Very good explanation. Nevertheless, what have left to be said is aerosol particle size spectrum ranges from about 1 nm to 100 micrometer and aerosol particle has to be be enough stable to be sufficiently detected. It is a bit cunning definition but reflects complexity of aerosol as multiphase system.

Thanks Dr. Jan and Dr. Constantin

Aerosol particles, within the atmosphere, are everything that is not just gas.

Size distribution is not really a matter: airplanes, birds, insects and other huge flying things will just fall down quickly.

The lifetime in the atmosphere of smaller particles does not qualify them as aerosol, it is just a specific property of those particles.

Condensation nuclei and rain droplets are aerosol, as well as ice. The only thing is that it is a special kind of aerosol: we suppose to know the composition.

Somehow, the definition could change depending on the context. The best definition, for me, is in the following reference.

Baron, P., and Willeke. K. Aerosol measurement: principles, techniques, and applications. Wiley, 2001..

@last answerer

I do not have the book; thus what is their definition?

As for an aircraft and flies and rain: those are NOT an aerosol: an aerosol is a gas-system with (homogeneously) dispersed particles

From they introduction:

The term aerosol refers to suspension of liquid or solid particles in a gaseous medium. The term originated as the gas-phase analogue to hydrosols and refers to suspension of particles in a liquid.

Aerosols are two-phase systems, consisting of the suspended solid or liquid phase, and the surrounding gas phase. Aerosols are formed by the conversions of gases to particles or by the disintegration of liquids or solid into finer constituents. Aerosols are quite ubiquitous; airborne particles from resuspended soil, cloud droplets, welding fumes[...]

I do not understand: you say that aerosol is a gas-system with homogeneously dispersed particles, but aerosols are constituted by single particles, that often can be seen one by one.

I understand that aircraft are somehow an extreme, that could require more effort to be considered aerosol, but insects can be found also in the stratosphere, hibernated or death, floating for long time before falling. Also those "particles" have their impact.

Rain and other meteorological events, with big particles falling and maybe evaporating long the path, or just floating following thermals are still aerosol. Their respect your definition within a reasonable volume of air.

The volume considered is a relevant issue, near a cumulus, different volumes have different size distributions, taking larger volumes the homogeneity is missing.

However, In ordinary conditions I agree that real big particles are not common, but what about a tornado?

Single particles are not often taken into account, but their group properties.

The way aerosols can be defined depends by many factors, especially by the way we measure them, and the effects we want to evaluate.

I am sorry

The definition of AN aerosol is the SYSTEM of gas plus particles. People use the term aerosols when they mean aerosol PARTICLES.

One of the criteria is that the particles are of a size that they can NOT be seen one by one because then the system is very likely not homogenized.

Rain is definitely beyond the limits of an aerosol and certainly a single insect.

Also, Baron is not a reliable source because he is an experimentalist. I would rather have a look at for instance Friedlander, "smoke dust and haze". He can be considered as one of the godfathers of aerosol science. Unfortunately I do not have a private copy here.

Thanks all for your valuable opinions but there are still doubts stand in my mind... I have read somewhere that Particulate matter/aerosol consists of tiny particles in the atmosphere that can be solid or liquid (except for water or ice) and is produced by a wide variety of natural and manmade sources. And Mr. Biavati is saying that even aeroplane is also a form of aerosol..... How it is possible?

http://www.learningace.com/doc/691356/ca52a1553a791d86d601e6792e51bc99/cuclis

https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CCoQFjAA&url=http%3A%2F%2Fwww.ruf.rice.edu%2F~eesi%2Fscs%2FCuclis.ppt&ei=2zpEUoj4H4rprQf8rYGQCA&usg=AFQjCNEuEUR9mHn1cADxDzYcf4TNrpYr_g

I am sorry for creating confusion,

but for me the definition of aerosol is a very important thing, and so I appreciate the discussion that we are developing here.

Probably the best and practical definition of aerosol is the one proposed by Dr Constantin Andronache. He refers to a system of colloidal particles dispersed in a gas.

You can live with this definition and refuse that ice and water are not aerosols, because in most of the conditions they fit to this definition.

now I will try to express some arguments to explain why one could consider unusual things as aerosols:

Measuring the optical depth of the atmosphere with passive remote sensing techniques ground based like with radiometers, we see attenuation of the sun light caused by the atmosphere. the measurement is performed with a certain time window, so the attenuation is estimated as an average attenuation of the air masses flowing over the instrument.

