Surprise! Lightning Has Big Effect On Atmospheric Chemistry

Typically over the U.S., fossil fuel burning is the main cause of nitrogen oxides (NOx), which lead to the formation of ozone near the Earth's surface. However, above the Earth's surface in the free troposphere (3-8 miles high), during the summer months, lightning activity increases NOx by as much as 90 percent and ozone by more than 30 percent.

Renyi Zhang of Texas A&M University, lead author of a paper that recently appeared in the Proceedings of the National Academy of Sciences, suggests lightning has distinct impacts on air chemistry over the U.S. Human activities dominate the creation of these gases near the Earth's surface, but lightning plays a bigger role in the free troposphere.

Depending on where ozone resides, it can protect or harm life on Earth. Most ozone resides in the stratosphere (a layer of atmosphere between 8 and 25 miles high), where it shields life on Earth from the sun's harmful ultraviolet radiation. At the surface, ozone is a harmful pollutant that causes damage to lung tissue and plants. In the tropopause (surface to 8 miles high) ozone also is a radiatively active gas that affects climate.

About 77 million lightning bolts annually strike the U.S. Measurements before and after lightning strikes have confirmed the generation of nitrogen oxides in the atmosphere.

"Ironically, over the United States lightning accounts for only about 5 percent of the total U.S. nitrogen oxide annual emissions and about 14 percent of the total emissions in July," said Zhang. Although the largest source of NOx over the U.S. is fossil fuel burning, lightning still plays a dominant role in influencing the regional air chemistry.

The explanation is NOx from fossil fuel burning is released close to the Earth's surface and is consumed rapidly by chemical reactions before being transported upward. Lightning, however, directly releases NOx throughout the entire troposphere. The lightening source over North America for NOx is sufficiently large, so that it has implications on free troposphere NOx over other parts of the world, most notably Europe, which is downwind of the U.S., given the prevailing westerly flow in the Northern Hemisphere mid-latitudes.

NASA funded this research, because one mission of NASA's Earth Science Enterprise is to assess and understand the primary causes of changes in Earth's system, including man-made and natural causes.

The objective of Zhang's work is to assess the impact of how the U.S. human-induced (mainly fossil fuel burning) and natural (lightning) sources contribute to air pollution in the lower and upper troposphere. He collaborated with Dr. Xuexi Tie of the National Center for Atmospheric Research (NCAR).

Zhang used lightning measurements from the ground-based National Lightning Detection Network and the Optical Transient Detector (OTD) instrument to obtain the number of lightning flashes over the U.S. The OTD, aboard the Microlab satellite, is the world's first space-based sensor capable of detecting and locating lightning events during day and night, with high detection efficiency.

This research was partially supported by NASA's New Investigator Program in Earth Science and the Texas Air Research Center. The National Science Foundation supports NCAR.