Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-1 September 24, 2010

Appendix L

Setting the Chesapeake Bay Atmospheric Nitrogen Deposition Allocations

Atmospheric Deposition Nitrogen Inputs Compared to Other Nitrogen 

Sources

Atmospheric deposition of nitrogen is the highest nitrogen input load in the Chesapeake

watershed (Figure L-1). Other nutrient input loads are fertilizer, manures, point sources, and 

septic systems. Over the 1985 to 2005 Chesapeake Bay model simulation period, the Chesapeake 

watershed average atmospheric deposition loads of nitrogen have been declining, particularly 

those of oxidized nitrogen.

0

100

200

300

400

500

600

700

800

900

1000

1980 1985 1990 1995 2000 2005 2010

Total Nitrogen Input Loads (millions of lbs/yr)

Point sources

Atmospheric Deposition

N Fertilizer

N Manure

Figure L-1. 20-year (1985–2005) time series of atmospheric, fertilizer, manure, and point source nitrogen 

input loads to the Chesapeake Bay Water Quality and Sediment Transport Model.

Atmospheric Deposition Inputs

Atmospheric loads of nitrogen are from chemical species of oxidized nitrogen, also called NOx, 

and from reduced forms of nitrogen deposition, also called ammonia (NH4

+

). Oxidized forms of 

nitrogen deposition originate from conditions of high heat and pressure and are formed from 

eutrophicly inert diatomic atmospheric nitrogen. The principle sources of NOx are industrialsized boilers such as electric power plants and internal combustion engines in cars, trucks, 

locomotives, airplanes, and the like.

Reduced nitrogen, or ammonia, is responsible for approximately one-third of the total nitrogen 

emissions that eventually end up as loads to the Bay. Ammonia sources are predominately 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-2 September 24, 2010

agricultural, and ammonia is released into the air by volatilization of ammonia from manures and 

emissions from ammonia based fertilizers. Minor sources include mobile sources, slip ammonia 

released as a by-product of emission controls on NOx at power plants and industrial processes.

Two types of deposition are differentiated and both are tracked through the Chesapeake models 

and atmospheric deposition monitoring networks as input daily. The first is wet deposition, 

which occurs during precipitation events and contributes only to nitrogen loads during days of 

rain or snow. The other is dry deposition, which occurs continuously and is input at a constant 

rate daily into the Bay Watershed and Bay Water Quality models.

Because the Bay Watershed and Bay Water Quality models are mass balance models, all sources 

of nutrient inputs to the tidal Bay have to be accounted for including phosphorus and organic 

forms of nutrients. For these minor nutrient inputs the models account for estimated loads of 

phosphorus and organic nutrients to open water only, on the assumption that all phosphorus and 

organic nutrients are derived from aeolian processes that result in no net change in organic 

nitrogen on terrestrial surfaces but result in a net gain when deposited on water surfaces.

Organic nitrogen is represented as wet fall only, i.e., dissolved organic nitrogen (DON). The 

magnitude of dry fall organic nitrogen is not well characterized in the literature, but the latest 

Community Multiscale Air Quality (CMAQ) model simulations with updated chemical 

mechanisms do include peroxyacyl nitrates (PAN, CH3COOONO2) and an organic nitrate group 

(NTR). The NTR represents several organic nitrates that are produced from ozone 

photochemistry. Both of these species are relatively small in magnitude and both are biologically 

labile. Therefore, the dryfall PAN and NTR are lumped into the oxidized nitrogen atmospheric 

deposition dryfall inputs. Table L-5 shows the estimated atmospheric deposition loads to the 

Bay’s tidal surface waters of the different nutrient species.

Air sources contribute about a third of the total nitrogen loads delivered to the Chesapeake Bay 

by depositing directly onto the surface of the Bay and onto the surrounding Bay watershed. 

Direct nitrogen atmospheric deposition to the Bay’s tidal surface waters is estimated to be 6 to 8

percent of the total (air and non-air) nitrogen load delivered to the Bay. The atmospheric nitrogen 

deposited onto the watershed and subsequently transported to the Bay is estimated to account for 

25 to 28 percent of the total nitrogen loadings to the Bay.

Two types of deposition are tracked in the Phase 5.3 Chesapeake Bay Watershed Model and 

input daily. Wet deposition occurs during precipitation events and contributes only to the loads 

during days of rain or snow. Dry deposition occurs continuously and is input at a constant rate 

daily.

