Questions on optical property calculations in AeroCom phase II models with the purpose of a correct interpretation of diagnosed AOD properties in the AeroCom database:

1) Do you assume and how internal mixing when computing optical properties of total aerosol? Is Size varying? Which mixing assumption?

2) Do you differentiate between clear-sky and all-sky relative humidity and which one do you use for the AOD calculation, reported to AeroCom?

2b) If you differentiate, have you investigated the difference?

3) Is the reported-to-AeroCom AOD as asked under 2) used directly for the RF calculations?

4) Would you consider the diagnosed AOD provided to the Aerocom database as representative for clear-skies? Eg for comparison to Aeronet and satellite retrievals in clear skies?

5) If you diagnose a clear-sky AOD ⇒ How do you compute it? What are the assumptions on RH? Any threshold of RH you use? Cloud fraction weighting to derive a monthly average Clear-sky AOD?

6) Which humidity growth factor and variation do you use ? How does it depend on RH? Do you include sub-grid variability in relative humidity?

7) Do you diagnose the aerosol water mass or aerosol water AOD? How?

8) Is the diagnosed sea salt and sulfate AOD computed at ambient RH? And eventually is that using clear-sky or all-sky RH?

9) Do you average nighttime and daytime values of AOD into daily averages?

10) What is the single-scattering albedo of your BC, POM, DUST? Does it vary as a function of size, RH? How roughly? Which range?

11) What is the absorption coefficient of BC? How is it calculated?

1) Do you assume and how internal mixing when computing optical properties of total aerosol?

4 out of 7 modes are internally mixed

Is Size varying?

Yes, variation of median radius for each mode accounted in calculation of optical properties

Which mixing assumption?

Submitted is volume weighted mixing of refractive indices. Differences owing to alternative mixing rules explored in Stier et al., ACP (2007) were negligible as compared to the uncertainty in BC refractive indices.

2) Do you differentiate between clear-sky and all-sky relative humidity and which one do you use for the AOD calculation, reported to AeroCom?

Clear-sky relative humidity, calculated from the grid-box mean assuming saturation in the cloudy-fraction, is used in the AOD calculation.

2b) If you differentiate, have you investigated the difference?

Yes, long ago…

3) Is the reported-to-AeroCom AOD as asked under 2) used directly for the RF calculations?

Yes, online in the model.

4) Would you consider the diagnosed AOD provided to the Aerocom database as representative for clear-skies? Eg for comparison to Aeronet and satellite retrievals in clear skies?

Well, in as much as satellite retrievals are able to separate clear-skies. Ultimately, we will need to work with satellite simulators to answer this question.

5) If you diagnose a clear-sky AOD ⇒ How do you compute it? What are the assumptions on RH? Any threshold of RH you use?

See above.

Cloud fraction weighting to derive a monthly average Clear-sky AOD?

“Clear-sky” AOD is reported even for fully cloudy cells. In this case the grid-box mean RH is at saturation which is then used in the water uptake calculation. It would be good to sample better.

6) Which humidity growth factor and variation do you use ?

Kappa-Koehler theory.

How does it depend on RH?

Obvious.

Do you include sub-grid variability in relative humidity?

No.

7) Do you diagnose the aerosol water mass or aerosol water AOD? How?

Mass based on Kappa-Koehler, aerosol water AOD based on volume weighted attribution of AOD to the components (this works well for AOD, but component specific AAOD cannot be accurately computed).

8) Is the diagnosed sea salt and sulfate AOD computed at ambient RH? And eventually is that using clear-sky or all-sky RH?

All based on RH as above.

9) Do you average nighttime and daytime values of AOD into daily averages?

Yes. We also have satellite overpass sampled AOD but this reduces sampling strongly for single year runs.

10) What is the single-scattering albedo of your BC, POM, DUST? Does it vary as a function of size, RH? How roughly? Which range?

Calculated from Mie theory based on volume weighted mixing of refractive indices and mode size. Details in Stier et al., ACP, (2007).

11) What is the absorption coefficient of BC? How is it calculated?

Not directly calculated. Can be diagnosed as for Phase I.