Wet deposition

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This page describes the current wet deposition scheme used in GEOS-Chem.

Overview

The Harvard Atmospheric Chemistry Modeling Group developed a wet deposition scheme (including scavenging of soluble tracer in convective updrafts, as well as rainout and washout of soluble tracers) for the GMI model. This scheme was then implemented into GEOS-Chem. Jacob et al [2000] describes the algorithm in full. This scheme is also described in Liu et al [2001].

A number of updates have been added since the implementation of the original wet scavenging algorithm. These include:

Allow both washout and rainout when precipitation forms

NOTE: This update was incorporated into GEOS-Chem v9-01-01.

Qiaoqiao Wang wrote:

When there is new formation of precipitation in lower layer k, rainout will be applied to the whole precipitation area: max(Fk,Fk+1), considering the contribution of precipitation formation overhead. This will overestimate rainout effect when Fk+1 is much larger than Fk. Therefore, we now apply rainout effect to precipitation area Fk and washout effect to the area: max (0, Fk+1-Fk) in the same grid box.

Updates for aerosol scavenging efficiency

This update was tested in the 1-month benchmark simulation v9-01-03e and approved on 02 Feb 2012.

Qiaoqiao Wang wrote:

The bulk below-cloud scavenging parameterization of Dana and Hales (k = 0.1P, where P is the precipitation rate mm h-1) used in the standard GEOS-Chem model integrates scavenging efficiencies over typical aerosol size distributions. This overestimates scavenging as it does not account for the preferential removal of the very fine and coarse particles over the course of the precipitation event, shifting the aerosol size distribution toward the more scavenging-resistant accumulation mode that accounts for most of aerosol mass. Now we use the below-cloud scavenging coefficients (k = a Pb constructed by Feng (2007, 2009)) integrated over accumulation mode for most aerosols and over coarse mode for coarse dust and sea salt.

--Bob Y. 15:50, 11 January 2011 (EST)

Updates for MERRA met fields

In GEOS-Chem v9-01-01, we have implemented an improved wet deposition scheme which uses the precipitation fields directly from the MERRA reanalysis product.

The wet deposition algorithm for GEOS-5 is relatively unchanged, except that we allow both rainout and washout to form within a grid box simultaneously. The wet deposition code has been partitioned into several new subroutines within wetdep_mod.F90 in order to allow for better compatibility with the wet deposition scheme used for the MERRA met fields.

--Bob Y. 14:12, 9 March 2011 (EST)

Add scavenging by snow

This update was tested in the 1-month benchmark simulation v9-01-03e and approved on 02 Feb 2012.

Qiaoqiao Wang wrote:

For in-cloud scavenging by rain droplets, we assume 100% of water-soluble aerosols are rained out. But in the case of snow, only dust and hydrophobic BC are considered to be IN and then could be rained out. Note that HNO3 is also assumed to be rained out by snow as it forms a monolayer in ice crystal. The below-cloud scavenging coefficients are also higher for snow than for rain droplets.

--Bob Y. 14:14, 9 March 2011 (EST)

Impaction scavenging for hydrophobic BC and homogeneous IN removal

These updates were validated with 1-month benchmark simulation v11-01b and 1-year benchmark simulation v11-01b-Run0. This version was approved on 19 Aug 2015.

From Wang et al. [2014]:

We modify the scheme by (1) scavenging hydrophobic aerosol (hydrophobic BC and OC) in convective updrafts, since this would take place by impaction [Ekman et al., 2004], and (2) scavenging water-soluble aerosol from cold clouds by homogeneous freezing of solution droplets at T < 237 K [Friedman et al., 2011]. We conducted 222Rn-210Pb simulation to test the general model representation of aerosol deposition and found a lifetime of tropospheric 210Pb aerosol against deposition of 8.6 days.

--Melissa Sulprizio 11:13, 26 November 2013 (EST)

Implementation of the species database

This update was validated with 1-month benchmark simulation v11-01d and 1-year benchmark simulation v11-01d-Run1. This version was approved on 12 Dec 2015.

We have created a new data structure—the [GEOS-Chem species database—that holds physical properties for each GEOS-Chem species]]. These physical properties include molecular weights, Henry's law constants, dry deposition parameters, and wet deposition parameters. The implementation of the species database will allow us to remove repetitive (and confusing) code, particularly in routines INIT_WETDEP, WETDEPID, COMPUTE_F, RAINOUT, and WASHOUT.

Implementation of the species database has begun in GEOS-Chem v11-01. This will be a multi-stage process. It should be noted that this will be a structural update, and will not change any science. But in the future, with the species database in place, updating physical properties for wet deposition will be much easier than it has been in prior GEOS-Chem versions. All that will be necessary is to update species physical properties in a single location, and they will be propagated to other locations in GEOS-Chem automatically.

As part of this update, the wet deposition module now calls the same Henry's law routines that are used by the HEMCO emissions component. This ensures that the same algorithm is used to compute the dimensionless Henry's law liquid/gas constant everywhere in GEOS-Chem.

--Bob Yantosca (talk) 21:45, 15 December 2015 (UTC)

Update SO2 scavenging in convective updrafts for consistency

Temporary fix to remove SO2 scavenging in convective updrafts

This update is slated for GEOS-Chem v11-02.