Atmospheric Deposition Input Trends

Between 1985 and 2005, the simulation period of the Phase 5.3 Bay Watershed Model, 

atmospheric deposition loads of nitrate have tended to decrease overall in the Chesapeake Bay 

watershed. Over that 20 year period, nitrate loads have decreased by about 30 percent (Figure L2); however, considerable variability exists across the Bay watershed, with the greatest 

reductions occurring in the northern and western portions. In Figure L-2, the average annual 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-3 September 24, 2010

concentration is used as an adjustment to smooth out the high and low rainfall years, which bring 

different amounts of deposition load to the Bay watershed, primarily from the volume of 

precipitation. Use of the dissolved inorganic nitrogen (DIN), nitrate (NO3), and ammonia (NH3) 

concentrations provides a reasonable estimate of the trend in atmospheric deposition.

Annual Concentrations in Atmospheric Deposition

y = 2E-06x + 0.1795

R

2

 = 7E-07

y = -0.0053x + 10.971

R

2

 = 0.5641

y = -0.0053x + 10.791

R

2

 = 0.7061

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1980 1985 1990 1995 2000 2005 2010

Year mg/l

NH3

NO3

DIN

Linear (NH3)

Linear (DIN)

Linear (NO3)

Figure L-2. Trend of estimated average NO3, NH3 and DIN deposition concentrations input to the Phase 5.3 

Chesapeake Bay Watershed Model.

Much of the reduction has been due to point source air emission reductions, particularly from 

electric generating units (EGUs) as shown in Figure L-3. Further, more rapid declines are 

expected between 2008 to 2010 as the Clean Air Transport Rule (previously the Clean Air 

Interstate Rule [CAIR]) controls on power plant emissions and the air quality standards for ozone 

and particulate matter come into enforcement deadlines by 2010 (Figure L-3). Further reductions 

are expected with the reduced ozone air quality standard announced in August 2010. Reductions 

from mobile sources are another large contributor to the downward trend. Reductions from 

mobile sources will continue past the year 2020 as large off-road diesel and marine diesel fleets 

are replaced.

Table L-1 shows the estimated portion of deposited NOx loads on the Chesapeake Bay 

watershed from four sectors including EGUs, mobile sources, industry, and all other sources. 

From 1990 to 2020, considerable reductions have been made in the power sector. In addition, 

both on road and off-road mobile sources have ongoing fleet turnover and replacement, which is 

putting cleaner spark and diesel engines in service; that is expected to continue beyond 2030. 

Note that some sources like mobile sources seem to increasing in percentage relative to other 

sources like EGUs. Both sources are actually decreasing and the total deposition load in 2020 is 

less than 1990, however, EGU emission reductions are relatively more than mobile reductions.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-4 September 24, 2010

Figure L-3. Estimated nationwide emissions of NOx and SO2 from EGUs since 1980 and estimated emissions 

to 2020.

Average ammonia atmospheric deposition loads over the Chesapeake Bay watershed have 

followed the trend in overall manure loads in the watershed and have remained steady over the 

1985 to 2005 simulation period (Figure L-2). Ammonia deposition is very site specific and 

strongly influenced by local emissions. Local and regional trends in manure, such as the rise of 

poultry animal units in the Eastern Shore and Shenandoah and dairy’s diminishment in the 

northern portions of the watershed in the late 1980s, affect regional ammonia deposition in the 

Bay watershed.

Table L-1. Estimated portion of atmospherically deposited NOx loads on the Chesapeake watershed from 

four sectors including EGUs, mobile sources, industry, and all other sources in 1990 and projected out to 

2020

21%

8%

30%

40%

1990 2020

Preliminary

31%

Other (off-road-construction; 

residential & commercial)

Industry 20%

Mobile Sources (on-road) 32%

Power Plants (EGU’s) 17%

Watershed

21%

8%

30%

40%

1990 2020

Preliminary

31%

Other (off-road-construction; 

residential & commercial)

Industry 20%

Mobile Sources (on-road) 32%

Power Plants (EGU’s) 17%

Watershed
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-5 September 24, 2010

The Bay’s NOx airshed—the area where emission sources that contribute the most airborne 

nitrates to the Bay originate—is about 570,000 square miles, or seven times the size of the Bay’s 

watershed. The ammonia airshed is slightly smaller (Figure L-4). Close to 50 percent of the 

nitrate (or NOx) deposition to the Bay is from air emission sources located in the seven Bay 

watershed jurisdictions. Another 25 percent of the atmospheric deposition load to the 

Chesapeake Bay watershed is from the remaining area in the airshed and the remaining 25 

percent of deposition is from the area outside the airshed. The ammonia airshed is similar to the 

NOx airshed, but slightly smaller (Figure L-4).