The instrument has a field of view, all the things entering within it reduce the energy received and will affect the estimated average optical depth.

The integration time and so the explored volume dilute all the particles, as well as the huge ones. They cant be discriminated from aerosols when their solid angle is small enough.

I hope now if my point of view is clear enough: everything depends on the size of the volume considered and by the distance we look at it.

I refer to the UNESCO encyclopedia for life sciences

I wrote the chapter on aerosol. The frist pages are on the web for free. I think I can call myself an authority here because I was asked to write this chapter

http://www.eolss.net/Sample-Chapters/C06/E6-13-02-01.pdf

I also have an advice for a next time:

1. Go to Wikipedia

2. Google

Then ask via ResearchGate

I'm not saying that you are creating any confusion Mr. Gionata Biavati .. Sorry for that but I was just clearing my doubts...

Pradyumn

I read myself the entry "Aerosol" in Wikipedia. The article is not very good. The problem with Wikipedia is that everyone can write and sometimes they are no experts.

The writer cites three books, but in these books the water-droplets (hydrometeors) are not described.

I liked this discussion because many people that I know always make confusion with all the technical terms. For example, they say that the "correct" term is particulate matter and not aerosol... or that particulate matter does not include water, and so on. The only thing I may say is that it seems that authors use the term aerosol and particulate matter as synonyms.

Dayana

Apologies; I have to add to the confusion or rather there is a formal difference between Particulate Matter and aerosol.

Particulate Matter (abbreviated as PM) is the suspended material in total and the measure is the mass. The unit is: microgram / m3. It is determined by collecting of the material on a filter by drawing air through a filter. It is almost like a vacuum cleaner.

Also: the mass is the DRY mass of the suspended material. This means that the mass is measured after the filter with the collected material on it has been dried.

Aerosol is seen as a system of particles in a gas (air). The unit used here is the number of particles per cm3. Furthermore the aerosol particles are characterized by the size distribution.

Also: the size of aerosol particles includes the so-called aerosol-water. This water is there because most particles in the atmosphere contain hygroscopic components. These components are seasalt and ammonium-sulphate. These attract water.

The water is important for the effect that aerosol particles have in the climate. Aerosol particles reflect sunlight. This is a cooling effect. The water in the aerosol also reflects sunlight.

This is a very simplified summary

Thanks for the explanation! Just another question to add a little more confusion... Does the new technologies to measure PM without using a "filter" may be called as particulate matter? I do not hope to understand every single measurement technology. Again thanks for the explanation, from now on I will read and analyze the works with another look. Hope everyone could read this explanation and not confuse the two terms.

Which new technologies do you mean?

TEOM and beta-gauge automated monitors also use filters.

CPM optical counters that estimates concentration or SMPS for example.

optical counters are indeed used as surrogates, but are not officially accepted as monitors. The reason is that such counters do not measure mass. They measure the light reflected by single particles or by the total of the aerosol but the relation between the amount of light scattered and the mass is not known. What is done is using the optical counters (best known is the Grimm instrument, also Osiris) together with a TEOM at the same station for a certain time and then see if there is a correlation. And then use this correlation for the rest of the time.

For a performance test see:

http://www.luchtmetingen.amsterdam.nl/pool/3/documents/Fieldexperimenton11automatedPMmonitorsfinalversion.pdf

CPM counters are used for emission measurements, I think

Thanks for the link. CPM is an optical counter that uses the same principle scattering, but cannot count nanoparticles (> 0.3 um).

Dayana

Cheap Optical counters do have this lower limit. Only the very advanced single particle counters, like the LAS-X have a lower detection limit of 0.1 um.

Still, I like to ask you if CPM has an instrument for atmospheric use.

Is a module used with a beta-gauge monitor for counting particles, a continuous measurement. There is a link showing the picture. It is used for atmospheric measurements... http://www.environnement-sa.com/products-page/en/air-quality-monitoring-en/particulates-samplers-and-analyzers-en/mp101m-with-cpm-option/?cat=88

Dyana

It appears that this French producer uses the CPM. I tried to read their manual but one needs to login and it is in French?! They also provide a number concentration in number per liter. This absolutely not common in atmospheric science

They do not mention which branch of beta-gauge monitor they have in conjunction with the optical counter.

