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Volcanic aerosol mass mixing ratio

Dear members,I am using CESM 1.0.4 with CAM4 for my study.I have a doubt regarding the volcanic aerosol file used in the model. In the CAM4 description, it is specified that the volcanic stratospheric aerosols are comprised of 75% sulfuric acid and 25% water.The variable details, as given in the input file 'CCSM4_volcanic_1850-2008_prototype1.nc', isdouble MMRVOLC(time, lev, lat) ;MMRVOLC:_FillValue = -9999. ;MMRVOLC:units = "kg kg-1" ;MMRVOLC:long_name = "layer volcanic aerosol mass mixing ratio" ;Can somebody clarify about what 'MMRVOLC' excatly represents in the file. Does it corresponds to total mass aerosol (75% sulfuric acid and 25% water) in the layer? Or is it only the 'SO4' part or just the 'S' part?Any help from the community would be extremely appreciated.Krishna


CSEG and Liaisons
Staff member
The mass is the total mass of aerosol (75% sulfuric acid and 25% water), as described in Neely et al. (2016): In CCSM4/CESM1(CAM4), stratospheric aerosol is treated by prescribing a single zonally averaged species. The prescription consists of a monthly mean mass (kg m−2) dis- tributed on a predefined meridional and vertical grid. The in- put time series from 1850 to 2010 is based upon Ammann et al. (2003). This aerosol mass is assumed to be comprised of 75 % sulfuric acid and 25 % water and have a constant log- normal size distribution with a wet effective radius (reff, i.e. the third moment divided by the second moment of the size distribution) of 0.426μm and a standard deviation (σ(lnr)) of 1.25. The standard CCSM4/CESM1(CAM4) forcing file is entitled “CCSM4_volcanic_1850-2008_prototype1.nc” and may be found in the CESM input data repository at “/glade/p/cesm/cseg/inputdata/atm/cam/volc/”.......Instead, similarly to CCSM4/CESM1(CAM4), the mass from the Ammann et al. (2003) data set is assumed to be comprised of 75 % sulfuric acid and 25 % water and have a constant log-normal size distribution with a wet effective radius of 0.426 μm and a standard deviation (σ (ln r )) of 1.8.Neely, R. R., III, A. J. Conley, F. Vitt, and J.-F. Lamarque (2016), A consistent prescription of stratospheric aerosol for both radiation and chemistry in the Community Earth System Model (CESM1), Geosci Model Dev, 9(7), 2459–2470, doi:10.5194/gmd-9-2459-2016.See also Table 1 from Neely et al. (attached). 
Thank you very much for the detailed answer Dr. Mills.I would like to clarify one more detail.The detials of the background sulfate aerosols as given in CAM4 documnent (Neale, 2010), is"The Mie calculations for sulfate assume that it is comprised of ammonium sulfate with a log-normal size distribution. The dry size parameters are a median radius of 0.05 µm and a geometric standard deviation of 2.0."
But in the aerosol input file 'aero_1.9x2.5_L26_1850clim_c091112.nc', the varible is specified asfloat SO4(time, lev, lat, lon) ;SO4:units = "kg/kg" ;SO4:long_name = "SO4 concentration" ;SO4:cell_method = "time: mean" ;
My questions are1) Is the mixing ratio related to the total mass of 'ammonium sulfate' or the only the 'SO4' component in it?2) There is no mention about the effective radius of background aerosols in the document. Is this because the optical peoperties are parametrised based on the variying relative humidty and corresponfing hygroscopic growth? (unlike the fixed wet effective radius of volcanic aerosols) 3) If the relative humidity is zero, is the dry radius the effective radius of the aerosol (0.05 µm)? (with no water content unlike the mixture of 75% sulfuric acid and 25% water)?
Thank you in advance,Krishna