Source: Chesapeake Bay Program Office

Figure L-4. The oxidized nitrogen airshed (blue line) is the principle area of NOX emissions that contribute 

nitrogen deposition to the Chesapeake Bay and its watershed. The reduced nitrogen airshed (red line) of 

ammonia deposition is slightly smaller.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-6 September 24, 2010

CBP Airshed Model

The Chesapeake Bay Airshed Model is a combination of a regression model of wet deposition 

(Grimm and Lynch 2007) and a continental-scale air quality model of North America called the 

CMAQ for estimates of dry deposition (Dennis et al. 2007; Hameedi et al. 2007). The Bay 

Airshed Model is represented in Figure L-5.

Combining 

a regression 

model of 

wetfall 

deposition...

...with 

CMAQ 

estimates 

of dry 

deposition 

for the 

base...

...and using the 

power of the 

CMAQ model for 

scenarios.

Combining 

a regression 

model of 

wetfall 

deposition...

...with 

CMAQ 

estimates 

of dry 

deposition 

for the 

base...

...and using the 

power of the 

CMAQ model for 

scenarios.

Combining 

a regression 

model of 

wetfall 

deposition...

...with 

CMAQ 

estimates 

of dry 

deposition 

for the 

base...

...and using the 

power of the 

CMAQ model for 

scenarios.

Figure L-5. The Chesapeake Bay Airshed Model is a combination of a regression model of wet deposition and 

the CMAQ model of dry deposition.

The regression and deterministic airshed models that provide atmospheric deposition input 

estimates, have gone through a series of refinements with increasingly sophisticated models of 

both applied over time (Linker et al. 2000; Grimm and Lynch 2000, 2005; Lynch and Grimm 

2003). The amount and timing of the wet atmospheric deposition input in the Phase 5.3 Bay 

Watershed Model is hourly, and is related to the timing and amount of hourly rainfall in the 

Phase 5.3 Bay Watershed Model precipitation input data. The dry deposition estimates are 

monthly constants that are input daily and are based on CMAQ (Dennis et al. 2007; Hameedi et 

al. 2007).

Wet Deposition Regression Model

Wet deposition is simulated using a regression model developed by Grimm and Lynch (2000, 

2005; Lynch and Grimm 2003). The regression model provides hourly wet deposition loads to 

each land segment on the basis of each land segment’s rainfall. The regression model uses 29 

National Atmospheric Deposition Program (NADP) monitoring stations and 6 AirMoN stations 

to form a regression of wetfall deposition in the entire Chesapeake Bay watershed over the entire 

simulation period (Figure L-6).
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-7 September 24, 2010

Figure L-6. Atmospheric deposition monitoring stations used in developing the wet deposition regression 

model.

To improve the accuracy of the regression estimates over previous regression analyses (Linker et 

al. 2000) a number of improvements in the sampling and representation of spatial and temporal 

patterns of land use activities and intensities and of emission levels were made. Also, detailed 

meteorological data were assimilated into the regression model to identify contributing emission 

source areas and to estimate the impact of the contributions on daily deposition rates on a perevent basis.

This version of the regression model included nine additional NADP/NTN sites in the regression

estimates (DE99, MD07, MD08, MD15, MD99, PA47, VA10, VA27, VA98, and VA99) that 

were placed in operation in and around the Chesapeake Bay watershed since 2001. The sites 

provided a more complete representation of agricultural influences than the station set used in the 

earlier analyses.

Refinements also involved developing a more accurate and comprehensive representation of the 

spatial and temporal distribution and intensity of livestock production and other agricultural 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-8 September 24, 2010

activities across the Bay watershed. An improved accounting of livestock production activities 

was achieved by combining county- and watershed unit-specific livestock production statistics 

with high-resolution (30 meters) land use data from the USGS’s National Land Cover Database 

(NLCD). Estimates of local ammonia emissions from fertilizers and manure applications to 

croplands were also assimilated into the model using EPA inventories and high resolution NLCD 

to identify likely cropland areas. Last, localized estimates for NH3 and NOx emissions for the 

Phase 5 domain and surrounding states were developed by combining facility and countyspecific emissions reports from the EPA’s National Emissions Inventory (NEI) database with the 

NLCD classifications.

