Aerosols and particle formation

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How do aerosols and particles form?

basis

Aerosols are solid (Greek: sol) and / or liquid particles that float in the air (Greek: Aero). The term aerosol encompasses a broad spectrum from sea salt particles, pollen and mineral dust to oil droplets and other micro particles. Separated from the aerosol are water droplets and ice crystals, which can usually only be formed by the presence of other aerosol particles.

Occurrence and formation

Atmospheric aerosols generally range in size from 1 nm (1 millionth of a millimeter) to about 100 μm (1 / 10th of a mm) and have a variety of sources. Their concentration is usually above the seas and oceans, as well as remote continental areas in the order of about 1000 to about 10,000 particles per cm³. In cities, it can rise to over 100,000 particles per cm³, especially in winter during the heating season or due to traffic (soot emissions or oxidation of aromatic hydrocarbons).

Depending on which process the individual aerosol particles arise, they dominate in a certain particle size range. A distinction is made here between the formation of particles from solids (bulk-to-particle conversion (BPC)), the formation of particles from gases (gas-to-particle conversion (GPC) and the formation of particles by reactions of solutes in cloud droplets (optical measurements, charge distribution measurements, etc.).

 

Particle formation from solids

Bulk-to-particle-conversion

 

Many particles are formed from already existing materials, e.g. Mineral dust (e.g., from deserts), clay minerals (soil erosion), or primarily biological particles (e.g., pollen, bacteria, spores, insect fragments from vegetation) by wind. As a further example, the sea salt is released into the air by foam formation on the sea or by the tearing of the sea surface by rising gases dissolved in the water and dries off to pure salt at low humidity (<30%). In contrast, mineral dust (such as Saharan dust) enters the atmosphere through the process of saltation, the impact of small particles with other rolling or bouncing particles (such as sand dune).

These particles, which are all formed from already existing material, as well as the diverse biological particles are essentially found in the size range above 0.1 μm in radius. This is the size range of the large (0.1 μm - 1 μm in radius) and the giant particles (1 μm and larger).

Particle formation from the gas phase

Gas-to-particle-conversion

 

If the aerosol substance must first be formed from the gas phase for a particle, it is found mainly in the lowest size range (less than 0.1 μm), the range of the Aitkenteilchen (1 nm - 0.1 microns in radius). In this case, a chemical reaction takes place in which relatively volatile substances such as e.g. SO2 and water a more volatile substance such as Sulfuric acid forms. This reacts trace gases, which are present in small quantities in the atmosphere, and water from the normal humidity. One speaks because of this conversion of substances also of secondary particles. The newly formed nonvolatile molecules then form clusters (molecular aggregates) on whose surface further substances condense. From 1 nm size, they are called aerosol.

The complex of particle formation by the GPC process is currently being intensively researched, since many factors have an influence on the amount and type of particle regeneration. In part, the educational paths are not yet known. There are two main ways of discussing this:
on the one hand sulfuric acid droplets from the reaction of water with already formed sulfuric acid and ammonia on the other hand, the formation of secondary organic particles from the oxidation of volatile hydrocarbons such. Terpenes (terpenes are released from the forests - plant fragrances) or aromatics such as toluene or benzene (mainly formed by human influence in the cities). The oxidation is carried out by the three main oxidizing agents of the atmosphere ozone, OH and NO3.

Formation of cloud droplets

Formation of aerosol particles in clouds

 

Finally, one must name the aerosol particles that are formed or changed by cloud droplets. If an aerosol particle (cloud condensation nucleus) dissolves in the absorbed water (cloud droplets) and then reacts with other substances dissolved in the droplets, it can form a new aerosol substance, from which a new aerosol particle forms when the water evaporates. The main part of the clouds does not settle as rain, but evaporates again.

 

Once the aerosols are in the atmosphere, they are removed from the air by various processes:

a) Diffusion (= striving for a uniform distribution): Smaller aerosol particles spread (diffuse) in the air, thereby encountering larger particles and unite with these large aerosol particles;

b) Sedimentation: With larger aerosol particles, the influence of gravitational acceleration increases and they fall to the ground;

c) raining and leaching: aerosols in cloud droplets or aerosols caught by cloud droplets are washed out of the atmosphere in rainwater;

d) Impact: Particles will adhere to obstacles when they hit obstacles.

The particles with a radius of 0.3 μm are the longest in the air, since these processes have the least effect on them.

e) and radiation emanating from the earth (especially infrared radiation): it absorbs, emits and scatters the radiation directly, through processes inside and at the surface of the particles. Indirectly, the aerosol particles have a further influence on the radiation balance on the above-mentioned effect of cloud droplet formation. The more aerosol particles that can form droplets of cloud, the more and the smaller the droplets form, which in turn absorb and scatter radiation. The indirect effect affects differently the incident and reflected and emitted by the earth radiation. However, both effects of the atmospheric aerosol and their effects on the climate are not yet well understood.

particle Sizing

Measurement

 

To measure aerosol particles really and also from compressed gases, many tasks have to be solved. The main task is the enormous size range of aerosols, which meets high requirements such as as the big difference in sensitivity, depending on the particle size, pressure range, places on the equipment.

As a trick you can, for example the particles grow by condensation, so that they can be counted by optical methods. Their mass can be e.g. determine by filter collection or impactor samples. In impactors, a gas flow is simply steered around the corner and the contained heavy particles do not get the curve, so to speak, and hit a baffle on which they are collected.

Many other measuring methods have been developed, which, however, can usually only be applied to a partial size range of the aerosol (optical measurements, charge distribution measurements, etc.).

 

TPS technology offers a complete range of Particle Sizers for purchase or, if needed, these measurements as a service.