-to-ground variety. The group compared only lightning and NO2 data collected under cloudy conditions so that the clouds would filter out anthropogenic NO2 coming up from the Earth.
“This is really hard to do. The data are really noisy just looking at NO2 and lightning production,” Lapierre said. “You have to be a little clever.”
The researchers found that on average, across the United States, each lightning flash produces 45.3 moles (about 2 kilograms) of NO2. These results fall on the lower end of the 10–1000 moles of nitrogen oxides described by other studies, according to Lapierre. In addition, they saw that cloud-to-ground lightning produced more NO2 than cloud-to-cloud lightning. He predicts that these results will vary across the country, depending on the types of storms that are prevalent in each area.
Lapierre is currently using his method to look at NO2 produced in specific regions of the United States, but his results are still preliminary. He is also using wind data to follow NO2 produced by specific storms.
Lapierre’s results are similar to those from a previous study that determined the amount of nitrogen oxides produced by lightning over the Gulf of Mexico. Because this study was over a body of water rather than over land, the authors didn’t need to separate out the man-made surface nitrogen oxides. Lapierre was pleasantly surprised that his results were so similar because it means that this method should be useful for studying lightning production of NO2 across populated regions.
Nitrogen Oxide Stays in the Atmosphere
Nitrogen oxides in the atmosphere hang around a lot longer than nitrogen oxides near the ground, according to Jim Crawford, an atmospheric chemist at NASA Langley Research Center in Hampton, Va., who was not involved with the study. “In terms of chemical impact, [it] can do a lot more chemistry…making ozone, doing its thing,” he said.
Unlike anthropogenic forms of nitrogen oxide, we have no control over how much nitrogen oxide lightning produces. “All we can do is just know how much is there, really,” said Lapierre. He told Eos that he hopes his data will help researchers to more accurately estimate the total nitrogen oxide produced by lightning across the globe and thereby to better quantify the human contribution to atmospheric ozone concentrations worldwide.
—Sarah McQuate (email: [email protected]; @potassiumwhale), Science Communication Program Graduate Student, University of California, Santa Cruz