For each day of rain, wetfall atmospheric deposition is estimated by the regression that has the 

general form

The daily precipitation nitrate and ammonium concentration models were developed using a 

linear least-squares regression approach and single-event precipitation chemistry data from the 

29 NADP/NTN sites and 6 AirMon stations in Figure L-6. The most significant variables in both 

models included precipitation volume, the number of days since the last event, seasonality, 

latitude, and the proportion of land within 8 km covered by forests or devoted to transportation 

and industry. (Local and regional ammonia and nitrogen oxides emissions were not as well 

correlated as land cover.) The abilities of these variables to predict wet deposition arise 

primarily from their relationship to either (1) the spatial and temporal distribution of emissions of 

ammonium and nitrate precursors from sources within or upwind of the Bay watershed; or (2) 

the chronology and characteristics of precipitation events. Modeled concentrations compared 

very well with event chemistry data collected at six NADP/AirMoN sites within the Chesapeake 

Bay watershed. Wet deposition estimates were also consistent with observed deposition at 

selected sites.

Log10(c) = bo + b1log10(ppt) + b2sseason + b3v3 + . . . + bnvn +e

where 

c = daily wet-fall ionic concentration (mg/L)

bo = intercept

ppt = daily precipitation volume (inches)

b1 = coefficient for precipitation term

season = vector of 5 binary indicator variables encoding the 6 bi-monthly seasons

b2s = vector of 5 coefficients for season terms

v3 . . Vn = additional predictors selected through stepwise regression

o National Land Cover Data (NLCD) 

 Within proximities of 0.8, 1.6, 3.2, 8.0, and 16.1 km of each NADP/NTN site: 

open water, forested, residential, industrial/transportation, croplands, and 

vegetated wetlands

o Local emission levels of ammonia and nitrous oxides from EPA National Emission 

Trends (NET)

 County emission totals 1985-2005

 County containing each NADP/NTN monitoring site and for the nearest three 

counties

b3 . . bn = coefficients corresponding to v3 . . Vn

e = residual error
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-9 September 24, 2010

Volume, duration, and frequency of precipitation events have obvious roles in determining wet 

deposition rates. However, these parameters alone do not completely describe all of the 

characteristics of a precipitation event. In particular the intersection of a precipitation event and a 

volume of air with a particular history is also important in determining wet deposition flux, so 

the interactions between storm trajectories and emission sources were also incorporated into the 

wet deposition regression model.

Using metrological data from the National Center for Environmental Prediction’s North 

American Regional Reanalysis (NARR), components were added to daily ammonium and nitrate 

wet deposition models that predict the rate at which emissions from area and point sources are 

emitted, dispersed, and transported to specific deposition locations. Surface and upper-level 

vertical and horizontal air movement data from the NARR allowed estimates of the extent to 

which emissions were transported and mixed into surface and upper-level atmospheric layers; 

and, thereby, enabled construction more realistic multilevel air mass trajectories with which to 

predict the movement of emissions from multiple source locations to deposition points of 

interest.

Dry Deposition - Community Multi-scale Air Quality Model (CMAQ)

The CMAQ Model is a fully developed air simulation of North American (Dennis et al. 2007; 

Hameedi et al. 2007). CMAQ simulates atmospheric deposition to the Chesapeake Bay 

watershed (indirect deposition) and tidal Bay (direct deposition) for every hour of every day for 

the representative year. A variety of input files are needed that contain information pertaining to 

the modeling domain which is all North America. Those include hourly emissions estimates and 

meteorological data in every grid cell and a set of pollutant concentrations to initialize the model 

and to specify concentrations along the modeling domain boundaries. The initial and boundary 

concentrations were obtained from output of a global chemistry model.

The CMAQ model simulation period is for one year, 2002, because 2002 is characterized as an 

average deposition year. The 2002 CMAQ simulation year was used to provide the monthly dry 

deposition estimate for all years of the 1985 to 2005 Phase 5.3 Bay Watershed Model simulation. 

