Dry deposition

This Guide describes the GEOS-Chem dry deposition scheme.

Overview

Here is a description of the GEOS-Chem dry deposition scheme from several journal articles:

From Section 4 of Alexander et al. [2005]:

Dry deposition velocities for sea-salt aerosols (and sulfate formed in sea-salt aerosols) are computed with the size-dependent scheme of Zhang et al. [2001] integrated over each model size bin and accounting for hygroscopic growth as a function of relative humidity Gerber [1985]. Dry deposition velocities for all other species are computed with a standard resistance-in-series scheme based on Wesely [1989] as described by Wang et al. [1998].

From Section 2.4 of Bey et al. [2001]

Dry deposition of oxidants and water soluble species is computed using a resistance-in-series model based on the original formulation of Wesely [1989] with a number of modifications (Wang et al. [1998]). The dry deposition velocities are calculated locally using GEOS data for surface values of momentum and sensible heat fluxes, temperature, and solar radiation.

From Section 6 of Wang et al. [1998]:

We use a resistance-in-series model (Wesely and Hicks [1977]) to compute dry deposition velocities of O₃, NO₂, HNO₃, PANs and H₂O₂. The deposition velocity \(V_i\) for species \(i\) is computed as:

\[V_i = \frac{1}{R_a + R_{b,i} + R_{c,i}}\]

where \(R_a\) is the aerodynamic resistance to transfer to the surface, \(R_{b,i}\) is the boundary resistance, and \(R_{c,i}\) is the canopy surface resistance. \(R_a\) and \(R_{b,i}\) are calculated from the GCM meteorological variables (Jacob et al. [1993]. Surface resistances \(R_{c,i}\) are based largely on the canopy model of Wesely [1989]] with some improvements, including explicit dependence of canopy stomatal resistances on LAI (Gao and Wesely [1995] and on direct and diffuse PAR within the canopy (Baldocchi et al. [1987]). The same radiative transfer model for direct and diffuse PAR in the canopy is used as in the formulation of isoprene emissions. Surface resistances for deposition to tropical rain forest and tundra are taken from Jacob and Wofsy [1990] and Jacob et al. [1992], respectively. The surface resistance for deposition of NO ₂ is taken to be the same as that of ozone (Erisman and Pul [1994]; Kramm et al. [1995]; Eugster and Hesterberg [1996]) and hence lower than specified by Wesely [1989]. Dry deposition of CO and hydrocarbons is negligibly small and not included in the model (Mueller and Brasseur [1995]).

Aerosol Dry Deposition

There are 3 dry deposition routines in GeosCore/drydep_mod.F90 that use the Zhang et al. [2001] scheme:

AERO_SFCRSII: Aerodynamic resistance for seasalt tracers. Hygroscopic growth is accounted for.
DUST_SFCRSII: Aerodynamic resistance of dust aerosol tracers. No hygroscopic growth. Used for dust species.
ADUST_SFCRSII: Aerodynamic resistance of non-size resolved aerosols. No hygroscopic growth. Based on DUST_SRFCRSII and activated by Pye et al. [2009]. Assumes particle diameter is 0.5\(\mu m\), density is 1.5 g cm^-3 (1500 kg m^-3). Used for all other aerosols.

Input Values for Dry Deposition

Land Cover Parameters

Land cover input variables for dry deposition
Variable	Read from	Description	Values
DRYCOEFF	`Olson_1992_Drydep_inputs.nc`	Local dependence of stomatal resistance on light intensity [1]	-0.358, 3.02, 3.85, …
IDRYDEP	`Olson_1992_Drydep_inputs.nc`	Indices for the 11 dry deposition land types. [2]	1, 2, 3 … 11
IOLSON	`Olson_1992_Drydep_inputs.nc`	Indices for the 72 Olson land types. [2]	1, 2, 3, …, 74

References

Aerodynamic Resistances

Aerodynamic resistances and maximum deposition velocity for aerosols (IVSMAX) for each land type are read from Olson_1992_Drydep_inputs.nc.

Parameter	Type 1	Type 2
DD type	1	2
Description	Snow/Ice	Deciduous forest
IRI	9999	200
IRLU	9999	9000
IRAC	0	2000
IRGSS	100	500
IRGSO	3500	200
IRCLS	9999	2000
IRCLO	1000	1000
IVSMAX	100	100

Dry deposition of organic VOCs

Parameters defined in the species_database.yml file:

OVOC Dry Deposition Parameters
Species	H* (moles L⁻¹ atm⁻¹)	\(f_0\)	Reference
NO2	0.01	0.1
Ox	0.01	1.0
PAN	3.6	0.1
HNO3	1.0d+14	0.0
H2O2	1.0d+5	1.0
PMN	as PAN
PPN	as PAN
PYPAN	as PAN
ISN2	as HNO3
R4N2	as PAN
CH2O	6.0e+3	1.0 (formerly 0)
GLYX	3.6d+5	1.0 (formerly 0)
MGLY	3.7d+3	1.0 (formerly 0)
GLYC	4.1d+4	1.0 (formerly 0)
MPAN, GPAN, GLPAN	as PAN
N2O5	as HNO3
HCOOH	1.67d+5	1.0 (formerly 0)
ACTA	1.14d+4	1.0 (formerly 0)
ISOPND	1.7d+4	0.0
ISOPNB	1.7d+4	0.0
MVKN+MACRN	1.7d+4	0.0
PROPNN	1.0d+3	0.0	Nitrooxyacetone (Sander table)
RIP	as H2O2
IEPOX	as H2O2
MAP	8.4d+2	1.0
MVK	4.4d1	1.0 (formerly 0)	Sander
MACR	6.5d0	1.0 (formerly 0)	Sander
SO2	1.0d+5	0.0

Updates to the original implementation

We have made several updates to the original implementation of dry deposition in GEOS-Chem. Here are some of the more important updates.

Cold-temperature Updates

The following updates were added, following Viral Shah:

Set HNO₃ bulk surface resistance to 1 s/m.
Limit increases in Rc at low temperature to a factor of 2.

Bug fix for Aerodynamic Resistance R_a

For the calculation of aerodynamic resistance R_a under very stable atmospheric conditions the integration of the stability function \(\phi_h\) (aka dimensionless vertical temperature gradient) from the roughness length to grid box center doesn’t take into account the discontinuity occurring at \(z/L = 1\) where \(\phi_h\) switches from \(1 + 5(z/L)\) to \(5 + z/L\) (Holtslag and Boville [1993]). The result is too high of a value for the integral of \(\phi_h\) and subsequently R_a.

A straightforward solution is to calculate RA under stable conditions (\(L > 0\)) using an integral form of the stability function, such as equation (12) of Holtslag and De Bruin [1988]. This fix (submitted by Brian Boys) was implemented this fix into GEOS-Chem v10-01 and later versions.

Aerosol dry deposition velocities over snow and ice

Modeled aerosol dry deposition velocities over snow and ice surfaces in the Arctic are much higher than estimated from measured values (e.g., Ibrahim et al. [1983]; Duan et al. [1988]; Nilsson and Rannik [2001]). In GEOS-Chem v9-01-02 and later versions we have imposed a dry deposition velocity of 0.03 cm/s for all aerosols over snow and ice surfaces.

Use local surface pressure instead of a constant value

Pressure comes into play in the mean free path calculation. For some reason the pressure was set at a constant value of 1500hPa. We have since replaced this constant pressure with the sea level pressure taken from the meteorology. The overall effect is small.