Mineral dust aerosols

Overview

From Fairlie et al, 2007:

We implemented two dust mobilization schemes in GEOS-Chem: (1) the scheme of Ginoux et al. (2004, hereafter G04), developed for the GOCART CTM and (2) the dust entrainment and deposition (DEAD) scheme of Zender et al. (2003a, b hereafter Z03a,b). Both schemes treat the vertical dust flux as proportional to the horizontal saltation flux. The DEAD scheme (Z03) follows MB95 in computing a total horizontal saltation flux, Qs, based on the theory of White (1979):

               RHOair                 (      U*,t  )   (      U*,t   )^2
     Qs = Cz * --------- * Ustar^3 *  ( 1 - ------ ) * ( 1 + ------- )      (1)
                  g                   (       U*   )   (       U*    )

where U* is the friction velocity, U*,t(D) is the threshold friction velocity, RHOair is the air density, g is the acceleration of gravity, and Cz is a global tuning parameter. Qs is computed at D = 75μm, where U*,t is a minimum, and the total vertical flux is given by:

     F = Am * Sz * ALPHA * Qs     (2)

where the sandblasting mass efficiency, ALPHA, depends on the fraction Mclay of clay in the soil, Am is the fractional area of land suitable for mobilization, and Sz is the ‘‘erodibility,’’ an efficiency factor that favors emissions from specified geographic features. We followed Z03b in using ‘‘geomorphic erodibility,’’ which depends on upstream runoff area, and set Mclay = 0.2 globally. We computed U from the 10-m wind speed assuming neutral stability below and used a roughness length Z0 = 100 mm, recommended by Z03a for dust mobilization candidate cells. F is distributed by particle size as a globally uniform tri-modal lognormal probability density function, which we project on to the selected size bins specified above.

The GOCART scheme (G04) follows Gillette and Passi (1988) in computing a size segregated vertical dust flux, Fp, for each size class, p:

     Fp = Cg * S * sp * U10^2 * ( U10 - U*,t )     (3)

where the ‘‘source function,’’ S, serves the same role as the product Am * Sz in DEAD (Eq. (2)), sp is the mass fraction applied to each size class, U10 is the 10-m wind speed, and Cg is a global constant. S confines dust emissions to topographic depressions in desert and semi-desert areas of the world (Ginoux et al., 2001, hereafter G01) and is time invariant....

Although the DEAD and GOCART schemes differ in detail, they differ most fundamentally in representing the role of vegetation. GOCART restricts emissions to persistent arid regions, whereas DEAD permits regions that become seasonally devegetated to mobilize....

Validation

See Fairlie et al, 2007 and Fairlie et al, 2010.

Source code and data

In GEOS-Chem v10-01 and higher versions

TBD

Prior to GEOS-Chem v10-01

The source code for the GEOS-Chem dust emissions & chemistry modules are contained in files dust_mod.f and dust_dead_mod.f.

For more information about the data, please see the README files in the following GEOS-Chem data directories:

1. 0.5 x 0.666 China nested grid: GEOS_0.5x0.666_CH/dust_200605/README
2. 0.5 x 0.666 North America nested grid: GEOS_0.5x0.666_NA/dust_200605/README
3. 2 x 2.5 global data: GEOS_2x2.5/dust_200605/README
4. 4 x 5 global data: GEOS_4x5/dust_200605/README

--Bob Y. 09:33, 24 February 2010 (EST)

Planned updates

Sub-micron dust mass partitioning change

This update was tested in the 1-month benchmark simulation v9-01-03d and approved on 12 Jan 2012.

David Ridley wrote:

Originally, dust mass in the smallest (0.1-1.0 micron) transport tracer was partitioned equally between four bins used for the optical calculations. Using data from aircraft PCASP measurements of Saharan dust (Highwood et al., 2003, Fig 4b) we have inferred a more realistic partitioning for the dust mass into the sub-micron size bins. The mass fraction in each bin is now 6%, 12%, 24%, 58% from smallest to largest, rather than 25% in each.