Phase 5.3 Bay Watershed Model dry deposition input estimates are derived from the CMAQ 

model as monthly average inputs expressed as a daily load.

An adjustment for the 20-year trend in atmospheric deposition loads was applied by using the 

trend developed in the wet deposition regression model and assuming the dry deposition trend to 

be the same as the wet in the separate nitrate and ammonia estimates. Figure L-7 shows the 12-

km grid used to provide better resolution of Phase 5 atmospheric deposition loads. The improved 

spatial resolution of direct atmospheric deposition of loads to tidal surface waters and the 

atmospheric deposition of loads to the watershed adjacent to tidal waters from metropolitan and 

mobile sources was an important improvement (STAC 2007).
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-10 September 24, 2010

Figure L-7. The CMAQ model’s 12-km grid over the Phase 5 Chesapeake Bay Watershed Model domain.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-11 September 24, 2010

Organic Nitrogen Deposition

The Phase 5.3 Bay Watershed Model accounts for estimated loads of atmospheric organic 

nitrogen to the open water land use only, on the assumption that all organic nitrogen is derived 

from aeolian processes that result in no net change in organic nitrogen on terrestrial surfaces but 

do result in a net gain when deposited on water surfaces. Organic nitrogen is represented as wet 

fall only, i.e., DON. The magnitude of dry fall organic nitrogen is unknown.

Dryfall Organic Nitrogen Deposition

The dryfall organic nitrogen is likely to be sorbed on to large and small particles or even to be 

particles themselves, like pollen. Such dryfall organic carbon species can be involved in longrange transport, as the pollens and organic nitrates found on the dust coming over from Africa, 

but EPA does not have a good estimates of the fraction of the dry deposition that these particles 

compose.

Also, the latest CMAQ simulations with updated chemical mechanisms do include peroxyacyl 

nitrates (PAN, CH3COOONO2) and an NTR. The NTR represents several organic nitrates that 

are produced from ozone photochemistry. Both of these species are relatively small in 

magnitude, and both are biologically labile. Therefore, the dryfall PAN and NTR are lumped into 

the oxidized nitrogen atmospheric deposition dryfall inputs.

Wetfall Organic Nitrogen Deposition

In the 1992 Phase 2 version of the Chesapeake Bay Watershed Model, organic nitrogen was 

assumed to be about 670 micrograms per liter (μg/L) (as nitrogen) based on data summarized by 

Smullen et al. (1982). The data showed considerable seasonal variability. The organic nitrogen 

load was constant in all watershed model segments. An equivalent annual load was used in the 

tributary model with application of the seasonal variability suggested by Smullen et al. (1982).

Organic nitrogen measurements from Bermuda are calculated at about 100 μg/L (as nitrogen) 

(Knap et al. 1986). Moper and Zika (year ?) reported an average DON concentration from the 

western Atlantic and Gulf of Mexico of about 100 μg/L (as nitrogen). That is consistent with the 

reported range from the North Sea and northeast Atlantic of between 90 μg/L to 120 μg/L 

(Scudlark and Church 1993). Scudlark et al. (1996) reported an annual volume-weighted average 

DON concentration in the mid-Atlantic coastal areas to be about 130 μg/L (as nitrogen). 

Measurements in this study are consistent with the interannual variation (maximum in spring) 

reported by Smullen et al. (1982).

A later study identified methodological problems with some of the previous studies and suggests 

the wet deposition of organic nitrogen in the Chesapeake watershed would be closer to 50 μg/L 

on an annual average basis (Keene et al. 2002). This study also documented the highest 

concentrations of organic nitrogen in the spring.

On the basis of Keene et al. (2002), a value of 50 μg/L (as nitrogen) was selected as 

representative of an average annual wet deposition concentration to the watershed and tidal 

waters with the seasonal loading pattern suggested by Smullen (1982) and Scudlark et al. (1996). 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-12 September 24, 2010

That applies an average concentration of 40 μg/L from July to March in rainfall and an average 

concentration of 80 μg/L from April to June. The load of organic nitrogen would depend on the 

precipitation in a particular land segment, but assuming 40 inches of precipitation, the load 

would be on the order of 0.4 lb/ac-yr.

Total Atmospheric Deposition Inputs of Nitrogen From Wet and Dry 

Deposition

The annual rate of total atmospheric deposition to Phase 5 land segments is shown in Figure L-8 

and Table L-2.

