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?

12) Please provide the refractive index at 0.55 microns for each aerosol species? And eventually further information on the hygroscopicity for each species.

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

50% BC from fossil fuel is internally mixed with POM. All BCs from the other sources are internally mixed with POM. The other aerosol species are completely in external mixture. Size of the internally-mixed BC is according to that of POM. It is assumed to mix homogeneously.

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?

The relative humidity is available only for the all-sky.

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

No. AOD is calculated at the specific wavelengths. The broadband information is essential to calculate the RF. The aerosol mass mixing ratio is transferred to the radiation code, then the RF is calculated in it.

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?

Yes.

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?

The clear-sky AOD can be calculated when the 2D cloud fraction is under 0.2 in each time step. If the cloud fraction is over 0.2, the clear-sky AOD is a missing value. The time-averaged AOD for the clear-sky is estimated by excluding the missing value.

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?

The growth factors are according to Tang and Munkelwitz (1994) for sulfate, Hobbs et al. (1997) for POM, and Shettle and Fenn (1979) for sea salt.

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

The aerosol water mass is diagnosed.

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

Yes. The all-sky RH is used.

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

Yes.

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?

At 550nm: 0.0165 for BC; 0.958 in 0%RH, 0.967 in 50%RH, 0.969 in 70%RH, 0.987 in 80%RH, 0.993 in 90%RH, 0.995 in 95%RH, 0.998 in 98%RH, 0.999 in 99%RH for POM; 0.987 in 0.13µm, 0.962 in 0.33µm, 0.987 in 0.82µm, 0.954 in 1.27µm, 0.983 in 3.20µm, 0.975 in 8.02µm for DUST.

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

2.774 m2/g at 550nm with the Mie theory (radius: 0.0118µm, standard deviation: 2.0, density: 2.3 g/cm3 with log-normal size distribution).

12) Please provide the refractive index at 0.55 microns for each aerosol species? And eventually further information on the hygroscopicity for each species.

At 550nm: (1.430,1e-8) for sulfate; (1.381,4.26e-9) for sea salt; (1.530,6e-3) for POM; (1.750,0.440) for BC; (1.530,2e-3) for dust; The homogeneous mixture with water is assumed in the hygroscopicity with the refractive index of (1.333,1.96e9) for water.

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.

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

External mixing only, size distributions prescribed for each aerosol component.

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?

The model differentiates between clear-sky and all-sky relative humidity, and AOD calculations use the clear-sky RH.

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

Quickly (unpublished).

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

No. Radiative flux calculations use waveband-averaged specific scattering and absorbing coefficients, and aerosol mass mixing ratios. AODs are diagnostic only.

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?

Yes, diagnosed AODs represent clear-sky conditions.

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?

Clear-sky AOD uses clear-sky RH, with no threshold on RH (i.e. calculations go up to 100% RH). Cloud fraction is only used when computing clear-sky RH (assuming cloudy-sky specific humidity is at qsat).

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?

Hygroscopic growth is parametrized as a function of RH following Fitzgerald [1975]. RH is gridbox mean only, although q = qsat is assumed in cloudy sky when computing clear-sky RH.

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

Aerosol water AOD is diagnosed as the difference between standard AOD and the AOD computed without accounting for hygroscopic growth (i.e. aerosol size and refractive index remain those of the dry aerosol).

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

Use clear-sky RH computed from ambient gridbox-mean RH and cloud fraction.

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

Yes.

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?

Please refer to Table 4 of Bellouin et al. (2011)

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

For FFBC, absorption coefficient is 3.2 m2g-1 at 0.55 microns. Computed using Mie theory from prescribed size distribution and refractive index.

12) Please provide the refractive index at 0.55 microns for each aerosol species? And eventually further information on the hygroscopicity for each species.

Please refer to Table 4 of Bellouin et al. (2011), and the Appendix of the manuscript for details on aerosol optical properties in HadGEM.

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

Internal vs. external mixing is determined on aerosol process level: coagulation, condensation, and cloud processing gives internal mixing with pre-existing particles, which grow in size due these processes. For mixing of absorbing and transparent constituent the Maxwell-Garnett mixing rule is applyed, otherwise volume mixing.

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?

