Study: Aerosol Pollution Influences Weather Systems

It is widely known that soot particles emitted from South Asia are spread across the northern Indian Ocean during the winter monsoon season.

These masses of soot and other forms of black carbon air pollution adversely affect human health and have been shown to play a significant role in regional and global climate change, second only to carbon dioxide.

New research led by Eric Wilcox, an atmospheric scientist at Nevada’s Desert Research Institute (DRI), and including Wilcox’s former advisor Veerabhadran Ramanathan, a distinguished climate and atmospheric scientist at Scripps Institution of Oceanography at the University of California San Diego, and others shows how these high concentrations of black carbon aerosols may also reduce atmospheric turbulence and enhance relative humidity near the earth’s surface, exacerbating damage to human health and extreme weather events.

“The finding that black carbon warming of the boundary layer is suppressing turbulence is a cause for concern,” said Ramanathan. “Since turbulence is one of the important ways by which pollution near the surface is transported away from human exposure, suppressing it can increase ground level pollution.”

The study, “Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer,” appears this week in the early edition of the journal Proceedings of the National Academy of Sciences (PNAS).

Utilizing a fleet of unmanned aerial systems (UAS) with specialized instrumentation developed by researchers at Scripps, Wilcox and his colleagues for the first time measured the atmospheric turbulence and vertical flow of latent heat above the ocean’s surface up to altitudes of 3,600 meters (12,000 feet), a level of the atmosphere known as the marine boundary layer.

“To completely understand the effects of these harmful aerosols on our climate and the modifications they cause to clouds and weather systems originating in the boundary layer, you have to first understand the turbulent dynamics of the air where the aerosols reside and the clouds form," said Wilcox.

The climate science community has long debated the impact of turbulence in the boundary layer and the research team set out to directly measure that, which hadn’t been done before, Wilcox added. 

The UAS flights took place during the National Science Foundation-sponsored 2012 Cloud Aerosol Radiative Forcing Dynamics Experiment or CARDEX. They originated at Hanimaadhoo in the northern part of the Maldives, a country made up of a string of islands south of India.

Specialized instrumentation aboard each aircraft measured turbulent kinetic energy (TKE), aerosol particle concentration, black carbon concentration, cloud droplet size and density, and temperature fluctuations. The turbulent flux measurement instrument package was developed by scientist Rick Thomas, during his time at Scripps as a postdoctoral researcher. Thomas is currently a research fellow at the University of Birmingham, School of Geography, Earth and Environmental Sciences. 

Aircraft data was combined with ground measurements obtained at the Maldives Climate Observatory on Hanimaadhoo, where Ramanathan and other scientists have conducted field experiments characterizing South Asian pollution for nearly two decades.

This study advances pioneering research performed earlier with the same lightweight UAS led by Ramanathan.

During the Indian Ocean Experiment (INDOEX) in the late 1990s, Ramanathan and colleagues first discovered that black carbon aerosols were a potent absorber of sunlight and therefore a major contributor to global warming.

The study also illustrates the continued advancement of UAS as valuable scientific research tools and showcases the ability of the aircraft to simultaneously measure multiple components of the same cloud system while only disturbing the cloud to a minor degree, added Ramanathan. 

Furthermore, said Wilcox, the unique instrumentation developed as part of this study can be applied to existing and planned UAS applications across the West that seek to better understand drought and the dynamics of winter precipitation events in the Sierra Nevada.  

The authors acknowledge that further study is needed to understand how the absorption of black carbon aerosols in the boundary layer may also impact the formation of cumulus (storm) clouds and higher elevation atmospheric events.

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