Update long_name descriptions of number fraction of dust aerosols for clarity

[mkavulich]

Description

This PR is a follow-up on #114, updating the long name descriptions for number fraction of dust aerosol in GLOMAP/UKCA for more clarity

Issues

None

[climbfuji]

Do you know what "coarse-mode" means? I don't.

[mkavulich]

Do you know what "coarse-mode" means? I don't.

It seems as if this is an unfortunately vague and confusing term. There are generally 3-4 "modes" of aerosols that have phenomenological distinctions but are also somewhat strictly separated by diameter through various thresholds.

I was able to find this thesis that reviews the various "modes" of aerosols based on diameter:

Mode	Definition	Diameter
Coarse	Large aerosol particles produced by mechanical processes	> 1.0 µm
Accumulation (Fine)	Aerosol particles formed by growth of smaller particles via coalescence and gas deposition, with some direct emissions by incomplete combustion	0.1 µm - 1.0 µm
Aitken	Small aerosol particles formed by gas oxidation and/or condensation, typically short-lived with growth into accumulation region	0.01 µm - 0.1 µm
Ultrafine (Nucleation)	Not always treated separately, poorly studied	< 0.01 µm

But other definitions vary wildly; some denote "Ultrafine" as anything < 0.1 µm, some denote "Accumulation" or "Fine" as d < 2.5 m and "Coarse" as d > 2.5 m (https://www.gfdl.noaa.gov/wp-content/uploads/files/user_files/pag/lecture2008/lecture2.pdf). Some definitions are frustratingly non-contiguous (e.g. NASA's algorithm for separating "accumulation mode" 0.1 - 0.25 µm from "coarse mode" 1.0 - 2.5 µm). CAM's own aerosol models does not give distinct diameter definitions, but seems to treat them phenomenologically.

To sum it up, this seems like a hefty can of worms. We should definitely get more clarity from UKMET if possible, and perhaps replace these vague/context-dependent terms with stricter definitions.

[climbfuji]

Do you know what "coarse-mode" means? I don't.

It seems as if this is an unfortunately vague and confusing term. There are generally 3-4 "modes" of aerosols that have phenomenological distinctions but are also somewhat strictly separated by diameter through various thresholds.

I was able to find this thesis that reviews the various "modes" of aerosols based on diameter:

Mode Definition Diameter
Coarse Large aerosol particles produced by mechanical processes > 1.0 µm
Accumulation (Fine) Aerosol particles formed by growth of smaller particles via coalescence and gas deposition, with some direct emissions by incomplete combustion 0.1 µm - 1.0 µm
Aitken Small aerosol particles formed by gas oxidation and/or condensation, typically short-lived with growth into accumulation region 0.01 µm - 0.1 µm
Ultrafine (Nucleation) Not always treated separately, poorly studied < 0.01 µm
But other definitions vary wildly; some denote "Ultrafine" as anything < 0.1 µm, some denote "Accumulation" or "Fine" as d < 2.5 m and "Coarse" as d > 2.5 m (https://www.gfdl.noaa.gov/wp-content/uploads/files/user_files/pag/lecture2008/lecture2.pdf). Some definitions are frustratingly non-contiguous (e.g. NASA's algorithm for separating "accumulation mode" 0.1 - 0.25 µm from "coarse mode" 1.0 - 2.5 µm). CAM's own aerosol models does not give distinct diameter definitions, but seems to treat them phenomenologically.

To sum it up, this seems like a hefty can of worms. We should definitely get more clarity from UKMET if possible, and perhaps replace these vague/context-dependent terms with stricter definitions.

Thanks very much for this summary. I agree, a clearer definition would be preferrable.

[nusbaume]

Thanks @mkavulich!