E8 Aerosols and Atmospheric Physical Chemistry

Topic

Atmospheric aerosols are solid or liquid particles suspended in the air, ranging in size from just one nanometer to one hundred micrometers in radius—a span extending from near-molecular particles to dust specks visible to the naked eye. These aerosols originate from diverse sources: combustion releases black carbon and dust; sea spray releases sea salt; vegetation emits volatile organic compounds that subsequently transform into particles; volcanic eruptions inject their own aerosols; and the air itself can generate new aerosols when gas condenses directly into tiny particles, bypassing any prior solid or liquid source. All these atmospheric aerosols influence the climate in two distinct ways: directly, by absorbing and scattering radiation; and indirectly, by acting as condensation nuclei and thereby altering the properties of the clouds that form around them.

The direct effect of atmospheric aerosols on radiation is characterized as "direct radiative forcing," a value that changes sign depending on the aerosol type: sulfate aerosols scatter solar radiation back into space, resulting in cooling; conversely, black carbon absorbs solar radiation rather than scattering it. This absorption warms the atmospheric layer where the black carbon resides while simultaneously depriving the surface below of sunlight; consequently, black carbon cools the surface even as it warms the air above it.

The indirect effect of atmospheric aerosols—known as the Twomey effect—follows a different chain of events: the more condensation nuclei available in the air, the more droplets form within the cloud, yet each individual droplet is smaller because the available water is distributed among a greater number of droplets. Those smaller, more numerous droplets give the cloud a higher albedo—because a cloud made of many small droplets reflects more solar radiation than one composed of a few large droplets—and also extend the cloud's lifespan, as small droplets take longer to grow large enough to fall as rain. The net result of this entire chain of events is negative forcing: through this indirect pathway, an increased number of condensation nuclei ultimately cool the planet.

Far above the altitude where most of these atmospheric aerosols float—in the stratosphere—a different type of chemistry occurs that also depends on solar radiation: stratospheric ozone chemistry. Stratospheric ozone is produced photolytically—that is, when sunlight breaks apart oxygen molecules and the resulting atoms recombine to form ozone, a process known as the Chapman cycle. Conversely, this same ozone is destroyed catalytically when it reacts with reactive chlorine and bromine compounds, nitrogen oxides, and hydrogen oxides; each of these substances can destroy many ozone molecules without being consumed itself in the process.

The Antarctic ozone hole represents the most extreme case of this catalytic destruction. There, on the surfaces of polar stratospheric clouds that form during the intense cold of the Antarctic winter, chlorine reservoirs that would otherwise remain inert become activated; once sunlight returns in the polar spring, these activated chlorine reservoirs cause massive destruction of stratospheric ozone. Thus, atmospheric aerosols and stratospheric ozone chemistry share the same stage—the sunlit atmosphere—even though each responds to that light differently: atmospheric aerosols scatter or absorb light or act as cloud seeds, while stratospheric ozone forms and breaks down depending on the reactive compounds it encounters.

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