Figure L-8. Annual average DIN atmospheric deposition on land segments in the entire Phase 5.3 

Chesapeake Bay Watershed Model domain.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-13 September 24, 2010

Table L-2. Annual average atmospheric deposition of reduced DIN, oxidized DIN and total DIN on land

segments in the entire Phase 5.3 Chesapeake Bay watershed model

Land-Segment NH4 NO3 Total DIN

A10001 2.50 3.21 5.71

A10003 1.68 2.87 4.55

A10005 5.62 4.55 10.16

A11001 0.24 0.44 0.68

A24001 0.41 1.37 1.78

A24003 1.02 2.99 4.01

A24005 2.02 4.42 6.44

A24009 0.40 1.29 1.69

A24011 1.60 1.64 3.25

C51071 0.17 0.53 0.69

C51165 0.45 0.28 0.72

TOTAL 264.07 556.59 820.66

Organic and Inorganic Phosphorus Deposition

The Phase 5.3 Bay Watershed Model accounts for estimated loads of atmospheric organic and 

inorganic phosphorus to the open water land use on the assumption that, like organic nitrogen, 

the load is derived from aeolian processes that result in no net change in organic nitrogen on 

terrestrial surfaces but do result in a net gain when deposited on water surfaces. Following 

Smullen (1982), annual loads of organic and inorganic phosphorus are set at 47 μg/L and 16 

μg/L, respectively. Seasonally, those loads are treated in the same way as organic nitrogen, 

assuming that organic phosphorus will follow a pattern similar to organic nitrogen and that an 

aeolian source of inorganic phosphorus might well increase during the bare ground of spring 

agricultural practices. Accordingly, organic and inorganic phosphorus concentrations are set at 

74 μg/L and 25 μg/L, respectively, from April to June, and at half those concentrations for the 

other nine months of the year.

CMAQ Airshed Scenarios

The CMAQ model also provides estimates of nitrogen deposition resulting from changes in 

emissions from utility, mobile, and industrial sources due to management actions or growth. For 

the CMAQ model the base deposition year is 2002 and scenarios include the management 

actions required by the Clean Air Act in 2010, 2020, and 2030. The future year scenarios reflect 

emissions reductions from national control programs for both stationary and mobile sources, 

including the CAIR, the Tier-2 Vehicle Rule, the Nonroad Engine Rule, the Heavy-Duty Diesel 

Engine Rule, and the Locomotive/Marine Engine Rule. Although CAIR has been remanded to 

EPA, it will remain in place pending a rulemaking to replace it. It unclear how the replacement 

rule will compare to the remanded rule. However, EPA anticipates that NOx emissions 

reductions close to those originally projected will occur.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-14 September 24, 2010

To develop a Bay watershed model scenario using one of the CMAQ model air scenarios below, 

a monthly factor is determined by CMAQ by comparing the CMAQ atmospheric deposition 

loads in the scenario year to the CMAQ 2002 base year. The CMAQ scenario factor is then used 

to adjust the base atmospheric deposition conditions in the Phase 5.3 Bay Watershed Model over 

the 1991 to 2000 scenario years.

CMAQ 2010 Scenario

The 2010 Scenario represents emission reductions from regulations implemented through the 

Clean Air Act authority to meet National Ambient Air Quality standards for criteria pollutants in 

2010. This includes National/Regional and available State Implementation Plans (SIPs) for NOx 

reductions. Other components of the 2010 Scenario include Tier 1 vehicle emission standards 

reaching high penetration in the vehicle fleet for on-road light duty mobile sources along with 

Tier 2 vehicle emission standards that were fully phased in by the 2006 model year and will 

begin to show an impact in 2010. For EGUs the 2010 controls assume that the NOx SIP call, 

NOx Budget Trading Program, and the CAIR program that regulates the ozone season NOx are 

all in place and that the CAIR program is designed for annual NOx reductions to match the 

ozone season reductions under the 2010 CAIR first phase conditions.