I have this "brochure"

I have this "brochure"

Dayana

Thank you. In this brochure I notice that in the DESCRIPTION it says:

numbers in nb/liter

Under CPA module specifications:

Max number of counted particles: 200,000 / cm3

This number sounds to high compared to the more advanced counters that have a lower upper limit. This limit is governed by "coincidence; this means that more than particle are in the sensing volume at the time

Actually the max number is very likely the number per liter

Few years back I wrote a blog about an article by Jaenicke. It discusses definition of the aerosol.

Jaenicke, R. (2008). Is atmospheric aerosol an aerosol?-a look at sources and variability. Faraday Discuss 137, 235-243

http://aerosols.blogspot.in/2008/01/is-atmospheric-aerosol-aerosol-comments.html

Harish

(Prof / Rupprecht) Jaenicke is an authority on atmospheric aerosol(s). Unfortunately the original publication is behind a pay wall.

Luckily, Prof. Jaenicke is available on ResearchGate. I guess he will be glad to share reprint. Following is link to the article through his profile.

Article Is atmospheric aerosol an aerosol?—A look at sources and variability

Harish

I know him well and his work, especially that done in the 60's and 70's when he was the first to document the atmospheric aerosol. We worked together leading a large European consortium some 15 years ago, as you can see at my site here.

My remark was general. Most publications that are cited in forums of ResearchGate are not open access and thus students who would like to read these cannot approach the publications:

also the publication you mention here has not a full text attached.

Harish

Thank you for the publication. Had not seen your mail before my above entry.

Duncan

First you mention aerosol of a size of 3 nanometers then you mention hydrometeors that are of the order of micrometers. Lifted dust is of the same size. Summary: consult a textbook on aerosols like that of Friedlander or Hidy before you make an entry

More than going trough a confusing literature about the definition of aerosol, I recommend to think first to the context of the measurements, then to the kind of measurements, then to what really matter. Aerosol is not a big molecule, but an aggregation, no matter the aggregated molecules. The phase state of the aggregation is in the most of the measurements totally irrelevant. However it could be also a huge molecule, because there is no definition of aerosol-s, or there are too many, this should give to anybody the ability to consider aerosols whatever aggregation in a gaseous environment.

I hope that at a certain point we will reach a definition more precise, but do we need it? Is aerosol definition on any relevance? Is it worth to think about it?

.

When something is defined in such a way, it it not defined at all, no matter who is the professor defining it, it remains a useless definition, bounded to the context of the measurement performed.

Gionata

Aerosol is NOT an aggregation. Aerosol is by a colloidal system, like a hydrosol. ONE particle of that system can be an "aggregation". You also say "aerosol is not a big molecule" and 2 sentences later: "it could also be a huge molecule". Please give a practical example of such an entity in air with sufficient number to call the total system an aerosol in the sense of a gaseous system with embedded molecules, not a few scattered molecules in a confinement

Jonathan

Very good remark; I think historically it was connected with the size that could be detected with an (Aitken?) condensation nucleus counter.

Yes. Small droplets or particles of water or ice suspended in air are considered aerosols.

The simplest form of aerosol is a solid or liquid suspended in a gas. Some other terms for solids or liquids suspended in a gas are dust, smoke, fume, fog, cloud, and mist. There is no strict definition of aerosol and there is no strict definition of suspended. The use of the term 'colloid' implies a uniform suspension, but there is no strict definition of uniform.

Suspended means not falling, but all particles have a settling velocity, as do gases. Particles of aerodynamic diameters greater than 100 micrometers have settling velocities greater than 0.02 m/s. Nevertheless, such particles remain suspended for considerable time.

There may be 'operational' definitions for particle size (aerodynamic), uniformity over a given volume, uniformity for a given time, settling rate, and other constraints, but there is no strict or agreed upon definition for an aerosol.

A water spray from a simple nozzle may have a size distribution from near molecular size to drops that are suspended only by momentum. A simple water spray produces aerosol sized particles, but may not meet some definitions of aerosol.

(It was interesting to learn that experimentalists are not reliable sources.)