Considerations for Accounting for Atmospheric Deposition in the
Nitrogen Loading Allocations
Chesapeake Bay Program Implementation Committee Issue Paper
September 4, 2003 Conference Call
Purpose: To request the Implementation Committee's decision on whether to account for
atmospheric deposition load reductions within state tributary strategies and to purpose a means
for accounting for reductions beyond existing regulations. To introduce to the Implementation
Committee some options for accounting for nitrogen emissions reductions due to NOx emissions
regulations and crediting voluntary emissions reductions in tributary strategies for upcoming
Implementation Committee decisions.
Background:
Currently atmospheric deposition contributes about 30 percent of the nitrogen1 load delivered to
the Chesapeake Bay and tidal tributaries. Atmospheric deposition, as a source/pathway2 of
nitrogen loadings [the atmosphere is not a significant source/pathway of phosphorus], ranks
about equal with the other two major source categories: point source discharges and agricultural
runoff. Air emissions and the resultant atmospheric nitrogen deposition are now considered by
the Chesapeake Bay Program partners as an integral component of the anthropogenic load on
which the basinwide and major tributary basin by jurisdiction cap load allocations were derived.
The tributary strategies now under development by the seven watershed jurisdictions and their
stakeholders need to account for current atmospheric deposition loads and future
anticipated/potential reductions from this "source/pathway".
Why Atmospheric Deposition is "Hard" to Account for
1 The classes of compounds considered are reduced nitrogen (NHx)-ammonia and ammonium
ion, a typical byproduct of agricultural activity, as well as many other sources; oxidized nitrogen
(NOx)-as nitric acid and nitrate ion, a typical byproduct of fossil fuel combustion; and organic
nitrogen-typically naturally occurring compounds such as amines in pollen.
Atmospheric deposition is referred to here as a source and a pathway at the same time. The
actual "sources" of the nitrogen deposited onto the lands or open water surfaces within the Bay
watershed are utilities, auto emissions, releases from agricultural activities, etc. The atmosphere and
the resultant dry or wet deposition is really the pathway through which the nitrogen is transported to
the land/open surface waters.

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The complexity inherent in assessing the contribution from atmospheric deposition to overall
nitrogen loads is the path by which the nitrogen reaches the Bay. Nitrogen compounds are
emitted into the atmosphere where they can be transported, mixed, and chemically changed by
atmospheric processes. Eventually, some fraction of these emissions is deposited onto the
landscape (e.g., the Bay watershed) or surface waters (e.g., free flowing river, tidal tributaries,
mainstem Bay). Physical processes move the deposited nitrogen through the landscape to
streams and rivers and eventually into the Bay or its tidal tributaries. Some landscapes will
move the atmospherically deposited nitrogen faster than others (e.g., impervious services vs.
forests). Modeling these processes is achieved through the Chesapeake Bay Program's
Watershed Model (WSM).
Modeling air emissions to atmospheric deposition to nitrogen loadings is also complex and time-
consuming, especially on the atmospheric side where emissions are transported, transformed and
deposited onto the landscape by a national air quality model, EPA's Community Multi-scale Air
Quality or CMAQ model-formerly the Regional Acid Deposition Model or RADM. While the
national model has provided a good assessment of the potential benefits of NOx emissions
regulations, a single computer run for a single "scenario" can take two months from start through
post processing of the model outputs. (Compare this with a 2 day turnaround for most watershed
model scenarios and a 5 day turnaround for most Bay water quality model scenarios.)
Another difficulty with using the national air quality model for Chesapeake Bay purposes is that
it does not allow for a direct association to be made between an emission reduction at a specific
source and the consequent reduction in the nitrogen load delivered to Bay tidal waters.
What states/tributary teams want to know is how much of a reduction in nitrogen deposition
(input) will occur to their tributary basins due to emissions regulations (e.g., NOx SIP) and how
does this reduction change their tributary basin's delivered nitrogen load to Bay tidal waters.
This "accounting" of decreased nitrogen deposition to each basin due to the regulations can be
done using the current modeling tools (CMAQ and CB WSM) in hand. However, since the basin
states are responsible for a significant percentage (-50%) of the nitrogen deposition to the
watershed, serious consideration should be given to going beyond the emissions regulations.
Important information to understand is how much do the emission reductions from sources
located in their own state change the nitrogen deposition to their own tributary strategy basin(s),
as well as to other basins in the watershed under varying source control actions.
Accounting Tool Developed by MD DNR
Maryland Department of Natural Resources has developed a tool that can establish an
association between the emissions from a source at a location in the Chesapeake Bay airshed and
the resulting load to Bay tidal waters due to those emissions. Once the association is set up, the
estimated reductions in nitrogen load (by tributary strategy basin) due to an estimated reduction
in nitrogen emissions can be easily determined. As each emission source is accounted for, each
source can be assessed for a particular reduction strategy. Many "what if " strategies can be
quickly tested and load reduction accounting is easily accomplished in a spreadsheet.
Collections of reduction strategies-both required by regulation and voluntary that have been
individually tested and ascribed a load reduction-can be accumulated. This accumulated

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information can be passed back through the CMAQ model and then to the Chesapeake Bay
watershed model for accounting of anticipated load reductions consistent with all other sources.
Options to Consider for Accounting and Crediting:
(Options are presented here only as background at this time—not requesting decisions on these
options during the September conference call.)
Option 1. Each Tributary Strategy Basin takes credit for all of the reductions in nitrogen
deposition that occurs to its region that translates into reductions in nitrogen load delivered to
Bay tidal waters.
Advantage: Straightforward in terms of accounting for reductions in atmospheric deposition.
Disadvantage: Can only account for national/state regulations that are imposed upon emission
sources. It is unlikely that an emission source would do more than what is required if they are
not given credit for their emission reductions
Option 2. Each Tributary Strategy Basin receives credit for only the reduction in atmospheric
nitrogen deposition and subsequent reduction in nitrogen loads delivered to Bay tidal waters
resulting from emission sources located its respective basin boundaries. In other words, credit is
given to the actual emission source and the state/tributary strategy basin where the source
resides. The allocation of credits associated with reductions from sources outside of the
watershed would need to be a policy decision. For example, if jurisdictions outside of the
watershed are given credit for actions taken, these could be allocated to those sources that reduce
their emissions. This would be an incentive for all sources in the airshed to participate in a
nutrient trading program. Another option may be to allocate the reduction in deposition derived
loads due to sources outside the watershed to the tributary strategy basin. A further option may
be for the Chesapeake Bay Program partners to use some or all of these credits for nutrient
auctions under a cap-and-trade program to develop price disclosure.
Advantage: This option will make it possible for emission sector-wide regulatory and individual
voluntary emission reduction actions to be credited to the actual entities making the emission
reductions.
Disadvantage: Will take a greater accounting effort than option 1. A suitable modeling tool will
be needed.
Option 3. A creative combination of the two above described options after further consultation
with all the state partners.
Requested IC Decisions
Should the Bay Program partners account for atmospheric deposition load reductions within the
state's tributary strategies?

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Should the Bay program partners pursue evaluation of and account for air emissions reductions
through pollution prevention, trading, and other "voluntary" means beyond existing Clean Air
Act regulations?
Requested IC Action
Charge the ad-hoc CBP air team to: 1) further develop options for accounting for air emission
reductions beyond existing regulations, 2) meet with each jurisdiction to walk them through the
options, 3) review options with the Nutrient Subcommittee and 4) bring forward a defined set of
options for Implementation Committee decision at the October meeting.

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