Aerosol formation and climate, Part I

Guest post by Bart Verheggen, Department of Air Quality and Climate Change , Energy research Institute of the Netherlands (ECN)

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.

Background

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.

Aerosol formation

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