Small Particles, Big Impact

High-resolution simulation for Mexico City (top), shows a more detailed and accurate picture of aerosol pollution compared to representations of a global climate model (bottom). The deep red to light green colors represent concentrations of aerosol pollution with red being highest, light green lowest.

Using systems at the National Energy Research Scientific Computing Center (NERSC), atmospheric scientists at the Pacific Northwest National Laboratory (PNNL) have found that small scale effects of aerosols—tiny particles of dust or pollution in the atmosphere—can add up and over time and lead to large, accumulated errors in climate prediction models.

“Aerosols like ozone, dust and sea salt in our atmosphere scatter and absorb sunlight. Depending on the type of particle and its elevation above Earth’s surface, these particles can tip the energy balance toward heating or cooling,” says Dr. William Gustafson, an atmospheric scientist at PNNL and principal investigator of the study, published in the July 2011 Journal of Geophysical Research Atmospheres.

Because global climate models typically calculate atmospheric processes at scales close to 100 by 100 kilometers (62 miles across), the characteristics of aerosols are averaged over a large area. This practice distorts the effects of aerosols in climate predictions because, in reality, these particles act on a much smaller scale and can vary according to local atmospheric or geographic features.

To quantify this error, Gustafson led a collaboration that looked at changes in the net flux of sunlight—the amount of sunlight that is reflected back into space, absorbed or allowed to hit the ground by aerosols—using a regional atmospheric model to emulate grids typical of coarse global climate models, as well as detailed grids at scales of 3 by 3 kilometers (1.6 miles across). The 30-day–long simulations were based on observations collected by the Department of Energy during March 2006.

“This net flux quantity is important because it directly relates to changes in the atmosphere’s energy balance, and thus the potential impact of the aerosol on atmospheric temperatures and climate,” says Gustafson. “In the short term, the impact of aerosol is less dramatic than clouds, but their effects add up in the long term and are important when considering mitigation and adaptation strategies for climate change due to the longer-lived greenhouse gasses, such as carbon dioxide.”

The team’s results revealed a 30 percent discrepancy between the coarse and detailed models for aerosol direct radiative forcing—rate of energy change at the top layer of the atmosphere due to the aerosols—over portions of Mexico.  

“Until recently, computers weren’t fast enough to incorporate detailed effects of aerosols in climate models. These calculations are very computationally expensive,” says Gustafson. “On average, if you are trying to predict climate without these particles, there may be 15 variables to consider. With aerosols, there are anywhere between 50 and 500 variables.” 

He notes that the detailed Mexico City models used about 250 computer cores, and were done over a several month period on NERSC’s Cray XT4 Franklin system. Ultimately, with more funding and compute power Gustafson says he would like to measure these effects on a global level, as well as be able to include the impact on clouds, which was excluded in the present study.  In addition to Gustafson, other authors of this paper include Drs. Yun Qian and Jerome Fast, also of PNNL.

This article was adapted from a story written by Jennifer Ovink, a communications specialist at PNNL’s Atmospheric Sciences & Global Change Division.