Difference between revisions of "Sea salt aerosols"

Revision as of 15:56, 23 November 2009

Updates to sea salt emissions algorithm

= Updated hygroscopic

Please see the dis GEOS-Chem_v8-02-04#Updated_hygroscopic_growth_factors_for_sea-salt

Bug fixes in seasalt_mod.f

Please see [[this discussion

Modification of size bins for coarse mode aerosols

Lyatt Jaeglé (jaegle@atmos.washington.edu) wrote:

I think that we should change the dry size bins for the coarse mode aerosols in input.geos. Instead of using:

   Online SEASALT AEROSOLS : T
    => SALA radius bin [um]: 0.1  0.5
    => SALC radius bin [um]: 0.5 10.0

we should use a smaller upper cut for the dry radius of the coarse mode aerosols (up to 4 microns dry size ==> 8-10 microns radius for wet sea salt).

   Online SEASALT AEROSOLS : T
    => SALA radius bin [um]: 0.1  0.5
    => SALC radius bin [um]: 0.5  4.0

The first two changes lead to a factor of 2 decrease in total sea-salt emissions (from ~8000 Tg/yr to ~4300 Tg/yr using GEOS-4 winds for 2003 2x2.5). The last change leads to another reduction by 40% in emissions. However the total burden of sea-salt aerosols (~12.5 Tg) remains nearly unchanged compared to the old formulation (~13.7 Tg) because of the strong non-linearity of the dry deposition velocity at sizes > 2 microns. Indeed the lifetime of coarse mode sea-salt aerosols (0.5-4um vs 0.5-10um) increases by a factor of almost 3.

Here is a summary of the changes:

Old formulation in GEOS-Chem (2x2.5 GEOS-4 winds 2003)

	0.1-0.5 um	0.5-10 um	Total: 0.1-10 um
Emissions (Tg/yr)	106	7865	7970
Dry deposition (Tg/yr)	4.7	5012	5016
Wet deposition (Tg/yr)	102	2859	2955
Burden (Tg)	0.73	13.01	13.74
Lifetime (hours)	60	14	15

New formulation, includes the following changes:

• Changing BETHA from 1.0 to 2.0 (as described on the GEOS-Chem v8-02-04 wiki page)
• Changing LOG to LOG10 in the expression for sea salt base emissions (as described on GEOS-Chem v8-02-04 wiki page)
• Capping of the coarse sea salt aerosol bin size at 4um

	0.1-0.5 um	0.5-4 um	Total: 0.1-4 um
Emissions (Tg/yr)	92	2633	2725
Dry deposition (Tg/yr)	4	689	693
Wet deposition (Tg/yr)	87	1944	2031
Burden (Tg)	0.63	11.92	12.55
Lifetime (hours)	60	40	40

I am also including calculations for a 3rd bin size 4-10 microns

While these larger aerosols (4-10 microns dry size) add another 50% to the emissions, they only contribute to 12% of the burden because of their short lifetime. So if we want to stick to 2 size bins, I think that it's fine to neglect these larger aerosols and limit the upper cut of the coarse mode sea-salt aerosols to 4 microns.

The overall sea-salt emissions ~3000 Tg/year is now similar to what other studies found when applying the Monahan formula: Monahan (1986), Spillane et al. (1986), Gong et al. (1998), Penner et al. (2001), etc... This is also within the range recommended by Lewis & Schwartz.

I also tried the Gong (2003) formulation which leads to a factor of ~2 decrease in emissions of accumulation mode aerosols but little change to the coarse mode aerosols. I am in the process of evaluating the sea-salt formulation against comparisons to cruise sea-salt observations from PMEL and find that both Gong (2003) and Monahan tend to overestimate sea-salt emissions at the high wind speeds in mid-latitudes and underestimate emissions in subtropical warmer waters. I am working on updating the Gong formulation based on SST.

Note that the optical properties currently used in the GEOS-Chem (jv_spec.dat) assume log-normal size distributions that lead to effective radii that are too large for coarse mode aerosols: ~9 microns at 50% RH! Based on observed sea-salt size distributions, this should be much smaller ~ 1-2 microns. This correction leads to larger AODs due to sea-salt. I think that Colette and Randall are working on updating jv_spec.dat and I will send them my recommendations.

