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Summer 1998

Sulfate aerosols: A really cool problem

Sulfate aerosols are a hot topic these days in global climate modeling. Although the particles are known to cool the atmosphere, models haven't included their effects until recently--and many still do so only partially and crudely. Many scientists believe that this cooling effect explains why the models have generally predicted that, given our greenhouse gas emissions, the earth should have warmed considerably more than it has in reality. Modeling the aerosols' effects is one of the bigger challenges for NCAR's Climate of the 20th Century and Climate of the 21st Century Projects.

(Photo by Carlye Calvin.)

But if sulfate aerosols are so important in the climate system, why haven't modelers been accounting for them all along? What makes them so hard to model?

Dirty, mixed-up, and short-lived

When Jeffrey Kiehl (NCAR Climate and Global Dynamics Division, or CGD) describes sulfate aerosols, you can understand why climate modelers might not be eager to tackle them. First, a real-world sulfate aerosol isn't pure. Kiehl says, "It's going to have some water attached to it, and what we call organics, which come from trees or human activity. In polluted places like the U.S. East Coast, 50% may be sulfate and another 50% may be organics. The organic molecules are extremely complex; people are just beginning to understand them.

"Everybody's concern is what role the organics are playing in the radiative forcing of the climate system," Kiehl continues. Scientists are trying to calculate this, but there are so many unknowns that the range of uncertainty is very large. Some researchers have even speculated that the organics have a greater effect than the sulfates themselves. However, the role of sulfates should not be diminished; they are still a key player in the climate system.

Also, compared to the greenhouse gases, sulfur has a short lifetime in the atmosphere. Therefore, it doesn't get distributed evenly over the whole globe. Kiehl says, "That's made quantifying the problem more difficult than [for] greenhouse gases; you have to know how much of the aerosol is located in a certain region of the globe. Global transport models were essential." And there's vertical movement as well. The species that react to produce sulfate may be carried upward in convective clouds before they convert into aerosol. "We really have very little information on what that vertical distribution is."

Modeling the uncertainties

Modelers have to deal with a lot of uncertainties in figuring out how to include sulfate aerosols in their models. One major problem is how to reproduce their effect on clouds, known as their indirect effect. Not only do the aerosols directly affect climate by absorbing and reflecting solar radiation when the sky is clear, they also serve as nuclei for new, small cloud droplets. Clouds that form on more nuclei are particularly dense and very good at reflecting incoming solar radiation back into space.

The amount of this increased reflectance depends on the number of aerosol particles. Models, however, only predict the amount of sulfate aerosol in the atmosphere, not the number of particles. "So what do we do?" Kiehl queries. "We rely completely on empirical relationships [between amount and particle number] observed in field campaigns." The problem with this approach is that "there's a huge spread in the data, so you could fit a number of curves through the data. Unfortunately, if you choose different curves you get different climate forcings."

An additional twist on the indirect effect comes from Bruce Albrecht (University of Miami). He theorizes that the denser clouds would have a longer lifetime, which means that there should be more clouds in the atmosphere. This increase in cloudiness would cause even more sunlight to be reflected back to space before it reached the earth's surface. "There are people who put that effect into climate models," says Kiehl, "and they get very large negative effects--more than enough to offset [greenhouse-gas-induced] global warming."

Another uncertainty is the size of the aerosol particles. To scatter light most efficiently, they must be a few tenths of a micron in size. Most sulfate aerosol particles fall into this size range, but there are larger and smaller ones. "We're not modeling the size distribution; we're not to that degree of sophistication," Kiehl explains. "We assume that all the aerosols are sitting in the same size range. Given all the other uncertainties, this is not that bad an assumption, but we would like to be able to model size."

Mary Barth. (Photo by Carlye Calvin.)

Finally, Mary Barth (NCAR Atmospheric Chemistry Division, or ACD, and Mesoscale and Microscale Meteorology Division, or MMM), a cloud chemistry modeler, points out that the composition of the aerosol particles themselves is a question. "We're assuming ammonium bisulfate globally, but there's really some ammonium sulfate and ammonium bisulfate in the lower troposphere, and in the upper troposphere there's sulfuric acid." The chemical reactions in the cloud drops depend upon the pH of the drops, which depends upon the aerosol composition.

Why now?

"The reason sulfate aerosols have gained so much attention is that the current rate of emission of sulfur into the atmosphere by human activity is much larger than natural emissions," says Kiehl. "And these emissions have increased dramatically over time. The chemistry is relatively well understood compared to, say, organics, and when people started to model the sulfur cycle, they were getting lifetimes of sulfur aerosols that seemed quite reasonable compared to observations. So that gave people an impetus."

There's a lot of activity at NCAR to nail down the uncertainties. For example, Barth is involved in cloud-chemistry modeling on four fronts:

But Kiehl warns that progress in understanding all of the effects of sulfate aerosols is likely to come slowly. "I think that's frustrating for policy people, who want the definitive answer now on the role of aerosols on climate. It's going to take a much longer time than it took us for greenhouse gases because of the complexities we're talking about. It makes it a very interesting problem for scientists."

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:55:01 MDT 2000