United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
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Research and Development
EPA/600/S3-88/025 Sept. 1988
&EPA Project Summary
Improved Parameterizations for
Surface Resistance to Gaseous
Dry Deposition in Regional-
Scale, Numerical Models
M. L. Wesely
Methods for estimating the dry deposi-
tion velocities of atmospheric gases in
the United States and surrounding areas
have been improved. The improvements
have been incorporated into a revised
module of computer coding for use in
numerical models of atmospheric
transport and deposition of pollutants
over regional scales. As before, the dry
deposition module computes deposition
velocities for five seasonal categories
and 11 landuse types specified in a land-
use map. The key improvement is the
computation of bulk surface resistances
according to three distinct pathways of
mass transfer: to the upper portions of
vegetative canopies, to the lower por-
tions of canopies or structures, and to
the ground (or water surface). This ap-
proach replaces the previous technique
of providing simple tables for looking up
bulk surface resistances. With the sur-
face resistances divided explicitly into
several pathways, the bulk surface
resistances for a large number of gases
can be computed in addition to those for
gases considered in the previous
module (SO2, 03, NOX, and HNO3), if
estimates of the effective Henry's Law
constants and appropriate measures of
the chemical reactivity of the various
substances are known. This has been
accomplished successfully for H2O2,
HCHO, acetaldehyde (to represent other
aldehydes), methyl hydroperoxide (to
represent organic peroxides), peroxy-
acetic acid, HCOOH (to represent
organic acids), NH3, PAN, and HN02.
Other factors considered include surface
temperature, stomatal response to en-
vironmental parameters, the wetting of
surfaces by dew and rain, and the cover-
ing of surfaces by snow. Surface emis-
sion of gases and variations of uptake
characteristics by individual plant
species within the landuse types are not
considered explicitly.
This Project Summary was developed
by EPA's Atmospheric Sciences Research
Laboratory, Research Triangle Park, NC,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see Pro-
ject Report ordering information at
back).
Introduction
Dry deposition of gases from the at-
mosphere provides a primary means of
cleansing the atmosphere and delivering
chemical doses to surface components.
Numerical simulation of dry deposition is
important in evaluating the relationships
between emissions in one area and deposi-
tion in another. The dry deposition module
of the Regional Acid Deposition Module
(RADM) has been used to compute the dry
deposition velocities for SO2, O3, NOX
(defined as the sum of NO and NO2),
sulfate (in submicron particles), and HNO3
in the United States and southern Canada.
The basis for the parameterizations of the
deposition velocities were micro-
meteorological formulas and, for SO2, O3,
and NOX, tables of resistances to uptake.
The purpose of the present work is to put
these tables on a more solid scientific
footing, within a framework that enables
logical extension to additional gaseous
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substances such as H202, HCHO, other
aldehydes, organic peroxides, peroxyacetic
acid, organic acids, PAN, NH3, and HNO2.
Other improvements include a more explicit
means to evaluate changes in surface
resistances caused by the effects of surface
wetness associated with dew and rain.
Procedures
The general approach commonly used
in resistance models of dry deposition of
gases is summarized by the following
formula:
= (ra + rb + rc)~
(D
where va is the deposition velocity, which
can be multiplied by the concentration at
a specific height to produce the deposition
rate; r& and rb are gas-phase resistances
to vertical transport, which are computed
by conventional means; and rc is the bulk
surface resistance and is the focus of this
work. In the previous version of the dry
deposition module, the estimates of rc for
SO2 and O3 were obtained from simple
tables for looking up surface resistances for
each of the 11 landuse types specified in
a computerized landuse map (urban land,
agricultural land, range land, deciduous
forest, coniferous forest, mixed forest in-
cluding wetland, water, barren land, non-
forested wetland, mixed agricultural and
range land, and rocky open areas occupied
by low-growing shrubs). The tables were
duplicated for each of five seasonal
categories (midsummer, autumn, late
autumn, winter with snow, and transitional
spring). The values of rc for NOX were
calculated with a simple function of the
values for O3, and values of rc for HNO3
were assumed to be very small in all cases.
