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August 13th, 2016

Study on formaldehyde scavenging in thunderstorms shows changing ideas about atmospheric processes

Schematic diagram of the 2012 DC3 Study showing the NASA DC-8 aircraft sampling the inflow of severe thunderstorms near the surface while the NSF/NCAR GV aircraft samples the outflow at high altitudes. Source: the NCAR/EOL DC3 Web page (

Thunderstorms are powerful things: their churning circulation can stir gases from the lower atmosphere into the upper atmosphere and even the lower stratosphere.  They can also scrub gases out of the air by dissolving them in raindrops, a process known as scavenging.  In a new study, INSTAAR scientists in collaboration with other scientists at CU and NCAR found that scavenging is not nearly as effective as previously believed for some soluble and highly reactive trace gases, a result that may change our views of atmospheric chemistry in a warming climate.

Formaldehyde and other trace gases in the atmosphere are ozone precursors—they combine with other ingredients to form ozone and reactive radicals.  Ozone acts differently in different layers of the atmosphere.  In the stratosphere it filters ultraviolet radiation; in the middle troposphere it’s a greenhouse gas; and in the lower atmosphere near the surface it is a powerful oxidant harmful to people and plants alike.  Because ozone plays different roles in different places, understanding how ozone precursors get moved around is key to a better picture of atmospheric chemistry.  As Earth’s climate warms and large thunderstorms become more frequent, atmospheric scientists have become increasingly interested in understanding the role of thunderstorms in transporting various precursors of ozone from their pollution sources near the surface to the upper atmosphere.

The NASA DC-8 airborne platform taken from the NSF/NCAR GV aircraft during wing-tip comparisons of measurements during the DC3 study.

Previously, conventional wisdom assumed that soluble gases like formaldehyde were efficiently scavenged during thunderstorms, effectively getting filtered out of the atmosphere.  However, a number of field campaigns over the past 20 years have shown that in some cases, significant levels of formaldehyde can be transported by thunderstorms to the upper atmosphere. In addition to large uncertainties in the results, these studies produced large variability from 4 to 78% for the amount of formaldehyde scavenged, and did not provide new information regarding the mechanisms responsible.  One of the biggest mechanism uncertainties relates to how much formaldehyde is retained in the ice phase as air masses ascend and cool during convective transport.  This is an important question for other ozone precursors as well.

To address the many questions regarding the role of thunderstorms in transporting ozone precursors like formaldehyde to the upper atmosphere, a group of scientists from the CU Advanced Laser Technology for Atmospheric Research (ALTAiR) Laboratory, Alan Fried, Dirk Richter, Petter Weibring, and James Walega, embarked on a series of airborne measurements in 2012 and 2013 with colleagues from CU, other universities, NCAR, and NASA to study transport of trace gases in thunderstorms in greater detail than any previous studies to date. These studies, the Deep Convective Clouds and Chemistry (DC3) campaign in 2012 and the 2013 Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign, involved both NASA and NSF/NCAR aircraft and deployed spectrometers developed by the ALTAiR laboratory.

The researchers devised new approaches that for the first time firmly established the coherence between aircraft measurements acquired in the outflow of each storm with those acquired in the storm inflow near the surface.  Also for the first time, the researchers found remarkably consistent scavenging efficiencies across several storms, using several different methods of analysis.  Their work continues to see if the consistency they observed across the range of storms studied also holds true for a wider range of storms. 

The team worked with NCAR Senior Scentist Mary Barth and ATOC graduate student Megan Bela to complete modeling studies that showed that formaldehyde is completely rejected out of ice and back into the gas phase.  These new results, which are continuing in the analysis of other storms, are changing our view of the role of thunderstorms in transporting precursors of ozone to the upper atmosphere.

The study by Fried, et al. has been published in the Journal of Geophysical Research—Atmospheres.