Aerosol and Particle Formation

Aerosols are solid (Greek: sol) and/or liquid particles, that float in the air (Greek: Aero). The term aerosols covers a wide spectrum, from sea salt particles, pollen and mineral dust to drops of sulfuric acid and other small particles. Water drops and ice crystals are not counted as aerosols, because they are mostly formed from aerosol particles.

Occurance and Origin

Atmospheric aerosols can be from 1 nm (1 millionth of a mm) to ca. 100 µm (1 tenth of a mm) and have many sources. Their concentration over seas, oceans and remote continental areas is generally in the range ca. 1000 to ca. 10,000 particles per cm³. In cities in winter during warming periods or because of traffic (soot emission and oxidation of aromatic hydrocarbons) the concentration can rise to over 100,000 particles per cm³.

The dominant size range of the aerosol particles depends on which process the individual aerosol particles are made by. Some particles are built from solids (bulk-to-particle-conversion (BPC)), some from gases (gas-to-particle-conversion (GPC)) and some from the reactions of dissolved materials in cloud droplets.

Bulk-to-particle-conversion (Formation from Solids)

Many particles are formed from already available materials, like mineral dust ( e.g. from desets), clay particles (from soil erosion) or primary biological particles (like pollen, bacteria, spores and parts of insects) through the influence of the wind. e.g. In this way sea salt gets into the air from foam on the sea surface, formed from the stirring-up of the sea water surface by rising gases dissolved in the water, and dries in conditions of low air humidity (< 30%) back into pure sea salt. Mineral dust (e.g. Sahara dust), manages to get into the atmosphere through the saltation process, when small particles are pushed by other rolling or jumping particles (e.g. in sanddunes).

These particles, which are all formed from already existing materials and also the many types of biological particles are generally to be found in the size range above 0.1 µm radius. This is the range of the large (0.1 µm - 1 µm radius) and giant (1 µm and larger) particles.

Gas-to-particle-conversion (Formation from the Gas Phase)

When the substance of aerosols first has to be formed from the gas phase, the aerosols will mainly be in the smallest size range (smaller than 0,1 µm), the range of the Aitken particles (1 nm - 0,1 µm radius). In this case a chemical reaction occurs in which relatively volatile substances such as SO2 and water form less volatile substances such as sulfuric acid. In this case, trace gases, which are available in small quantities in the atmosphere, and water from the normal air humidity, react together. One speaks now of 'secondary particles' because the particles were formed from other substances. The newly formed, less volatile molecules then form clusters (molecules balling together), on the surfaces of which further substances condense. From a size of 1 nm upwards they are classed as aerosols.

Complexes involved in particle building through the GPC-Process are currently being intensively researched because a lot of factors can have an influence on the number and kind of particles which are formed. Some paths of formation are still unknown. Here we will discuss two of the pathways:
One is the formation of a sulfuric acid droplet from the reaction of water with already existing sulfuric acid and ammonia.
The other is the formation of secondary organic particles from the oxidation of volatile carbohydrates such as terpenes (terpenes are emitted from forests and are a kind of 'plant perfumes') or aromatics such as toluene or benzene (mainly formed by Man in cities). The oxidation takes place through the three main oxidative substances of the atmosphere; ozone, OH and NO₃.

Formation of Aerosol Particles in Cloud Droplets

Lastly we must give a name to the particles which are formed or changed by cloud droplets. If an aerosol particle (cloud condensation nucleus) dissolves in taken-up water (a cloud droplet) and then reacts with other substances dissolved in the droplet then it can build new aerosol subsances, and form a new aerosol particle when the water evaporates. (The main part of clouds doesn't come down to Earth as rain, but actually just evaporates again.)

Removal from the Atmosphere

Once aerosols are in the atmosphere, they can be removed from it by several processes;

a) Diffusion (= striving for equal distribution): smaller aerosol particles share themselves out (diffuse) in air, and in doing so bump into bigger aerosol particles and join together with them.
b) Sedimentation: Bigger particles are influenced more by gravity and fall back to the Earth's surface.
c) Rain-out and washout: Aerosols in cloud droplets, that is, aerosols which have been caught in cloud droplets, can be washed out of the atmosphere in raindrops.
d) Impaction: Particles stick to obstacles they bang into.
Particles with a radius of 0.3µm stay in the atmosphere for the longest time because the processes above have the least effect on them.

Effects of Aerosols in the Atmosphere

Formation of Cloud Droplets

Aerosols are particularly interesting because they build water droplets in the atmosphere. If the water vapour in the atmosphere had to build droplets without aerosols, the formation of new surfaces would be so limiting that a relative humidity of 600 % would be necessary, which wouldn't actually happen. Some aerosol particles (cloud condensation nuclei), like salt, reduce the limit for droplet formation in the atmosphere to ca. 101 % relative humidity, because water only has to condense on the surface of the particle. These drops are to be found in the atmosphere with a radius of 50 nm and more. The size is strongly dependant on the composition of the individual particles. In this area there is still a lot to be discussed and researched. However, we do know that aerosol particle formation and composition has a great influence on the formation and disappearance of cloud droplets through rain-out and evaporation.

Radiation

Aerosols also have an influence on the radiation from the Sun (above all UV and visible light) and from the Earth (above all infra-red radiation).They absorb, emit and scatter the direct radiation, through processes happening both inside the particles and on their surfaces. Indirectly, aerosol particles also affect the radiation balance through their above-mentioned ability to promote cloud droplet formation. The more aerosol particles available that can help to form water droplets, the more and the smaller the water droplets. These droplets can also absorb and scatter radiation. The indirect effect influences the incoming (from the Sun), reflected and outgoing (from the Earth) radiation differently. The effects of atmospheric aerosols upon climate are not yet well understood.

Measurement

To really be able to measure aerosol particles, many problems must be solved. The main problem is the enormous size range of aerosols., The large differences in sensitivity, depending on particle size, put a high demand on the equipment. As a trick, one can let the particles grow from condensation, so that they can be counted by optical methods. Their mass can for example be determined through filter collection such as impactor samples. Put simply, in impactors a gas stream is run around a corner and the bigger particles, which don't make the curve, hit a collection area.

Many other measurement methods have been developed, which generally can only be used for one particle size range (optical measurement, load dispertion methods etc.).