The impacts of aerosols on climate are significant, but also very uncertain. There are several reasons for this, one of which is the uncertainty in how and how fast they are formed in the atmosphere by nucleation. Here, in part I, I’ll review some of the basic processes that are important in determining the climate effects of aerosols, focusing in particular on their formation. This is also relevant in order to better understand –and hopefully quantify- the hypothetical climate effects of galactic cosmic rays which I’ll discuss in a follow-up post.
Aerosols are liquid or solid particles suspended in the atmosphere (but not including water droplets or ice crystals). They can either be directly emitted into the atmosphere (primary aerosols like dust), or they can be formed in the atmosphere by condensation (secondary aerosol like sulfates). Almost all of their properties, and thus effects, are size dependent: The particle size governs the rate at which they fall out (and thus atmospheric lifetime), their interaction with radiation, their impact on clouds, or even their health effects. And they come in very different sizes, ranging from a few nanometers to tens of micrometers. Some sites with good introductory explanations to aerosols and their climate effects are here, here and here (German). RC also had some posts on the same generic topic here and here.
Climate effects of aerosols
Aerosol particles can influence climate in several ways: They scatter and absorb (in the case of black carbon) solar radiation (direct effects). They also act as cloud condensation nuclei (CCN) around which clouds can form, and thereby influence cloud reflectivity and cloud lifetime (indirect effects). Black carbon can have another indirect effect by changing the albedo of snow and ice, but that’s not the topic of this post. The aerosol indirect effects are the greatest source of uncertainty in assessing the human impact on climate change (reviewed here. The main idea is that more CCN causes liquid clouds to consist of more, but smaller, droplets. The resulting cloud is more reflective (first indirect effect). Due to the smaller size of cloud droplets, the formation of precipitation may be suppressed, resulting in a longer cloud lifetime and larger cloud cover (second indirect effect).
The mass of a freshly nucleated aerosol particle is more than 100,000 times smaller than that of an ‘aged’ aerosol of a size optimal to affect climate. As a rule of thumb, particles have to grow past 100 nm (1 nm = 10-9 meters) in order to become climatically active; below this size they are not easily activated into a cloud droplet and they don’t scatter solar radiation very efficiently. It is thus not immediately obvious that the climate effects of aerosols will depend very strongly on nucleation; the dependence is likely considerably damped, because a lot can happen to the aerosol particle as it comes of age.
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.
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)
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.)
So what is needed for nucleation to occur? Favorable conditions include a strong source of condensable vapor; high UV radiation intensity; low aerosol surface area; high relative humidity; low temperature; presence of ions; and atmospheric mixing processes. Under different environmental conditions, different nucleation mechanisms may be at work. For example, in industrial plumes and over urban areas enough sulfuric acid may be present to form new particles and have them grow to a stable size. Ammonia may neutralize the acidic cluster, and thereby help stabilizing it. Over forested areas, the relative role of organic compounds is expected to be much larger (though a strong correlation of nucleation events with sulfuric acid remains). In coastal areas, iodine compounds are likely involved in the nucleation process. In the upper troposphere, the ion density is usually larger, whereas the sulfuric acid concentration is lower. The relative role of ion induced nucleation may therefore be larger up there. The dominant role of sulfuric acid has remained a steady conclusion over the years, whereas the potential roles of organic compounds and ions are still hotly debated.
In part II, I’ll discuss the potential importance of nucleation and of galactic cosmic rays for climate change.