Earth's atmosphere might be out of sight, out of mind, but it is actually a dynamic place with layers of distinct characteristics that are constantly vying for real estate in the sky. And while the atmosphere's layers can't be seen with the naked eye, scientists have the incentive to determine where they lie. 

For example, knowing the height of what's called the planetary boundary layer – the layer of the atmosphere that interacts directly with the planet's surface – compared to the mixing layer, which contains thoroughly mixed air that steadily cools with increased elevation, can help meteorologists predict air quality. 

The planetary boundary layer and mixed layer usually have nearly the same height. When they do, lidar laser technology can be used to effectively detect their height. Lidar uses pulsed lasers to measure light scattering and absorption by aerosol particles in the atmosphere. 

But sometimes, atmospheric conditions arise in which the mixed layer is significantly lower than the planetary boundary layer, a phenomenon called decoupling. Lidar has not been used to retrieve the height of both layers under these conditions. This is something scientists would like to be able to do because, for example, the more shallow the mixing layer is, the more concentrated pollution will be over a region.

A team of University of Arizona-led researchers published a technique in the Journal of Geophysical Research Atmospheres that combines new scientific insights with the revision of an existing computer algorithm to better detect different layers of the atmosphere using lidar, enabling scientists to explore the still mostly uncharted atmospheric world and enhancing the value of lidar measurements. Yike Xu is the paper's first author and a graduate student in the UArizona Department of Hydrology and Atmospheric Sciences.

The results were made possible by a  $30 million NASA award to a UArizona-led project in 2018. The project team – led by Armin Sorooshian, a professor in the Department of Chemical and Environmental Engineering – collected data on aerosols, clouds and meteorology and synthesized their interactions over the northwestern Atlantic Ocean using lidar and other instruments flown on airplanes. The funding comes from NASA's Earth Venture Class program, which funds projects investigating important, but not-well-understood aspects of Earth system processes. 

Parsing out the boundaries

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While there's a wide range in the height of the planetary boundary layer, it averages just over 3,000 feet globally. During hot desert summers, it can swell to over 13,000 feet as the sun beats down on the desert floor, heating and puffing up the air above. In contrast, at the frigid poles, the planetary boundary layer shrinks to less than 500 feet, said study co-author Xubin Zeng, professor in the Department of Hydrology and Atmospheric Sciences and the Agnese Nelms Haury Endowed Chair in Environment.

Sometimes, an inversion layer develops above the mixing layer, which traps the mixing layer well beneath the planetary boundary layer. While this decoupling isn't obvious to the naked eye, there are some hints. 

"On a clear day in the wintertime in Tucson, if you drive into the city from the suburbs, you can see a brownish fog," Zeng said. "That air is trapped near the surface by warm air above. That's an inversion." 

Lidar from aircraft or satellite can measure atmospheric inversion height under clear-sky conditions or cloud tops at the top of the planetary boundary layer, but that's where lidar capabilities have ended, until now.

The revised algorithm and new insights applied to lidar data uses holes in the clouds or clear areas between clouds reveal both the lower inversion layer height (indicated by cloud bottoms) and the higher inversion layer height (indicated by cloud tops). Importantly, the lower inversion layer also demarcates the upper limit of the mixing layer.

The team assessed the accuracy of its work against data collected by a weather balloon dropped from an aircraft. As the balloon fell, it captured information on temperature, humidity and winds. 

"From this comparison, we verified that our new techniques are a reliable way of determining the height of the planetary boundary layer and mixing layer," Xu said. 

Although the study was conducted over the Atlantic Ocean, Zeng said that the method can be applied globally. 

There are also implications for climate change mitigation.

Some researchers have turned to the controversial practice of pumping aerosols into the air off the coast of California in order to make low-level clouds brighter to reflect more solar radiation and help cool the warming world. But ejecting aerosols upward isn't always effective at brightening the clouds.

"If they release aerosols when the planetary boundary layer and mixing layer are decoupled, the aerosols can't overcome the boundary between the two to reach the clouds," Xu said. "This significantly reduces the effect the aerosols can have on cloud-whitening solar radiation management."

Zeng and his team hope that their results will contribute to the development and launch of a new satellite mission focusing on the planetary boundary layer in the next decade.