RFM Aerosols
Contents
10JUL26

Introduction

The RFM only considers 'aerosol' as an absorber/emitter (i.e. no scattering), which is realistic if the aerosol particle size is much smaller than the wavelength.

In the absence of scattering, aerosol behaves, radiatively, like any molecular absorber. The difference is that the concentration and absorption cross-sections are expressed in different units (i.e., not in terms of molecules, or even aerosol particles).

Implementation

Aerosol extinction (=scattering+absorption) is expressed in units of inverse length e.g., if extinction κ=0.01/m then the intensity of a beam I is reduced by approximately 1% for every metre along path s travelled through the aerosol.
I = I0 exp ( − κ s )

Comparing the aerosol extinction with the extinction (or absorption) due to a molecule of mole fraction v (≃mixing ratio) and absorption cross-section σ (m2/mole) in air density ρ (moles/m3)

I = I0 exp ( − σ v ρ s )

Thus κ/ρ for aerosol is equivalent to σv for a molecule. Note that while the molecular absorption has separate components for concentration (v) and spectral dependence (σ), these are combined in the aerosol extinction κ since ρ is just the air density.

This equivalence is the basis of the RFM handling of aerosols. Internally, the RFM takes the extinction κ (in km-1) and converts it to a VMR-like quantity v' (in ppmv)

v' = 10 κ / ρ
where ρ is the local air number density in molecules/cm3. The factor 10 is the product of 10-5 from the conversion of extinction/km to extinction/cm, and 106 from conversion of VMR to ppmv.

Aerosol is then handled like any cross-section molecule, i.e., with absorption coefficient defined by a .xsc file.

Adding Aerosol to the RFM

As with other cross-section molecules, the driver table should contain the following:

At a particular height and wavenumber, the extinction is then calculated as the product

κ(z,ν) = κ(z,ν0) × κ'(ν)

Adding More Aerosol to the RFM

The RFM can only handle one aerosol explicitly, i.e. one profile and one spectral dependence of absorber AEROSOL.

If you need two or more aerosols there are two options.

1. Use p dependence of aerosol.xsc
Using the pressure axis of the .xsc to create different absorption cross-sections to be used at different pressure levels in the atmosphere, each cross-section being a sum of the individual aerosol extinction coefficients at that pressure. Effectively the aerosol relative concentrations as a function of altitude are fixed by the .xsc file.

2. Treat additional aerosol as a cross-section molecule
A more flexible alternative is to add extra aerosol as a cross-section molecule.
  1. Select name (up to 7 characters), e.g. Sulfate (case does not matter).
  2. Add this as an absorber in the *GAS section of the driver table.
  3. Create an .atm file containing profile *Sulfate of VMR-like values v' using the above equation, and add this file to the *ATM section.
  4. Create a .xsc file containing the spectral scaling factors, eg Sulfate.xsc, and add this file to the *XSC section. This has to contain the name Sulfate in the MOLEC field.