The improved procedures for the revised
module divide the bulk surface resistance
into component resistances for three ma-
jor pathways of mass flux, as is shown in
Figure 1. To implement this model, a table
of numerical values of the component
resistances (except for rm) is provided for
the landuse types and seasonal categories
specified above. The component
resistances are as follows:
(i) rs, the leaf stomatal resistance for
water vapor (the table lists minimum
values, and an algorithm is supplied
to compute rs as a function of solar ir-
radiation and surface temparature);
(ii) rm, plant mesophyll resistance (com-
puted entirely from algorithms);
(iii) nu, resistance at the outer surfaces of
leaves in the upper canopy;
(iv) rdc, a gas-phase dynamic resistance
from the top of the canopy to the SUr-
fa, aerodynamic
/V sublayer
Vegetation
r
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Consistent with experimental observa-
tions, the resistances for NO2 for surfaces
other than sunlit vegetation appear to be
quite large, a result of the relatively low
solubility and chemical reactivity of NO2.
The resistances for NO indicate practical-
ly no surface uptake. It is recommended
that the sum of NO and NO2 should be con-
sidered, not either alone, because rapid in-
air chemical reactions can change the ver-
tical fluxes of each of these substances but
do not change the sum of the two fluxes.
Another factor that should be considered
for these nitrogen oxides, as well as NH3,
is that surface emissions due to biological
activity can obviate the usefulness of the
air-surface exchange rates estimated with
the present scheme. More realistic
estimates would be produced if an internal
concentration corresponding to a compen-
sation point, typically a few ppbV, were
asumed for Cm, C|U, Cci, or Cg in Figure 1,
rather than the value of zero implied when
deposition velocities are used with air con-
centrations to estimate fluxes.
The resistances computed for other
substances are predictions that usually
have few, if any, supporting field data.
Hydrogen peroxide has the unusual proper-
ties of being both moderately soluble in
water and a strong oxidizing agent. Rapid
removal takes place at wet surfaces, and
moderately rapid deposition occurs over
vegetation. Many surfaces that may seem
somewhat inert, such as those in
unharvested agricultural areas, remove
H202 fairly efficiently. Solubility alone is
highlighted in the surface resistances
calculated for formaldehyde (HCHO), for-
mic acid (HCOOH, or ORA to represent
organic acids), and acetaldehyde (CH3CHO,
or ALD to represent aldehydes other than
HCHO). Formaldehyde is taken up rather
rapidly at liquid water surfaces and by sunlit
vegetation, but has much less interaction
with soils and senescent vegetation. The
rather large solubility of formic acid allows
it to be taken up rapidly at many different
types of surfaces. Variations on the same
theme are seen for NH2 and HNO2.
The remaining three substances, methyl
hydroperoxide (CH3O2H, or OP to represent
several organic peroxides), peroxyacetic
acid [CH3C (0) O2H, or PAA], and peroxy-
acetyl nitrate [CH3 (O) O2NO2, or PAN],
have slightly limited solubility and are
moderately reactive as oxidants. PAN is the
least soluble and thus has the largest sur-
face resistance of the three substances for
sunlit green vegetation, although the low
solubility of PAN is offset somewhat by its
relatively higher reactivity. This behavior of
PAN is consistent with laboratory
observations.
In conclusion, the dry deposition module,
which is available along with the landuse
map in computer-compatible form, provides
a means to estimate the dry deposition
velocities for many substances. Limitations
include the fact that the module categorizes
all surfaces by only 11 landuse types and
considers only five general seasonal
categories. Estimates of dry deposition
velocities are probably not very accurate for
short periods of time or for a particular
small area. Rather, the estimates are in-
tended for long-term averages over at least
several weeks and for rather large areas,
over which the individual variations of plant
species composition and factors such as
soil moisture content are smoothed. For
vegetation, uptake resistances by individual
plant species have not been identified, and
the influence of varying amounts of leaf
area (green or senescent) has not been tied
explicitly to a measurable quantity such as
LAI (leaf area index). The following factors
are considered in a general fashion:
vegetation height, aridity or soil moisture
content, surface temperature, and varia-
tions of leaf stomatal resistance with solar
radiation and temperature. A number of fac-
tors that can strongly influence air-surface
exchange are not considered. These in-
clude the differences between sea water
and fresh water, the effects of fog or impac-
ting cloud water at high elevations, soil
alkalinity or acidity, and natural surface
emissions.
M. L Wesely is with Argonne National Laboratory, Argonne, IL 60439..
James M. Godowitch is the EPA Project Officer (see below).
The complete report, entitled "Improved Parameterizations for Surface Resistance
to Gaseous Dry Deposition in Regional-Scale, Numerical Models," (Order No. PB
88-225 099/AS; Cost: $ 14.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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