Atmospheric Aerosols: Fossil Fuel and Biomass Burning
There is yet another impact of biomass burning on the energy budget of the planet—such burning produces tiny smoke particles called "aerosols." These aerosols can be either cooling or warming depending on how much solar radiation they absorb versus how much of it they scatter back to space. Close to the burning area, smoke clouds can look almost black, indicating strong absorption, while further downwind they may look white, indicating weaker or no absorption. Fossil fuel burning in automobiles, factories, and power plants also add aerosols to the atmosphere while polluting clouds (Figure 4). These aerosols are thought to absorb less solar radiation than those produced by biomass burning, but the exact amount of absorption from each type of aerosol is not yet known. Overall, we do not even know if aerosols are warming or cooling the planet! This depends critically on how much aerosol is present, how absorptive the aerosol is, how large the aerosol particles are, and how high the aerosol is in the atmosphere. For example, the aerosols emitted by the Mt. Pinatubo eruption in 1991 were very small particles that absorbed very little sunlight and primarily reflected it. As predicted, the aerosols from Mt. Pinatubo acted to cool the planet for a couple of years before settling out of the atmosphere. Man-made aerosols tend to be processed out of the atmosphere by clouds within a few weeks.

Figure 4: Clouds polluted by aerosols from factories have more numerous and smaller drops, causing the cloud to become brighter and reflect more of the Sun's radiative energy.

As the number of aerosols increases, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere, and decrease the amount of rainfall. In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts. Even the tiny aerosols (typically less than 50 millionths of an inch or 1 micron across) can affect the clouds, which in turn can change the radiation balance of the planet. Thus, aerosols can have both a direct effect on the energy balance, as well as an indirect effect (through clouds). It is thought that the indirect effect of aerosols can be even larger than their direct effect, but at present it is not known whether such an effect is a net cooling or warming of the planet.

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Why isn't Earth as hot as an oven?

Introduction

Clouds: A hot topic or are we made in the shade?

Surface Absorption and Reflection

Atmospheric Aerosols: Fossil Fuels and Biomass Burning

From Measurements to Climate Models

Related Data Sets:

Surface Temperature

Outgoing Longwave Radiation