With respect to aerosol treatment, no. Both OD550_AER (all-sky AOD) and OD550CS_AER (all-sky AOD) are calculated at ambient relative humidities (RH), but OD550CS_AER is a clear-sky fraction (=1-CLDTOT) weighted AOD.

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

For CAM4-Oslo the two are found to differ quite significantly, especially at mid to high latitudes. Negligible differences in areas with low RH. Globally averaged, the relative difference is 16% (clea-sky 0.128 vs. all-sky 0.153).

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

All-sky AOD is used in the RF-calculations*. The only RH limitation is an upper cut-off value of RH=99.5% in the optics look-up tables. *More precicely, spectrally resolved 3-dimensional extinction coefficient (unit m**-1), single scattering albedo and asymmetry parameter are used in the radiative transfer code.

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?

Since AERONET uses cloud-screening, I would consider some kind of clear-sky AOD estimate to be better than the all-sky AOD.

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?

OD550CS_AER is a clear-sky fraction (=1-CLDTOT) weighted OD550_AER, but with no explicite RH screening.

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?

Growth factors for pure (unmixed) aerosol constituents are based on the GADS data, but assuming linear growth factors in the hysteresis range for sea-salt and ammonium sulfate. For internally mixed aerosols the growth factor is calculated from the Kohler theory, taking the hygrocopicity of each mixed constituent into account.

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

Yes, both. AOD: od550aerh2o is calculated as OD550_AER (ambient RH) - OD550_AER (RH=0%). Water mass: mmraerh2o is the mixing ratio for aerosol-water (from hygroscopic growth).

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

Yes, using all-sky assumptions (for RH and cloud cover).

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

Yes, 24h AOD values are used.

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?

For a given wavelength, SSA varies both with size (distribution) and state of mixing with other aerosol constituents and condensed water (RH). Approximate range for BC: [Will fill in later] POM: [Will fill in later] DUST: [Will fill in later]

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

It varies with size and state of mixing (also with RH when internally mixed) [Will fill in example/range later] It's calculated using Mie-theory and Maxwell-garnett mixing for refractive index when internally mixted.

12) Please provide the refractive index at 0.55 microns for each aerosol species? And eventually further information on the hygroscopicity for each species. Refractive index at 0.55 micron is (2.0,1.0), (1.53,0.006), (1.56,0.0025), (1.5,4.1.e-8), (1.43,1.e-8) for BC (with monomer density), POM, dust, sea-salt, sulfate, respectively. Hygroscopicities: [Will fill in later]

1) Do you assume and how internal mixing when computing optical properties of total aerosol? Is Size varying? Which mixing assumption? internal mixing within each of 3 modes. size varies with mass/number. volume mixing of refractive indices of components within mode.

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? No.

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? Yes.

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? Sort of. The grid cell mean RH is used, which is an approximation. Caveat: we only treat optics of interstitial aerosol.

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? Kappa Kohler theory using volume mean kappa. Dry if RH<RH_crystalization. Wet if RH>RH_deliquescence. Linear in RH between. No subgrid variability in RH.

7) Do you diagnose the aerosol water mass or aerosol water AOD? How? Aerosol water from volume mean wet radius.

8) Is the diagnosed sea salt and sulfate AOD computed at ambient RH? Not sure what is meant by ambient RH. And eventually is that using clear-sky or all-sky RH? All-sky RH is used.

9) Do you average nighttime and daytime values of AOD into daily averages? No. Day only.

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? Properties diagnosed from volume mean refractive index and wet surface mode radius.

11) What is the absorption coefficient of BC? How is it calculated? Diagnosed from Mie theory assuming refractive index of (1.95,0.79). Depends on size. Emitted with 0.08 micron number mode radius and 1.8 geometric standard deviation.

12) Please provide the refractive index at 0.55 microns for each aerosol species? And eventually further information on the hygroscopicity for each species. Refractive index at 0.55 micron is (1.95,0.79), (1.53,0.005665), (1.53,0.00627), (1.53,0.005665), (1.43,1.e-8), (1.49,1.e-8) for BC, POM, dust, SOA, sulfate, respectively. Hygroscopocity is 0, 0.10, 0.068, 0.14, 0.507, 1.16 for BC, POM, dust, SOA, sulfate, sea salt, respectively.