There is a change to the code in aerosol_mod.f file. At line 450, this:

           ! Lump 1st dust tracer for het chem
           DO N = 1, 4
              SOILDUST(I,J,L,N) =
    &              0.25d0 * STT(I,J,L,IDTDST1) / AIRVOL(I,J,L)
           ENDDO

has become this:

           ! Lump 1st dust tracer for het chem
           SOILDUST(I,J,L,1) =
    &              0.06d0 * STT(I,J,L,IDTDST1) / AIRVOL(I,J,L)
           SOILDUST(I,J,L,2) =
    &              0.12d0 * STT(I,J,L,IDTDST1) / AIRVOL(I,J,L)
           SOILDUST(I,J,L,3) =
    &              0.24d0 * STT(I,J,L,IDTDST1) / AIRVOL(I,J,L)
           SOILDUST(I,J,L,4) =
    &              0.58d0 * STT(I,J,L,IDTDST1) / AIRVOL(I,J,L)

The result of this change in dust particle size is a reduction in the AOD of up to 25% close to source and up to 40% downwind where sub-micron aerosol dominate (Ridley et al., 2011).

--Melissa Payer 14:20, 3 January 2012 (EST)

Surface chemistry on dust

This update is currently slated for inclusion into GEOS-Chem v9-02.

T. Duncan Fairlie wrote:

I have developed GEOS-Chem modules to include additional sources of sulfate and nitrate, associated with the uptake of SO2, and HNO3, on dust, limited by dust alkalinity, and uptake of H2SO4, limited by competition with other surfaces.

The modules I am working on entail the addition of 12 new tracers, represent nitrate on dust, sulfate on dust, and dust alkalinity. Each of these 3 constituents is represented in 4 size bins, corresponding to the 4 dust size bins in the current version of the model. Each of these new tracers is transported, and subject to wet and dry deposition. Dust alkalinity is introduced at the point of emission and corresponds Ca2+ and Mg2+ cation equivalents of 3.0% and 0.6% respectively of the dust by mass. Uptake of acid components consumes dust alkalinity. Uptake of H2SO4 may continue after dust alkalinity titration. Uptake of acid components is represented by a standard first order uptake parameterization. Details are given by Fairlie et al. (ACP, 2010)

I am using the routines that Becky Alexander and Rokjin Park developed for acidic uptake on sea salt as a template. Those routines are associated with the coarse mode SO4s and NITs constituents.

Uptake of acid components is represented by a standard first order uptake parameterization. A thermodynamic equilibrium condition may be appropriate for the fine mode sulfate and nitrate, as is done in the case of sea salt, but results of our study indicate that this is not appropriate for supermicron components. Here, we use RH-dependent functions for γ(HNO3) and γ(SO2), as shown in Fig. 1 of Fairlie et al. (2010). The RH-dependences are based on laboratory results for uptake on calcite particles (Liu et al., 2008, for HNO3; Preszler-Prince et al., 2007, for SO2),

Notes:

1. Hygroscopic growth affects the dry deposition of aerosols. Dust is typically not subject to hygroscopic growth. However, when submicron dust is coated with sulfate or nitrate hygroscopic growth might be expected, and hence to affect deposition frequencies. This is not done in the model. We compute deposition frequencies for dust-sulfate, dust-nitrate, in addition to those for dust and dust alkalinity, and store in DEPSAV.
2. Updates are based on v8.01.01 of the code. I ran at 2x2.5 resolution on my SGI machine "crunch". I had some issues running tpcore_fvdas with v8.01.01, associated with the memory capacity of my I’ve circumvented some of the issues by eliminating the static declaration of 4-d flux arrays in tpcore associated with ND24->26. However, transition to crunch was necessary to run at 2x2.5.

--Bob Y. 11:55, 6 September 2011 (EDT)

Improved dust size distribution scheme

This update is currenty slated for inclusion in GEOS-Chem v10-01.

Li Zhang wrote:

This is Li Zhang from University of Colorado Boulder. We implemented a new dust size distribution scheme into GEOS-Chem, which significantly improves the PM2.5 surface dust concentration over western U.S.. Corresponding results have been published in GRL:

• Zhang, L., J. F. Kok, D. K. Henze, Q. Li, and C. Zhao (2013), Improving simulations of fine dust surface concentrations over the western United States by optimizing the particle size distribution, Geophys. Res. Lett., 40, 3270–3275, doi:10.1002/grl.50591.

