1. The Aerosol Microphysics Model M7
IACETH ECHAM/HAM presentation series
Nr. 2,

2. M7: background and short history
M7 was conceived as a model capable of simulating the time evolution of a multi-component, size-resolved aerosol population
Originally 0-D (a box model)
First developed at JRC, Ispra by J. Wilson and E. Vignati with contributions from J. Feichter (MPI-M, Hamburg)
Adapted to 3 dimensions and set into the ECHAM5/HAM framework by P. Stier as part of his PhD work
Further developments: new nucleation schemes (J. Kazil, ), new SO4 condensation scheme (J. Kazil, 2007), new water uptake scheme (D. O’Donnell, 2008), greater flexibility with respect to included species (D. O’Donnell 2009), some optimisation for scalar machines (D. O’Donnell 2009).
Basic aerosol dynamics scheme unchanged since inception

3. M7: lognormal modes and why 7?
Observations show that ambient aerosol numbers can be well characterised as a set of one or more lognormal distributions, termed ‘modes’.
n: number of particles in the size range r, r+dr as dr→0
Ni: number of particles in mode i
ri : median radius of particles in mode i
σi: geometric standard deviation of mode i
Kavlitis et al. ACP 2008
Aerosols are observed over several orders of magnitude in size, ranging from nm to many μm
The largest particles (>> 1 μm) are of less interest due to their short lifetime (because of rapid sedimentation)
Particles may be soluble (able to take up water) or insoluble (except: not observed in finest (nm) mode)

4. Composition matters, too…
So, the lognormal distribution can tell us about number. What about aerosol composition ?
We know that ambient aerosol generally contains a mixture several chemical components
Since each component has its own density, it follows that aerosol mass can vary independently of aerosol number, but it also lognormally distributed with the same geometric standard deviation if the particles are assumed to be composed of identical mixtures (aka internally mixed)
Thus a lognormal distribution of an internally mixed aerosol population can be characterised by :
The total aerosol number
The total aerosol mass
The geometric standard deviation
In M7, geometric standard deviation is prescribed for each mode. For the fine (<1 μm) modes, σi = 1.59, for the coarse modes σi = 2
Soluble
N(D)
Nucleation
Aitken
Accumulation
Coarse
Insoluble
Sulphate
Black Carbon
Organic Carbon
Sea Salt
Dust
Log D

5. Where M7 fits in ECHAM5/HAM
SO2 OH
SO4
NOx O3
Atmospheric transport
(ECHAM)
Dry deposition
(HAM/HAMMOZ)
Chemistry (SO4 and optionally SOA)
(HAM/HAMMOZ)
Emissions (HAM)
Nucleation
Aitken
Accumulation
Coarse
Wet deposition
(HAM/HAMMOZ)
Sedimentation
(HAM/HAMMOZ)
M7 Microphysics

6. Processes modelled by M7
Calculation of the median dry particle radius per mode
Water uptake on soluble modes and hence the median wet particle radius per soluble mode
Condensation of gas phase sulphate
Nucleation of new particles
Ageing of insoluble particles (coating by sulphate and transfer to soluble modes)
Coagulation of particles
Particle growth (transfer of mass and number to larger modes)

7. M7: inputs, outputs ECHAM Full, half level pressure
Temperature & tendency
Specific humidity & tendency
Tracer concentrations & tendencies
Fractional cloud cover ….
ECHAM M7
Updated tracer concentrations
& tendencies
Diagnostics:
Median particle dry radius per mode
Median particle wet radius per soluble mode
Particle density per mode
Water content per soluble mode
Nucleation rate
Condensation of SO4

8. M7: BEWARE OF UNITS! (and know about the RH)
ECHAM M7
SO4 tracers: kg(S) / kg(air)
Other HAM mass tracers: kg / kg(air)
HAM aerosol numbers: # / kg(air)
SO4 tracers: molecules (S) / cm3
Other HAM mass tracers: μg / m3
HAM aerosol numbers: # / cm3
Mostly cgs units
Gridbox mean specific humidity q
Cloud-free mean specific humidity qcf :
q = f qs + (1 – f)qcf
where qs = saturation specific humidity wrt water
f = cloud fraction

