Aerosol formation and climate, Part I

The most prevalent trace gases do not generally nucleate new aerosols (or even condense onto existing ones), because they are too volatile (i.e. they have a high saturation vapor pressure and thus evaporate readily). They first have to be oxidized (usually under the influence of sunlight) to produce a compound with a lower vapor pressure. The prime example of this is the oxidation of sulfur dioxide (SO2) into sulfuric acid (H2SO4), which has a very low vapor pressure. The H2SO4 can then condense together with water vapor (and perhaps organic compounds and/or ammonia) to form a stable cluster of molecules: A new particle is typically 1-2 nanometers in diameter. Ions can also play a role, by lowering the energy barrier that needs to be overcome: The attractive forces between the molecules are stronger when one of them is charged. See here and here for a review of atmospheric nucleation processes.

Instead of nucleating into a new particle, H2SO4 could also condense on an existing aerosol particle, making it grow in size. Because of this competition for the vapor, nucleation is more likely to happen when there is only a little aerosol present.

Aerosol growth

Condensation of more vapor onto the nucleated aerosol makes it grow in size. However, other processes hamper its possibility to grow large enough to substantially influence the climate: Two aerosols can collide together, in a process called coagulation. Coagulation is particularly efficient between very small nano-particles and larger particles (of a few hundred nanometers). It causes the bigger one to grow in size, whereas the smaller (recently nucleated) one disappears. When there are a lot of very small aerosols around (i.e. after a nucleation event), they can also coagulate together. This causes them to grow in size, but decreases their number concentration. The loss processes for the number of aerosols (deposition and coagulation with bigger particles) are stronger when they’re very small.

Figure 1: Different factors influence the extent to which nucleation contributes to the number of cloud condensation nuclei (CCN). (Figure partly based on AGU presentation by Jeff Pierce)

Measurements

New particle formation has been observed all over the globe, from the Poles to the Tropics, from urban to remote areas, and from surface sites to the upper troposphere (see here for a review of such observations). Of these locations, only nucleation in the free troposphere and in the vicinity of clouds seems to agree with theoretical predictions. In most other cases the number of aerosol particles produced is under-predicted. This has led to the development of semi-empirical approaches to describe nucleation. Laboratory studies have typically found much stronger dependencies on H2SO4 than atmospheric measurements. A confounding factor is that newly formed particles of 1 to 2 nanometers can not be directly measured by commercially available instrumentation (though there are new developments in this area). Nucleation takes place in a kind of no-man’s land between the gas and the liquid phase, about which we know surprisingly little.

Figure 2: Measurements of an atmospheric nucleation and growth event in the Lower Fraser Valley, Canada. The color gives the (normalized) number concentration, where the red color indicates the enhanced concentration of nucleated particles, growing into the CCN size range. (from Mozurkewich et al.)

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