--Bob Y. 10:56, 23 November 2009 (EST)

Double-substitution bug in routine GET_ALK

Becky Alexander (beckya@u.washington.edu) wrote:

The code in GET_ALK (in routine seasalt_mod.f) as it is now is wrong. I did a substitution twice by mistake, that should have been applied only once. This is calculated for both accumulation and coarse mode seasalt, for both SO2 and HNO3, so there are 4 places in the code that must be fixed.

The correct code should be as follows:

            !----------------------------------
            ! SO2 uptake onto fine particles 
            !----------------------------------
 
            ! calculate gas-to-particle rate constant for uptake of 
            ! SO2 onto fine sea-salt aerosols [Jacob, 2000] analytical solution
            CONST1 = 4.D0/(V*GAMMA_SO2)
            A1     = (RAD1/DG)+CONST1
            B1     = (RAD2/DG)+CONST1
   !-----------------------------------------------------------------------------
   ! Prior to 7/18/08:
   ! Becky Alexander's fix to remove double-substitution (bec, bmy, 7/18/08)
   ! Remove these lines:
   !         TERM1A = (((B1/DG)**2)+(2.0D0*CONST1*B1/DG)+(CONST1**2)) -
   !     &            (((A1/DG)**2)+(2.0D0*CONST1*A1/DG)+(CONST1**2))
   !         TERM2A = 2.D0*CONST1*(((B1/DG)+CONST1)-((A1/DG)+CONST1))
   !         TERM3A = (CONST1**2)*(LOG((B1/DG)+CONST1) -
   !     &            LOG((A1/DG)+CONST1))
   !         KT1    = 4.D0*PI*N1*(DG**2)*(TERM1A - TERM2A + TERM3A)
   !-----------------------------------------------------------------------------
            TERM1A = ((B1**2)/2.0d0) - (((A1**2)/2.0d0)
            TERM2A = 2.D0*CONST1*(B1-A1)
            TERM3A = (CONST1**2)*LOG(B1/A1)
            KT1    = 4.D0*PI*N1*(DG**3)*(TERM1A - TERM2A + TERM3A)

            !----------------------------------
            ! SO2 uptake onto coarse particles 
            !----------------------------------
        
            ! calculate gas-to-particle rate constant for uptake of 
            ! SO2 onto coarse sea-salt aerosols [Jacob, 2000] analytical solution
            CONST2 = 4.D0/(V*GAMMA_SO2)
            A2     = (RAD2/DG)+CONST2
            B2     = (RAD3/DG)+CONST2
   !------------------------------------------------------------------------------
   ! Prior to 7/18/08:
   ! Becky Alexander's fix to remove double-substitution (bec, bmy, 7/18/08)
   ! Remove these lines:
   !         TERM1B = (((B2/DG)**2)+(2.0D0*CONST2*B2/DG)+(CONST2**2)) -
   !     &            (((A2/DG)**2)+(2.0D0*CONST2*A2/DG)+(CONST2**2))
   !         TERM2B = 2.D0*CONST2*(((B2/DG)+CONST2)-((A2/DG)+CONST2))
   !         TERM3B = (CONST2**2)*(LOG((B2/DG)+CONST2) -
   !     &             LOG((A2/DG)+CONST2))
   !         KT2    = 4.D0*PI*N2*(DG**2)*(TERM1B - TERM2B + TERM3B)
   !------------------------------------------------------------------------------
            TERM1B = ((B2**2)/2.0d0) - (((A2**2)/2.0d0)
            TERM2B = 2.D0*CONST2*(B2-A2)
            TERM3B = (CONST2**2)*LOG(B2/A2)
            KT2    = 4.D0*PI*N2*(DG**3)*(TERM1B - TERM2B + TERM3B)
            KT     = KT1 + KT2

            !----------------------------------
            ! HNO3 uptake onto fine particles 
            !----------------------------------