CMAQ 2020 Scenario

The 2020 Scenario has all components of the 2010 Scenario and includes the Clean Air Mercury 

Rule (CAMR), the Best Available Retrofit Technology (BART) used for reducing regional haze

and the off-road diesel and heavy-duty diesel regulations. The 2020 scenario represents emission 

reductions from regulations implemented through the Clean Air Act authority to meet National 

Ambient Air Quality standards for criteria pollutants in 2020. Those include

On-Road mobile sources: For On-Road Light Duty Mobile Sources this includes Tier 2 vehicle 

emissions standards and the Gasoline Sulfur Program which affects SUVs pickups, and vans, 

which are now subject to same national emission standards as cars.

On-Road Heavy Duty Diesel Rule – Tier 4: New emission standards on diesel engines starting 

with the 2010 model year for NOx, plus some diesel engine retrofits.

Clean Air Non-Road Diesel Rule: Off-road diesel engine vehicle rule, commercial marine 

diesels, and locomotive diesels (phased in by 2014) require controls on new engines.

Off-road large spark ignition engine rules affect recreational vehicles (marine and land-based).

EGUs: CAIR second phase in place (in coordination with earlier NOx SIP call); Regional Haze 

Rule and guidelines for BART for reducing regional haze; CAMR all in place.

Non-EGUs: Solid Waste Rules (Hospital/Medical Waste Incinerator Regulations).

CMAQ 2020 Maximum Feasible Scenario

The 2020 Maximum Feasible scenario includes additional aggressive EGU, industry, and mobile 

source controls. Emissions projections were developed that represented incremental 

improvements and control options (beyond 2020 CAIR) that might be available to states to meet 

a more stringent ozone standard. The more stringent standard is due to a reconsideration of the 

national ambient air quality standards for ozone that were promulgated in 2008 along with a 

review of the secondary national ambient air quality standards for oxides of nitrogen and sulfur. 

The new 2010 ozone standard and is expected to be between 0.070 ppm and 0.060 ppm. The 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-15 September 24, 2010

2020 Maximum Feasible Scenario was designed to meet a 0.070 ppm ozone standard, which is 

less than the 0.075 ppm ozone standard in place since 2008.

Incremental control measures for five sectors were developed:

EGUs: lower ozone season nested emission caps in OTC states; targeting use of maximum 

controls for coal fired power plants in or near non-attainment areas.

Non-EGU point sources: new supplemental controls, such as low NOx burners, plus increased 

control measure efficiencies on planned controls and step up of controls to maximum efficiency 

measures, e.g., replacing SNCRs (Selective Non-Catalytic Reduction) with SCRs (Selective 

Catalytic Reduction) control technology.

Area (nonpoint area) sources: switching to natural gas and low sulfur fuel.

On-Road mobile sources: increased penetration of diesel retrofits and continuous. Inspection and 

maintenance using remote onboard diagnostic systems.

Non-Road mobile sources: increased penetration of diesel retrofits and engine rebuilds.

Reduced NOx emissions from marine vessels in coastal shipping lanes.

The 2020 Maximum Feasible Scenario also includes a reduction of ammonia deposition of 15 

percent from estimated ammonia emission programs in the Bay watershed jurisdictions. 

Estimates of up to about 30 percent ammonia emission reductions from manures can be achieved 

through rapid incorporation of manures in to soils at the time of application, biofilters on poultry 

houses, and other management practices (Mark Dubin 2009, personal communication). From a 

state and sector analysis of NOx emissions and deposition, an estimated 50 percent of emissions 

from Bay states becomes deposition to the Chesapeake Bay watershed, along with a further 50 

percent of the ammonia deposition load coming from outside the Bay watershed. Assuming that 

only 50 percent of the emissions are from watershed sources, a 30 percent reduction of emissions 

results in an estimated 15 percent decrease in wet and dry ammonia deposition for the Maximum 

Feasible Scenario from ammonia emission control management practices in the Bay watershed 

jurisdictions.

CMAQ 2030 Scenario

The 2030 scenario is in some areas a further decrease in emissions beyond the 2020 Maximum 

Feasible Scenario due to continuing fleet replacement of heavy diesels, off road diesels, and 

mobile sources of all types. These emission decreases are offset by continued growth in the 

Chesapeake Bay region. The emissions projections assume continued stringent controls are in 

place, such as

Tier 2 vehicle emissions standards fully penetrated in the fleet.

Heavy Duty Diesel vehicle fleet fully replaced with newer heavy-duty vehicle that comply with 

new standards.

On-Road mobile sources: Increased penetration of diesel retrofits maintained.

Non-Road mobile sources capped at 2020 Maximum Feasible Scenario levels.

