AIRMoN Dry Deposition
Click here for AIRMoN-Dry measurement
data.
Background.
ARL is a leader in the development and
operation of dry deposition networks. Since 1984, the Atmospheric
Turbulence and Diffusion Division in Oak Ridge has
been operating a network specifically designed to get around
the major problem confronting dry deposition monitoring
activities -- there is no existing method suitable for
routine direct measurement. The nested network that was
developed consisted of a small number of research sites
supporting a larger array of stations making simpler but
more routine observations. The Dry Deposition Inferential
Method (DDIM) that was developed remains the central routine
analytical tool of the ongoing NOAA dry
deposition trial network, now identified as the dry deposition
component of the Atmospheric
Integrated Research Monitoring Network (AIRMoN). This
network started with six sites; thirteen stations are now
operating (as shown in the AIRMoN network map elsewhere
in this series of documents).
Why NOAA, why now, and why ARL?
Air-surface exchange is a specialty of ARL. Monitoring and coupling with research in an integrated
fashion is also an ARL specialty. These
two interests and specializations combine in the case of
dry deposition, where it is the desire to derive estimates
that are scientifically credible even though there is no
universally-accepted methodology to make the measurements.
In practice, dry deposition of some compounds is about
the same as wet, but equality of the two would be unexpected
and fortuitous. We know, for example, that the small particles
that carry atmospheric radioactivity are deposited about
90% by rainfall, leaving only about 10% for the dry deposition
contribution. For many trace gases, the dry contribution
is now recognized to be much higher.
The question of dry deposition contributions to ecosystem
health requires urgent attention, since the matter is high
on the horizon of policy makers and regulators. At present,
the relevant science is beginning to become understood.
It is the intent of the NOAA programs (conducted by ARL,
and also including programs conducted by ARL for
the EPA) to provide
the best possible quantifications
of the rates of dry deposition at selected sites. It is
not the intent to provide such data at all locations that
are specified on other grounds, since the techniques now
available are tentative and exploratory, and are not yet
widely applicable.
Network Investigations of Dry Deposition
Bruce Hicks (bruce.hicks@noaa.gov)
The NOAA/ARL Atmospheric Integrated Research Monitoring Network (AIRMoN)
is in two halves -- one wet and the other dry. Both halves
are made up of nested arrays of simple and sophisticated
stations. The intent is to conduct research as necessary
to improve the measurements and the scientific understanding
of controlling processes at the more advanced sites, and
to conduct field tests of the methodologies that are developed
at the larger array of simpler locations. In the case of
AIRMoN-dry, the program is set up with a small number of "CORE" sites,
which service a larger array of "satellite" stations. AIRMoN-dry
has been in operation since 1985. Dry deposition rates
are derived by applying an "inferential model" to measurements
of air concentrations of the chemical species of interest,
using as input measurements of key atmospheric and surface
variables known to be indicative of the controlling processes.
The dry deposition estimates that are yielded are thus
the products of a
site-specific
model, driven by local data. It is basic to this that the
estimates derived are themselves site-specific. This is
not a program designed to yield areal averages, or even
to provide estimates of areal averages through time smoothing.
It is now clear that wet deposition data obtained at a
single location are indeed representative of the surrounding
region if the site is adequately selected. This cannot
be the case for the dry deposition contribution, since
the controlling factors are surface-driven and are not
themselves regionally representative. This concept is conveniently
summarized in a single statement -- wet deposition is an
ergodic process, whereas dry deposition is not.
Coupled with the AIRMoN operation
is the EPA-sponsored
Clean Air Status and Trends Network (CASTNET), which is
the present-day continuation of the EPA National
Dry Deposition Network initiated under the
National
Acid Precipitation Assessment Program in 1988. The
same inferential methodology mentioned above is the underpinning
of the CASTNET dry deposition program. The EPA and
the NOAA activities are wholly collaborative:
both operations are structured to apply new understanding
as rapidly as possible, and to quantify the uncertainty
associated with the indirect (inferential) estimates that
the routine monitoring networks produce. In simple concept,
the Collocated Operational Research Establishments ("CORE" stations)
of AIRMoN-Dry provide a continuing infrastructure for exploratory
research. The much larger CASTNET array of the EPA is the routine network in which the
results are applied. The tier of simpler "satellite" stations
of the NOAA AIRMoN-dry
array serves as a transition, testing the transferability
from the research environment to routine application.
The Dry Deposition Inferential Method.
Tilden Meyers
(tilden.meyers@atdd.noaa.gov)
Interest, both national and international, continues in
the NOAA Inferential Method, initially developed
under NAPAP auspices
for estimation of dry deposition fluxes. Requests have
been received for details of the technique and for the
latest version of the inferential model for dry deposition
velocity from organizations that are now too numerous to
count, from universities and other government agencies
to foreign organizations such as in Central Europe, Spain,
and South Africa.
