Fundamentals of Atmospheric Science

4.6 Where do cloud condensation nuclei (CCN) come from?

Now that you know everything about the atmosphere’s gas-phase composition, it is time to look at its particle composition. We are interested in atmospheric particles for several reasons:

  • smaller ones can get into the lungs and cause serious health problems;
  • smaller ones can absorb or scatter sunlight, thus affecting climate;
  • some of them are good cloud condensation nuclei, which are essential for the formation of clouds.

Atmospheric aerosol is most obvious to us on warm muggy summer days. Under these conditions, there are lots of aerosol particles and they absorb water and swell up to a size that is quite efficient at scattering sunlight. The following picture was taken over Maryland on a flight between Washington Dulles airport and State College airport. Above the fair weather cumulus clouds is blue sky in the free troposphere. Below the clouds is the atmospheric boundary layer, which is filled with aerosol that has been well mixed by warm, moist air parcels rising and stirring the boundary layer air. The haze is so thick that it is a little hard to see the ground.

aerial view of summertime haze over Maryland as described in the text above
Summertime haze over Maryland.
Credit: W. Brune

Atmospheric particles come from many different sources. Good cloud condensation nuclei (CCN) must be small particles, so that they do not settle too fast, and must be hydrophilic, which means that water can stick. They can be either soluble (i.e., dissolvable in water), or insoluble, but most are soluble.

Most particles originate from emissions from Earth’s surface. Primary aerosols are emitted directly from the source, although the smaller ones start off as hot gases that rapidly condense to form particles even before they leave the smokestack or tailpipe. Secondary aerosols are gaseous emissions that are converted to aerosol particles by chemical reactions in the atmosphere. Some of these become CCN. This process is often called gas-to-particle conversion. Most CCN are secondary aerosols.

The sources are both natural and anthropogenic (human-made). Seaspray, volcanoes, forests, and forest fires, as well as gas-to-particle conversion of naturally occurring gases such as sulfur dioxide (SO2) and some naturally occurring VOCs, such as α-pinene (which gives the pine smell) are important natural particle sources. Industry, power plants, using fires to clear cropland, transportation, and gas-to-particle conversion of anthropogenic SO2 and numerous other gas emissions are important anthropogenic particle sources.

Note that we must pay attention not only to the aerosol sources but also the aerosol sinks, as shown in the diagram below.

atmospheric aerosol sources and sinks
Sources and sinks of atmospheric aerosol particles.
Credit: NOAA

The different sources make particles of different sizes. The typical size distribution (i.e. number of particles in a volume of air, plotted as a function of size) has bumps in it, with more particles at some sizes than at others, as seen in the diagram below. Reading these bumps tells us a lot about how the particles were made.

The nucleation mode (there are other designations for this) includes particles that are made by gas-to-particle conversion. A low-volatility vapor is one that will condense onto particles or other surfaces when its vapor pressure exceeds its low saturation vapor pressure. This situation is analogous to water.

Coarse mode includes particles made by mechanical processes. The hydrophilic coarse particles can be CCN, but they settle out pretty fast.

Accumulation mode particles are usually made when nucleation particles collide and stick (called coagulation) or when gases accumulate on a nucleation mode particle. They neither settle fast nor coagulate, so they tend to hang around in the atmosphere for a few weeks. They make pretty good CCN.

Typical aerosol size distributions and their sources and sinks. Nucleation range particles quickly grow into accumulation range particles, which fall slowly to the ground and stay a week or two in the atmosphere. The larger particles, called coarse particles, fall to the ground within hours or sometimes days. Gas-to-particle conversion means that the particles start as gases but are converted by reactions to sticky chemicals that form particles.
Credit: W. Brune

PM2.5 - Secondary Particles from Gas-to-Particle Formation

PM2.5 is a particle size designation that means “Particle matter smaller than 2.5 µm in diameter”. Another common term is PM10, which is "particle matter smaller than 10 µm in diameter". PM2.5 particles are the ones that are most important for human health and climate, and, in many cases, cloud formation because of their longer lifetime in the atmosphere.

Secondary particles start with the emission of VOCs or sulfur compounds, which react mainly with OH to start a sequence of reactions. These reactions tend to add oxygen to the molecules, which chemically makes them stickier (with a lower saturation vapor pressure) and more water soluble, which is just what is needed to make them better cloud condensation nuclei.

For particles that start as gaseous sulfur compounds, such as sulfur dioxide (SO2), the reaction sequence starts with OH and the reaction product is sulfuric acid, a compound that has a very low vapor pressure and is very sticky.

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Sulfuric acid is easily taken up into cloud drops and raindrops and then can be deposited on Earth’s surface when it rains. The good news is that the rain cleans the atmosphere. The bad news is that the rain is very acidic and has earned the name “acid rain” because of its harmful effects on forests and on buildings, memorials, and statues.

If sulfur sources are upwind of an area, the particles in that area will contain some sulfur. But almost all atmospheric particles also contain some organic compounds and sometimes particles are mainly made up of carbon-containing organic compounds. Some of these organic particles are primary, but most of the small ones are made by gas-to-particle conversion, which is just a simple way to say the volatile organic compounds react in the atmosphere with OH or O3 to form less volatile organic compounds that become aerosol particles. The chemicals in these particles can continue to oxidize, thus making them even better CCN.

We can demonstrate gas-to-particle conversion of a VOC that is often emitted into the atmosphere by trees. This compound is limonene and also comes from oranges. In the video (4:47) below entitled "Demonstration of Gas to Particle Conversion," I will use orange peel to demonstrate this effect.

Click here for a transcript of Gas-to-Particle Conversion Demo Video

Quiz 4-2: Atmospheric constituents and chemistry.

  1. Please note: there is no practice quiz for Quiz 4-2 because the answers come right from the text and videos. Read and re-read the material (and ask questions in the discussion board in Canvas) until you understand it.
  2. When you feel you are ready, take Quiz 4-2. You will be allowed to take this quiz only once. Good luck!