Duncan Fairlie wrote:

It was indeed the instantaneous titration of H2O2 by SO2 and its subsequent scavenging as sulfate Which I deduced was leading to excessive removal of SO2 in wet convective updrafts, and consequent Deficits of both SO2 and sulfate in the UTLS above the Asian monsoon.(It’s a problem also in GOCART, Which I think is the origin of the G-C approach).
My solution (which may or may not be valid) was to to treat SO2 as partially soluble, via the effective Henry’s Equilibrium, so that only the fraction of SO2 in the cloud condensate would be subject to wet scavenging. i.e. Treat SO2 in the same way as H2O2 are CH2O are currently treated in COMPUTE_F routine (see attached wetscav module). Note, I am not considering subsequent in-cloud sulfate production from H2O2 or O3 here. That is done in sulfate_mod. I’m essentially focusing on the fraction of SO2 taken up in the cloud water, and hence subject to wet scavenging.
As I say, I am now treating this quite separately from the sulfate production from in-cloud SO2 oxidation by H2O2 And O3 that is done in the sulfate_mod module. In sulfate_mod, the solubility of SO2 in cloud water IS Treated using the Henry’s Law constant; the sulfate produced is then subject to wet scavenging as aerosol.
So, it seems to me, in the standard code, that there is potential multiple counting of SO2 loss in wet Convective updrafts - instantaneous titration of H2O2 and subsequent scavenging, and also what’s going on in sulfate_mod where the dissolved SO2 is oxidized by H2O2 and O3 to sulfate which is then scavenged.
What you propose sounds like a more appropriate solution. My approach seemed quick and simple, an improvement I think, but perhaps not the correct thing to do.

Final solution to account for SO2 oxidation in updrafts

Daniel Jacob wrote:

I agree with Duncan that the current formulation in the standard code double-counts SO2 oxidation in precipitating clouds since this oxidation is done in both sulfate_mod and in scavenging. That should be a relatively small effect since 90% of clouds don't precipitate. However, I think it's essential to separately account for SO2 oxidation in convective updrafts, since otherwise SO2 scavenging in these updrafts will be negligible and all the SO2 will be detrained at the top. We should place a kinetic limitation on SO2 oxidation in convective updrafts, following the formulation in sulfate_mod, rather than assume instantaneous titration of SO2 by H2O2 as is applied at present. This would allow more SO2 to be detrained at the top of the updraft while still recognizing the potential for SO2 scavenging due to oxidation in the updraft.
In the meantime, should we implement [Duncan's] current code that doesn't allow for SO2 scavenging in convective updrafts? That should be better than what we have right now (100% scavenging).

--Melissa Sulprizio (talk) 21:51, 10 November 2015 (UTC)

Validation

See Liu et al [2001].

References

  1. Amos, H. M., D. J. Jacob, C. D. Holmes, J. A. Fisher, Q. Wang, R. M. Yantosca, E. S. Corbitt, E. Galarneau, A. P. Rutter, M. S. Gustin, A. Steffen, J. J. Schauer, J. A. Graydon, V. L. St. Louis, R. W. Talbot, E. S. Edgerton, Y. Zhang, and E. M. Sunderland, Gas-Particle Partitioning of Atmopsheric Hg(II) and Its Effect on Global Mercury Deposition, Atmos. Chem. Phys.,12,591-603, 2012. PDF
  2. Dana, M.T., and J.M. Hales, Statistical aspects of the washout of polydisperse aerosols, Atmos. Environ, 10, 45-50, 1976.
  3. Domine, F., and E. Thibert, Mechanism of incorporation of trace gases in ice grown from the gas phase, Geophys. Res. Lett., 23, 3627-3630, 1996.
  4. Giorgi, F., and W.L. Chameides, Rainout lifetimes of highly soluble aerosols as inferred from simulations with a general circulation model, J. Geophys. Res., 91, 14,367-14,376, 1986.
  5. Jacob, D.J., Heterogeneous chemistry and tropospheric ozone, Atmos. Environ., 34, 2131-2159, 2000. PDF
  6. Jacob, D.J. H. Liu, C.Mari, and R.M. Yantosca, Harvard wet deposition scheme for GMI, Harvard University Atmospheric Chemistry Modeling Group, revised March 2000. PDF
  7. Levine, S.Z., and S.E. Schwartz, In-cloud and below-cloud scavenging of nitric acid vapor, Atmos. Environ., 16, 1725-1734, 1982.
  8. Liu, H., D.J. Jacob, I. Bey, and R.M. Yantosca, Constraints from 210Pb and 7Be on wet deposition and transport in a global three-dimensional chemical tracer model driven by assimilated meteorological fields, J. Geophys. Res., 106, 12,109-12,128, 2001. PDF
  9. Mari, C., D.J. Jacob, and P. Bechtold, Transport and scavenging of soluble gases in a deep convective cloud, J. Geophys. Res., 105, 22,255-22,267, 2000. PDF
  10. Selin, N.E. and D.J. Jacob, Seasonal and spatial patterns of mercury wet deposition in the United States: North American vs. intercontinental sources, Atmospheric Environment, 42, 5193-5204, 2008. PDF
  11. Wang, Q., D.J. Jacob, J.A. Fisher, J. Mao, E.M. Leibensperger, C.C. Carouge, P. Le Sager, Y. Kondo, J.L. Jimenez, M.J. Cubison, and S.J. Doherty, Sources of carbonaceous aerosols and deposited black carbon in the Arctic in winter-spring: implications for radiative forcing, Atmos. Chem. Phys., 11, 12,453-12,473, 2011. PDF
  12. Wang, Q., D.J. Jacob,J.R Spackman, A.E. Perring, J.P. Schwarz, N. Moteki, E.A. Marais, C. Ge, J. Wang and S.R.H. Barrett, Global budget and radiative forcing of black carbon aerosol: constraints from pole-to-pole (HIPPO) observations across the Pacific, J. Geophys. Res., 119, 195-206, 2014. PDF
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