            ! calculate gas-to-particle rate constant for uptake of 
            ! HNO3 onto fine sea-salt aerosols [Jacob, 2000] analytical solution
            CONST1N = 4.D0/(V*GAMMA_HNO3)
            A1N     = (RAD1/DG)+CONST1N
            B1N     = (RAD2/DG)+CONST1N
   !-----------------------------------------------------------------------------
   ! Prior to 7/18/08:
   ! Becky Alexander's fix to remove double-substitution (bec, bmy, 7/18/08)
   ! Remove these lines:
   !         TERM1AN = (((B1N/DG)**2)+(2.0D0*CONST1N*B1N/DG)+(CONST1N**2)) -
   !     &             (((A1N/DG)**2)+(2.0D0*CONST1N*A1N/DG)+(CONST1N**2))
   !         TERM2AN = 2.D0*CONST1N*(((B1N/DG)+CONST1N)-((A1N/DG)+CONST1N))
   !         TERM3AN = (CONST1N**2)*(LOG((B1N/DG)+CONST1N) -
   !     &             LOG((A1N/DG)+CONST1N))
   !         KT1N    = 4.D0*PI*N1*(DG**2)*(TERM1AN - TERM2AN + TERM3AN)
   !-----------------------------------------------------------------------------
            TERM1AN = ((B1N**2)/2.0d0) - (((A1N**2)/2.0d0)
            TERM2AN = 2.D0*CONST1N*(B1N-A1N)
            TERM3AN = (CONST1N**2)*LOG(B1N/A1N)
            KT1N    = 4.D0*PI*N1*(DG**3)*(TERM1AN - TERM2AN + TERM3AN)

            !----------------------------------
            ! HNO3 uptake onto coarse particles 
            !----------------------------------

            ! calculate gas-to-particle rate constant for uptake of 
            ! HNO3 onto coarse sea-salt aerosols [Jacob, 2000] analytical solution
            CONST2N = 4.D0/(V*GAMMA_HNO3)
            A2N     = (RAD2/DG)+CONST2N
            B2N     = (RAD3/DG)+CONST2N
   !-----------------------------------------------------------------------------
   ! Prior to 7/18/08:
   ! Becky Alexander's fix to remove double-substitution (bec, bmy, 7/18/08)
   ! Remove these lines:
   !         TERM1BN = (((B2N/DG)**2)+(2.0D0*CONST2N*B2N/DG)+(CONST2N**2)) -
   !     &             (((A2N/DG)**2)+(2.0D0*CONST2N*A2N/DG)+(CONST2N**2))
   !         TERM2BN = 2.D0*CONST2N*(((B2N/DG)+CONST2N)-((A2N/DG)+CONST2N))
   !         TERM3BN = (CONST2N**2)*(LOG((B2N/DG)+CONST2N) -
   !     &             LOG((A2N/DG)+CONST2N))
   !         KT2N    = 4.D0*PI*N2*(DG**2)*(TERM1BN - TERM2BN + TERM3BN)
   !-----------------------------------------------------------------------------
            TERM1BN = ((B2N**2)/2.0d0) - (((A2N**2)/2.0d0)
            TERM2BN = 2.D0*CONST2N*(B2N-A2N)
            TERM3BN = (CONST2N**2)*LOG(B2N/A2N)
            KT2N    = 4.D0*PI*N2*(DG**3)*(TERM1BN - TERM2BN + TERM3BN)

Please make the fix in your version, or you may download it from ftp://ftp.as.harvard.edu/pub/geos-chem/patches/v8-01-01/seasalt_mod.f_w_getalk_fix.

Also see this document by Becky Alexander which describes the analytical solution in more detail.

Duncan Fairlie (t.d.fairlie@nasa.gov) replied:

Thanks for taking a look at this. I will look at your code corrections and integrate them into my dust code.

Since we're looking back at the analytical solution, I think the last line should read

   Kt = 4.pi.N.D(cubed)[  ]  ,

the extra factor of D coming from

   dr = D.dx,

and the limits of the integral

  ( r=[a,b] ) 

become

   X = [a/D+c, b/D+c] 

I'll recheck my math, and look back at the code.....

NOTE: This fix has been incorporated into GEOS-Chem v8-01-02.

--Bob Y. 16:15, 7 August 2008 (EDT)