Colette Heald wrote:

We have discussed this update with Daven Henze and Li Zhang. The Aerosols Working Group recommends that this update be implemented in the standard code, but there is one small caveat. This code has been evaluated for US/Asian dust, but not for Africa. So we'd like to keep an eye on Africa in the benchmarks and if there is any substantial change in AOD over the region, we may consider reverting to the "old" code for this region (since it's such an important dust region). I have asked Li and Daven to ensure that the code provided to you is easily revert-able. The Aerosols Working Group will review this change in the benchmark and we'll make a recommendation on Africa at that time.

--Bob Y. 10:12, 23 January 2014 (EST)

References

1. Chin, M., P. Ginoux, S. Kinne, B. Holben, B. Duncan, R. Martin, J. Logan, A. Higurashi, and T. Nakajima, Tropospheric aerosol optical thickness from the GOCART model and comparisons with satellite and sunphotometers measurements, J. Atmos Sci., 2001.
2. Chin, M., et al., Aerosol distribution in the Northern Hemisphere during ACE-Asia: results from global model, satellite observations, and Sun photometer measurements, J. Geophys. Res., 109, D23S90, 2004.
3. Fairlie, T.D., D.J. Jacob, J.E. Dibb, B. Alexander, M.A. Avery, A. van Donkelaar, and L. Zhang, Impact of mineral dust on nitrate, sulfate, and ozone in transpacific Asian pollution plumes, Atmos. Chem. Phys., submitted, 2010. PDF
4. Fairlie, T. D., D.J. Jacob, and R.J. Park, The impact of transpacific transport of mineral dust in the United States, Atmos. Environ., 41, 1251-1266, 2007. PDF
5. Gillette, D.A., Passi, R., Modeling dust emission caused by wind erosion, J. Geophys. Res., 93, 14,233–14,242, 1988.
6. Ginoux, P., et al., Sources and distributions of dust aerosols simulated with the GOCART model, J. Geophys. Res, 106 (D17), 20255–20274, 2001.
7. Ginoux, P., et al., Long-term simulation of global dust distribution with the GOCART model: correlation with North Atlantic oscillation, Environmental Modeling and Software, 19, 113–128, 2004.
8. Ridley, D. A., C. L. Heald, and B. J. Ford, North African dust export and deposition: A satellite and model perspective, J. Geophys. Res., doi:10.1029/2011JD016794, in press, 2011.
9. White, B.R., Soil transport by winds on Mars, J. Geophys. Res., 84, 4643–4651, 1979.
10. Zender, C.S., Bian, H., Newman, D., The mineral dust entrainment and deposition (DEAD) model: description and 1990s dust climatology, J. Geophys. Res, 108 (D14), 4416, 2003a.
11. Zender, C.S., Newman, D., Torres, O., Spatial heterogeneity in aeolian erodibility: uniform, topographic, geomorphic and hydrologic erodibility, J. Geophys. Res., 108 (D17), 4543, 2003b.
12. Zhang, L., Gong, S., Padro, J., Barrie, L., A size-segregated particle dry deposition scheme for an atmospheric aerosol module, Atm. Environ., 35, 549–560, 2001.

--Melissa Payer 14:35, 3 January 2012 (EST)

Known issues

Low bias at 4x5 resolution

The following comments are in the documentation header to source code file dust_dead_mod.f, which is the implementation of Zender's DEAD scheme in GEOS-Chem:

T. Duncan Fairlie wrote:

NOTE: The current [dust] code was validated at 2 x 2.5 resolution. We have found that running at 4x5 we get much lower (~50%) dust emissions than at 2x2.5. Recommend we either find a way to scale the U* computed in the dust module, or run a 1x1 and store the the dust emissions, with which to drive lower resolution runs.

-- Duncan Fairlie, 1/25/07

Daniel Jacob wrote:

NOTE: [We'll] implement the [dust] code in the standard [GEOS-Chem] model and put a warning about expected low bias when the simulation is run at 4x5. Whoever is interested in running dust at 4x5 in the future can deal with making the fix.

-- Daniel Jacob, 1/25/07

--Bob Y. 10:34, 22 February 2010 (EST)