In the last two years, a new multi-layered model has been
adapted for dry deposition inferential application, replacing
the initial "big leaf model" that had its origins in agricultural
meteorology. The new model provides a greatly improved
capability to simulate the effects of several natural variables
known to be important in the dry deposition context. However,
some tests comparing the two models indicated little improvement
while the newer model was in its early stages of development
(for example, results obtained in Europe as part of the
EUROTRAC program). Field tests to study the performance
of the multi-layered models have been informative. Recent
results indicate that a key requirement is to assimilate
data as required to get the surface water budget correct.
Research Activities.
Tilden Meyers
(tilden.meyers@atdd.noaa.gov)
Peter
Finkelstein (finkelstein.peter@epamail.epa.gov)
The major goal of dry deposition research conducted by ARL
scientists relates to the need to identify and understand
the processes that cause dry deposition, in order to quantify
dry deposition rates at locations where direct measurement
is not possible. The focus is on the improvement of models,
whether for site-specific application using local observations
of key variables as input or for regional application using
model "data" fields to drive the deposition routines. ARL presently
focuses its attention on
- the development of systems for quantifying dry deposition,
- the measurement of dry deposition using micrometeorological
methods,
- the development of techniques for assessing air-surface
exchange in areas (such as specific watersheds) where
intensive studies are not feasible, and
- the extension of local measurements and understanding
to describe areal average exchange in numerical models.
Improved estimates of dry deposition rates for sites in
the NOAA AIRMON-Dry network were generated.
Testing of DDIM Predictions
Tilden Meyers
(tilden.meyers@atdd.noaa.gov)
Peter
Finkelstein (finkelstein.peter@epamail.epa.gov)
There has been a long history of field tests of the methodsn
used to infer dry deposition rates from routinely collected
data. Most of these tests have been conducted at sites
of the AIMoN-dry array -- the so-called CORE sites (State
College., PA; Oak Ridge, TN; Argonne, IL; and recently
Bondville, IL). It was on the basis of the experience gained
in these studies that the original "big leaf" single-layer
model was recently replaced by a multi-layer version. The
reason for the change was rthat the ranges of conditions
encoutered in field studies demonstrated that a single
version of the big-leaf model could not be expected to
work well everywhere (although clearly it could be "tuned" to
local conditions if adequate local data were available).
It has been a well-recognized philosophy of the team developing
these models to make their products available immediately,
to all who ask for them, on the basis that these techniques
are still in development and that progress towards a high-quality
product will be hastened if many groups give attention
to the need, independently. In practice, ozone is frequently
seen as a key indicative variable. Early studies conducted
in Europe with the single-layer model indicated that the
model worked well for about five days out of ten; for the
others it was way off. More recent studies have indicated
that for a pine forest in Holland the single-layer version
worked better than the multiple-layer. Clearly, there is
more work to do.
The ARL team at Research
Triangle Park, working
with colleagues at Oak Ridge, has developed a movable system
for direct measurement of dry deposition fluxes. A first
test of this system was conducted near Beaufort, NC. The
system provides direct eddy correlation measurements of
sulfur dioxide, ozone, and carbon dioxide fluxes, and gradient
measurement of nitric acid flux. The system also measures
the surface energy budget. The system was subsequently
deployed at the Bondville CASTNET (and also AIRMoN and
ISIS) site in Illinois, and later at a coastal site in
New Jersey. Deployment at other sites is planned. In essence,
the flux data obtained have been used to assess uncertainty
and to improve the inferential dry deposition models being
widely used in analysis and modeling of dry deposition.
The ASMD team has also conducted an evaluation of existing dry deposition
algorithms for gaseous pollutants to identify an algorithm
for implementation into the ISC-COMPDEP (Industrial Source
Complex - COMPlex terrain DEPosition) model. Model predictions
were compared against O3, SO2, and
HNO3 field data sets. Sensitivity tests showed
that the models were most sensitive to land-use type and
time of day (day/night), so the data sets were stratified
based on these classifications for use in the evaluation.
The question of how to handle the fact that dry deposition
processes differ greatly from day to night has yet to be
well resolved.
Collaborations
Three ARL teams are heavily involved in
these studies -- Oak
Ridge,
Research Triangle Park, and
Silver Spring. In addition, a large number of
other organizations are partners with ARL in the research that
underpins AIRMoN-dry and its developmental programs:
University of Vermont |
Vermont Monitoring Cooperative |
Pennsylvania State University |
US Geological Survey |
The University of Maryland |
Environmental Protection Agency |
The University of Illinois |
Department of Energy |
The State University of New York, Albany |
Argonne National Laboratory |
The State University of New York, Syracuse |
State of Vermont |
University of Maine |
National Park Service |
US Army |
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