Pulse of the Planet: Aerosols Continue to Puzzle

Well, I'm back from Hong Kong. The visit went well but it's good to be home. After posting on Hong Kong air quality and China's seemingly constant haze and cloudiness, I arrived home to find an interesting new paper on the link between pollution and clouds.

Scientists refer to the tiny particles suspended in the atmosphere as particulate matter or aerosols. A typical aerosol might have a diameter of 1 to 10 microns; a micron is one-millionth of a meter so millions of aerosol particles could fit on the head of a pin. Pollution is a major source of atmospheric aerosols. Think, for example, of smoke from a fire. It can be composed of sulfates, nitrates, organic carbon molecules, and soot. When aerosols are heavy in the atmosphere, everything looks hazy (or smoky). The reason is simple: those tiny particles get in the way of light traveling through the atmosphere, sending it in every which way and making things in the distance lose their definition. We call this process light scattering.

Light scattering has an important climate effect. Aerosols scattering sunlight back to space (where it came from) cools the atmosphere. This is a small but not insignificant counter to warming from greenhouse gases. Steve Schwartz of Brookhaven National Labs coined the term the whitehouse effect to describe this process and differentiate it from the greenhouse effect's warming.

Where Do Clouds Come In?

But aerosols are versatile when it comes to climate. In addition to producing the whitehouse effect they can affect clouds. And because clouds play a huge role in climate, the aerosol-cloud interaction could be major.

There are two opposing ways aerosols work. They can invigorate clouds or inhibit them:

1. Invigorate Clouds - Aerosols help form clouds by serving as sites where tiny cloud droplets can gather. Check out this NASA video on "Cloud Makers" (you'll need Windows Media Player to view it). The more aerosols in the atmosphere, the more cloud droplets there are in a cloud. The more cloud droplets, the more reflective the clouds are and the smaller the droplets are -- this makes the cloud less apt to rain and therefore longer lasting. Both of these effects make clouds reflect more sunlight back to space and thus, like the whitehouse effect, they cool the atmosphere.
2. Inhibit Clouds - Aerosols can absorb sunlight. This absorbed radiation heats the atmosphere and makes clouds evaporate. The net result is a warming rather than a cooling.

Scientists have been trying for years to figure out which of these two effects dominates. I have been in the invigorate/cooling camp -- I even published a paper on the subject. And this makes sense to me whenever I visit China -- lots of aerosols from pollution, lots of haze, AND lots of cloudiness.

Now, researchers affiliated with NASA and the Weizmann Institute have shaken up both the invigorate/cooling camp and the inhibit/warming camp. They carefully analyzed the strength of the two opposing aerosol effects using a theoretical model and satellite data of clouds and aerosols over the Amazon. They found that both camps are wrong some of the time and both are right at other times.

Both cloud invigoration and inhibition play out at the same time -- with the relative importance of the two waxing and waning. The researchers found that at lower concentrations of aerosols the invigoration effect dominates, and thus adding aerosols to the atmosphere makes clouds cool the atmosphere more. But as the aerosol concentration continues to grow, the inhibition effect becomes dominant, and so adding more aerosols causes warming.

Interesting result and quite possibly correct. We'll have to wait to see what further research brings to light. Don't you get tired of hearing scientists say that? But that is the way things work in Scienceland. In the meantime, I will be pondering the hazy, cloudy skies of China and trying to jibe my observations from there with this new set of findings.

Dr. Bill Chameides is the dean of Duke University's Nicholas School of the Environment. He blogs regularly at www.thegreengrok.com.