Take a huge inhale of the salt spray while standing on the beach and you’ll smell the unmistakable strong perfume of the sea. That ripe, almost rotting smell? That’s sulfur.
When water vapor, an invisible gas, condenses into liquid water droplets, clouds form. These water droplets develop on microscopic particles in the air, such as dust. Depending on the temperature and weight of the air, or the atmospheric pressure in a certain place, the air can only carry a fixed amount of water vapor. The more water vapor the air can retain, the greater the temperature or atmospheric pressure. A volume of air is considered to be “saturated” when it contains all of the water vapor it can retain.
Every year, marine plankton releases more than 20 million tons of sulfur into the atmosphere, the majority of which is in the form of dimethyl sulfide (DMS). This molecule may change into sulfuric acid in the air, which aids cloud formation by providing a place for water droplets to develop. This process has a global impact on the climate since it occurs on the size of the world’s seas.
However, a new study from the University of Wisconsin-Madison, the National Oceanic and Atmospheric Administration, and others shows that more than a third of the DMS emitted by the sea is lost to the clouds themselves, preventing new clouds from forming. The new discoveries have a big impact on how scientists think about how marine life affects clouds, and they might transform how scientists estimate how cloud formation responds to changes in the seas.
It turns out that this story of cloud formation was really incomplete.
Tim Bertram
Clouds influence the global climate by reflecting sunlight back into space and influencing rainfall. Understanding the implications of climate change requires being able to accurately forecast them.
“It turns out that this story of cloud formation was really incomplete,” says Tim Bertram, a UW-Madison professor of chemistry and senior author of the new report. “Over the last three or four years, we’ve been questioning parts of that story, both through laboratory experiments and with large-scale field experiments. Now we can better connect the dots between what’s emitted from the ocean and how you form these particulates that encourage cloud formation.”
Gordon Novak, a graduate student at UW-Madison, collaborated with 13 other schools to create the study that will be published in the Proceedings of the National Academy of Sciences on Oct. 11.
This group of colleagues, lead by Patrick Veres of NOAA, found a few years ago that DMS first transforms into a molecule known as HPMTF, which had never been recognized previously, on its journey to producing sulfuric acid. The researchers took extensive measurements of these compounds over the open ocean, both inside clouds and under sunny skies, using NASA-owned, instrument-laden aircraft.
“This is a massive DC-8 aircraft. It’s a flying laboratory. Almost all of the seats have been removed, and highly accurate chemical instrumentation has been installed, allowing the team to analyze both the released molecules in the atmosphere and all of the chemical intermediates at very low quantities,” says Bertram.
The scientists determined from the flight data that HPMTF rapidly dissolves in the water droplets of existing clouds, thereby removing sulfur from the cloud nucleation process. In locations where there are no clouds, more HPMTF survives to produce sulfuric acid and aid in the formation of new clouds.
The scientists accounted for these new findings in a massive, worldwide model of ocean-atmospheric chemistry, led by partners from Florida State University. They determined that this method loses 36% of the sulfur from DMS to clouds. Another 15% of sulfur is lost through other processes, leaving less than half of the sulfur released by marine plankton as DMS available to assist nucleate clouds.
“This loss of sulfur to the clouds reduces the formation rate of small particles, so it reduces the formation rate of the cloud nuclei themselves. The impact on cloud brightness and other properties will have to be explored in the future,” says Bertram.
Researchers have generally overlooked the impact of clouds on chemical processes above the ocean until recently, in part because accurate data from the cloud layer is difficult to come by. However, recent research demonstrates both the importance of using the correct instruments to collect data and the important functions clouds may play, including affecting the processes that give birth to clouds.
“This work has really reopened this area of marine chemistry,” says Bertram.
