Yu leads study on aerosol import to North America

A study conducted by ESSIC Associate Research Scientist Dr. Hongbin Yu and a team of scientist that assesses the contributions of cross-ocean aerosol transport to North America was published in Science Magazine (3 AUGUST 2012, VOL. 337, #6094).

In the study, titled “Aerosols from Overseas Rival Domestic Emissions over North America”, the team estimates, from NASA satellite observations, that the mass of aerosols arriving at North American shores from overseas is comparable to the total mass of particulates emitted domestically.

Aerosols, the tiny particles suspended in the air, have plenty of time for trans-continental transport due to their relative longevity, which means aerosols emitted in one region can travel thousands of miles downwind into far-away regions.

A concern the study presents is the possibility that imported aerosols will cause the surface dimming, increase atmospheric stability, alter cloud and precipitation processes through acting as ice nuclei and accelerate snow melting in the Sierra Nevada mountain range through deposition on snow.

“We hoped to get a complete picture of contributions of trans-Pacific aerosols to North America so we looked into dust,” Yu said, noting he and his colleagues published a paper on the trans-Pacific transport of combustion aerosols in 2008.

“Surface monitoring networks have detected dust from Asia arriving in North America and adding to surface PM concentrations, mostly on episodic basis. People didn’t know the amount of dust import in the whole atmospheric column and on an annual basis,” he said. “The beauty of satellites is that they can provide measurements of the whole atmospheric column on a daily basis around the globe over many years”.

Using a combination of advanced aerosol satellite measurements from NASA’s MODIS and CALIPSO lidar instruments to estimate the dust transport across the Pacific Ocean, the team found that roughly half of all particulate matter found in the air above North America originates from sources overseas. Most of the total aerosol import is dominated by the trans-Pacific dust transport, with the remaining almost evenly divided between trans-Pacific transport of combustion aerosols and trans-Atlantic dust transport.

According to the study’s summary, curbing domestic emissions of particulates and precursor gases is not sufficient to mitigate the arrival of aerosols in North America.

“Even a reduction of industrial emissions in the emerging economies of Asia could be overwhelmed by an increase in dust emissions due to changes in meteorological conditions and potential desertification,” Yu said.

He emphasized that implications of the findings from the paper are important, but should not be misinterpreted.

“For example, as we emphasize in the paper, ‘trans-Pacific’ dust and pollution should not be interpreted as “Asian” dust and pollution. A significant amount of trans-Pacific dust and pollution comes from regions outside of Asia,” he said.

Yu worked with NASA Physical Scientist Dr. Mian Chin, UMBC/JCET Research Scientists Dr. Lorraine Remer, Dr. Huisheng Bian, and Dr. Tianle Yuan, and GESTAR/USRA Research Scientists Dr. Qian Tan and Yan Zhang on the study.

He said the team started work on the paper in 2010. The paper is tied into a three-year project funded by NASA in early 2011 under The Science of Terra and Aqua Program and another project funded by NASA under the Atmospheric Chemistry Modeling and Analysis Program (ACMAP).

That project, he said, focuses on assessing air quality impacts of foreign aerosols in the U.S. with high resolution NASA Unified WRF-Chem model. The satellite measurements reported in the paper will be used to constrain the model by providing chemical boundary conditions.

The team uploaded a YouTube video that clearly illustrates dusts (illustrated in green) hitting the shores of North America throughout a year, including both the MODIS observation and GOCART model.

Yu said he wants dedicate the paper to his mentor:

“We would like to dedicate this paper to Yoram Kaufman, an outstanding aerosol scientist and great mentor who was tragically killed in a bicycle accident in May 2006. Yoram encouraged me to pursue in this direction and challenged me to write a concise paper just a few months before the tragedy,” he said. “At the time, we just finished a comprehensive paper on the aerosol direct radiative forcing that later served as one of major inputs to the U.S. Climate Change Science Program’s (CCSP) Synthesis and Assessment product 2.3. I am glad that we are fulfilling tasks he assigned to me.”