# Geometric mean diameter of sea salt aerosols in CMAQv5.0.2

Hello everyone! I would like to know the properties related to the accumulation of modal sea salt aerosols in CMAQ 5.0.2, such as geometric mean diameter and standard deviation. I see some arrays at the beginning of SSEMIS.F, each containing 136 values. I understand that this is calculated from the relative humidity RH. So is the geometric mean diameter of the accumulation mode sea salt aerosol the range of these 136 values? Or do I then take the geometric mean or arithmetic mean of them? Is the geometric standard deviation the same?

Also I see in AERO_DATA.F:
`Real, Parameter :: def_diam( n_mode ) = (/ 1.0E-8, 7.0E-8, 1.0E-6 /) ! default background mean diameter for each mode.`
Does this mean that the default accumulation mode mean diameter in the CMAQ is 7.0E-8 (0.07um)? Is this related to sea salt aerosols?
Thank you all!

Hello,

These lines in SSEMIS.F show how the RH is mapped to the mean diameters and standard deviations:

`
C Find position in the data arrays that corresponds to ambient RH in this
C grid cell and set the appropriate data values

``````              IF ( BLKRH .LE. 0.45 ) THEN
IPOS = 1
ELSE IF ( BLKRH .LE. 0.90 ) THEN
IPOS = NINT( 100.0 * BLKRH - 44.0 )
ELSE IF ( BLKRH .LE. 0.99 ) THEN
IPOS = NINT( 1000.0 * BLKRH - 854.0 )
ELSE
IPOS = 136
END IF
``````

`
IPOS is then the index used to lookup the value of the diameter or standard deviation in the lookup tables. As you can see, the first index is for 45% RH and below, the next 45 indices correspond to increasing RH by 1%, then the next 90 indices correspond to increases by 0.1%. The values in the table are not averaged together, neither geometrically nor arithmetically.

The default diameters prescribed in AERO_DATA.F are not relevant for sea spray aerosol emissions. Those diameters are used mainly for boundary conditions when size distributions from the boundary data appear to be non-physical.

Best regards,
Ben Murphy

Thank you! According to your reply, if I want to express the particle diameter of sea salt aerosol in the accumulation mode in CMAQ, I should express a range, right? For example, “The geometric mean diameter of the accumulation mode sea salt aerosols in CMAQ ranged from 0.2651 to 0.8187 um.” Is there any problem with this expression?
Wish you all the best!

Thanks for clarifying. Yes, that would be an accurate description for a manuscript or technical documentation, for example.

However, if you’re wanting to apply the CMAQ approach to a particular calculation for a particular application, I would encourage using a known or characteristic RH relevant for the situation and using a narrower range in aerosol properties corresponding to that RH.

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Thanks for your advice! I have another small question, does the geometric mean diameter here refer to the wet or dry diameter? This may sound like a stupid question, but I didn’t find a note about it in the program.

Hi! That is definitely not a stupid question. If you follow the operations in lines 803-888 of SSEMIS.F, you can see how the term is being interpreted and used.

In lines 880-888 for example, the wet and dry 3rd and 2nd moments are calculated. The wet moments are directly related to the diameter, while the dry moments are adjusted down by the mass of water present. So we know from these lines that the original diameter corresponds to the wet diameter.

Another point of potential confusion for me is confirming whether the diameter is for the number or volume distribution. We can see in lines 820-821 that it is the number distribution mean diameter since it must be adjusted to calculate the volume-based diameter.

Hope this helps!

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I sincerely appreciate your response, it has been immensely helpful. I found that in SSEMIS.F, sea salt aerosol emissions are treated separately for AH2OJ and AH2OK for the accumulation and coarse modes. Do I have to include AH2O in the calculation of total sea salt mass emissions?
Also, I would like to know how the response of AH2O to radiation, clouds, etc. differs from other sea salt aerosols (e.g. ASO4J, ACLJ, etc.)? Could you please provide some information on these aspects? I tried searching the CMAQ documentation and tutorials but didn’t find much explanation.

Thanks again for your valuable insights and help!
Best regards.

Thanks for your question. And thank you to @wong.david-c and @slfarrell for the helpful information below.

Currently the WRF-CMAQ system accounts for aerosol-shortwave radiative interactions for 5 lumped aerosol categories: black carbon, sea salt, water, water-insoluble species, and water-soluble species. Ultimately, the complex refractive indices will be different for all categories – which are defined in the module_ra_rrtmg_aero_optical_util_cmaq.F file, MODULE module_ra_rrtmg_sw_cmaq, which is pulled in from the WRF repository when one builds WRF-CMAQ. The real part of the complex refractive index represents the scattering of radiation, where the imaginary component represents the absorption of radiation. Plots showing the real and imaginary refractive indices of all of the aforementioned lumped aerosol categories currently in WRF-CMAQ are attached. The model also takes into consideration particle size when calculating radiative scattering and extinction (in CCTM/src/twoway/twoway_rrtmg_aero_optics.F90)

In SSEMIS.F H2O species are included in the calculation but aerosol radiative calculation and SSEMIS calculation are two independent routines and the way to handle H2O species are little bit different. Species that are defined for the 5 categories already mentioned can be found in twoway_cgrid_aerosol_spc_map_module.F90.

Cloud activation of sea spray and other aerosol components is a different problem altogether. I can share more information on that, but want to confirm that you are interested in that process and not primarily focusing on the direct cloud-aerosol interactions.

-Ben

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Dear Ben:

Sincerely thank you for the reply! I carefully read the programs and I think I see what you mean，thanks again！