9. M7: work flow ECHAM interface, unit conversion
Mean particle mass, all modes, and mean radius and density, insoluble modes
Mean particle mass, all modes, and mean dry radius and density, insoluble modes
Water uptake; median radius and density, soluble modes
Water uptake; median radius and density, soluble modes
Redistribution of mass and number between modes
Calculate H2SO4 condensation sink
Calculate SO4 condensation flux on each mode
Mean particle mass, all modes, and mean radius and density, insoluble modes
Calculate coagulation kernel
Water uptake; median radius and density, soluble modes
New particle nucleation
Diagnostics
Unit conversion and return to ECHAM
Aerosol number dynamics
Insoluble → soluble transfer through SO4 coating

10. M7 : water uptake Two alternatives for water uptake on aerosols:
Petters and Kreidenweis, ACP 2007 (SO4, SS, organics), aka Kappa-Köhler theory approach (default)
Vignati, Wilson and Stier (JGR 2004), sulphate, and Jacobson et al. (1996) for particles containing sea salt
Alternative 1:
Petters and Kreidenweis showed that the hygroscopic growth factor (gf) of a multicomponent aerosol can be expressed as a function of temperature (T), relative humiidty (RH), dry diameter (Dd) and κ :
This is solved offline for gf using T, RH, κ and Dd of atmospheric relevance and the results stored in a lookup table (lut_kappa.nc).
In runtime, we first calculate the dry diameter Dd and the κ value for each soluble mode, then simply look up the growth factor to get the wet diameter (see: m7_kappa, mo_aero_kappa)
Alternative 2:
For particles with negliglible sea salt, a parameterisation of water uptake by sulphuric acid derived by J. Wilson is used (polynomial in log [SO4], T, RH) (See: m7_equiz, m7_equimix)
For particles containing sea salt, the ZSR approach is applied: for calculation of binary molalities an empirical approach is also used (See: m7_equil)

11. M7: nucleation Alternatives:
nucleation of new particles from sulphuric acid and water (Vehkamaeki et al., 2002)
neutral and charged nucleation of H2SO4 and water (Lovejoy et al, 2004, Hanson and Lovejoy, 2006)
nucleation in the presence of organics (Kulmala et al., 2006, Riipenen et al. 2007)
Vehkamaeki et al is based on classical nucleation theory, accounting for hydration. Known to give too low nucleation rates in the lower troposphere.
Neutral and charged nucleation scheme uses a lookup table with inputs T, RH, [H2SO4], condensation sink strength of H2SO4, and ionisation rate and output formation rate of clusters of ≥ 15 molecules. Now the default.
Ionisation rate is calculated as a function of galactic cosmic ray intensity, taking into account the solar cycle
Organic nucleation schemes actually depend on [H2SO4] or [H2SO4]2 rather than directly on the concentration of organics in the atmosphere in accordance with observations a Hyytiälä. The organics are supposed to be represented by a multiplicative constant. In M7, this is applied only in the boundary layer in the forested fraction. Default = neither of the organic schemes

12. M7: now for the difficult part
The general aerosol dynamics equation (GADE) as applicable to M7 can be written
where
Ni = number of particles in mode i
aii = intra-modal coagulation coefficient
bij = inter-modal coagulation coefficient
ci = nucleation rate (ci = 0 for i > 1)
To save CPU, some aii and bij are set to zero.
calculation of coagulation coefficients follows Fuchs (1964) in terms of diffusion, thermal velocity and mean free path.
For insoluble modes, condensation of SO4 can cause transfer to the soluble modes
Vignati et al, JGR, …4: soluble modes, 2i…4i: insoluble modes

13. M7 aerosol number microphysics: implementation
m7_dnum
The “master” of the procedure
m7_coaset
Calculates the coagulation coefficients
m7_delcoa
Solves the GADE for soluble modes
m7_concoag
Calculates transfer from insoluble to soluble modes with the help of m7_coat
m7_coat
Calculates the number of molecules of sulphate needed to coat a particle of given radius with a monolayer of sulphate

14. M7 : final steps
The aforementioned processes can change the size distribution substantially
To re-balance the coexisting modes, export of mass and number takes place from smaller to larger modes
We integrate the number distribution function up to the mode upper boundary. “Excess” particles (in terms of both mass of each species and particle number) are exported to the next larger mode
Implementation: m7_dconc, m7_cumnor
Finally, repeat the water uptake calculation, store the diagnostics, do unit conversions and return to ECHAM

15. Thank you for your attention!