Ariel Stein

Tracking pollutants in the atmosphere with a world-renowned model

by Sarah Fesenmyer and Caroline Mosley (NOAA Research Communications)

One versatile model helps the world track airborne dust from the Sahara Desert, forecast smoke dispersal across the West from wildfires in California, and predict the spread of radiation through the atmosphere from accidents like the Fukushima Daiichi nuclear disaster. Dr. Ariel Stein leads the group at NOAA’s Air Resources Laboratory (ARL) in College Park, Maryland, that maintains and continually improves this model.

HYSPLIT model run

Example of a HYSPLIT model simulation of radioactive Cesium particles from the Fukushima Daiichi Nuclear Power Station.

Stein grew up in Argentina. His interest in science was sparked during a high school visit to a nuclear power plant, where he was astounded by the concept of extracting energy from atoms. After college, Stein traveled to the United States to pursue his master’s in environmental pollution control and Ph.D. in meteorology from Pennsylvania State University. He studied the chemical transformation of sulfur dioxide and nitrogen oxide—gases emitted by coal-burning power plants—into the main components of acid rain.

One day in 1997 while in graduate school, Stein found himself sitting around a table with a group of NOAA scientists. The group was responsible for maintaining a model that could track how particles move and disperse in the atmosphere. Stein was inspired by the versatility of the model, which could be used to look at the movement of polluted air masses locally and around the globe. From that point on, he has been involved in developing and continually improving NOAA’s Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model.

HYSPLIT’s 30-year evolution

Stein and his colleagues published a major report on HYSPLIT, featured on the cover of the December 2015 Bulletin of the American Meteorological Society.

“It’s a tool that can be used worldwide by decision-makers,” explains Stein. “You can not only simulate where a pollutant is going, but also work backwards and track the pollutant back to its source.” For example, the model can predict the daily spread across a region of smoke from wildfires, and it can also pinpoint a distant source for elevated levels of wind-blown dust. HYSPLIT is one of the most widely-used models around the world for calculating the trajectory and dispersion of pollutants in the atmosphere. Scientists have used HYSPLIT to forecast the spread of airborne allergens such as pollen, analyze the fate of mercury emissions into the atmosphere, and track volcanic ash plumes, among many other applications.

The sophisticated model that Stein manages started out over 60 years ago as hand-based calculations. In 1949 the U.S. detected radioactive material in the atmosphere near the Kamchatka Peninsula in Russia. U.S. meteorologists used wind data from weather balloons to back-calculate the trajectory of the radioactive debris and locate the site of the first Soviet atomic test. The results turned out to be highly accurate, and this original model has since evolved into HYSPLIT, providing complex computations of the transport, mixing, chemical transformation, and deposition of a wide range of air pollutants and hazardous materials.

In December 2015, Stein and his NOAA colleagues published a report describing the evolution of the HYSPLIT model over time and important recent technical improvements. The report catalogs the wide range of HYSPLIT applications, conducted by scientists at ARL and around the globe in Spain, Argentina, Australia, and China. An important application for Stein has been tracking the origin of dust events in Spain back to the Sahara Desert. He explains that knowing that dust particles in the atmosphere over Southern Europe come from the Sahara Desert means that we can more easily predict when the next dust intrusion will arrive. This advance knowledge allows decision makers to implement additional abatement strategies for controllable pollutants so that air quality stays within healthy limits.

Por orden del doctor (By Doctor’s Orders)

Dr. Ariel Stein (bass and voice, center) with bandmates Dr. Aldo Badano (drums, left) and Dr. Bob Lyons (guitar), performing with their rock band. (Credit: Ariel Stein)

Stein is dedicated to making NOAA’s model widely available to serve public health and safety around the world. His team at ARL provides workshops and support for HYSPLIT. He also teaches a master course every year in Spain on the transformation of pollutants in the atmosphere, such as the formation of tropospheric ozone, a key component of smog. Stein trains the scientists in his course on how to use HYSPLIT to track pollutants and ultimately provide decision-makers with crucial information on the spread of pollutants through the atmosphere.

Stein loves his work with NOAA. “The thing I enjoy most,” he says, “is accomplishing state-of-the-art science while at the same time sharing a good laugh with the members of the HYSPLIT team.” In his free time, Stein sings and plays guitar and bass in a rock band of fellow scientists.  All the band members have Ph.D.’s, and their band name is “Por orden del doctor” (By Doctor’s Orders). His favorite song to perform: “Comfortably Numb” by Pink Floyd.