EGUs and Non-EGUs emissions capped at 2020 Maximum Feasible Scenario levels.

Area sources emissions capped at 2020 Maximum Feasible Scenario levels, assuming energy 

efficiency and control efficiencies keep up with growth.

Marine Vessels: Further reductions in NOx emissions from marine vessels in coastal shipping 

lanes.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-16 September 24, 2010

Atmospheric Deposition Loads to the Watershed and Tidal Bay

Nitrogen loads atmospherically deposited to the Chesapeake Bay watershed by jurisdiction and 

by nitrogen species of wet and dry deposition for key scenarios are tabulated in Table L-3. Table 

L-4 lists the loads delivered to the Bay from the key scenarios, in millions of pounds, using the 

Phase 5.2-August 2009 version of the Bay Watershed Model.

All the scenarios in Table L-4 use the 2002 scenario as a base year. The point sources, human 

and animal populations, septic system loads and so on, are the same 2002 levels in all these 

scenarios. Only the atmospheric deposition changes. The 1985 CMAQ scenario uses the trend of 

atmospheric deposition described in Figure L-2, and the same trend was used for the 2002 

atmospheric deposition in the 2002 scenario. The scenarios of 2010, 2020, 2020 Maximum 

Feasible, and 2030 used estimated atmospheric deposition loads from CMAQ.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-17 September 24, 2010

Table L-3. Atmospheric deposition loads of nitrogen (millions of pounds as nitrogen) to the 

Chesapeake watershed for key scenarios by jurisdiction

STATE Chesapeake

Total Nitrogen DE DC MD NY PA WV VA Watershed

1985 Scenario 7.8 0.8 97.4 53.7 221.7 30.6 179.8 591.8

1985-2000 Calibration 7.1 0.7 84.0 46.0 192.2 26.2 159.3 515.4

2002 Scenario 6.5 0.6 73.0 39.5 167.3 22.5 142.3 451.6

2010 Scenario 6.3 0.5 59.6 30.6 133.3 17.2 112.8 360.2

2020 Scenario 6.6 0.4 54.6 26.2 117.6 15.3 99.9 320.6

2020 Maximum Feasible 6.5 0.4 51.9 24.8 111.2 14.5 95.0 304.3

2030 Scenario 7.4 0.4 56.9 26.1 121.4 15.4 100.0 327.6

Dry NOx Deposition 

1985 Scenario 3.1 0.5 51.0 23.1 102.1 15.7 97.5 293.0

1985-2000 Calibration 2.6 0.4 42.2 19.2 84.9 13.1 83.2 245.4

2002 Scenario 2.2 0.3 35.2 16.2 71.3 10.9 71.8 207.8

2010 Scenario 1.6 0.2 23.1 10.8 46.2 6.7 46.7 135.4

2020 Scenario 1.3 0.1 16.6 7.9 32.5 4.8 33.3 96.5

2020 Maximum Feasible 1.1 0.1 14.3 6.9 28.2 4.2 29.6 84.5

2030 Scenario 1.0 0.1 13.7 6.7 27.0 4.1 28.9 81.6

Dry NH3 Deposition 

1985 Scenario 2.1 0.1 12.2 5.0 25.3 2.9 18.2 65.8

1985-2000 Calibration 2.2 0.1 12.1 4.7 25.3 2.8 18.5 65.7

2002 Scenario 2.3 0.1 12.1 4.5 25.4 2.8 18.7 65.7

2010 Scenario 3.0 0.1 15.8 5.3 32.0 3.7 24.8 84.7

2020 Scenario 3.7 0.1 18.7 5.6 36.5 4.4 29.2 98.3

2020 Maximum Feasible 3.9 0.1 19.4 5.8 37.2 4.5 29.8 100.7

2030 Scenario 4.8 0.1 23.9 6.6 45.5 5.2 34.0 120.3

Wet NOx Deposition 

1985 Scenario 1.6 0.1 22.2 17.0 63.4 8.1 42.0 154.4

1985-2000 Calibration 1.3 0.1 17.9 13.9 51.7 6.6 35.4 126.9

2002 Scenario 1.1 0.1 14.1 11.0 40.9 5.2 29.4 101.8

2010 Scenario 0.7 0.1 9.4 7.3 26.7 3.4 19.6 67.2

2020 Scenario 0.6 0.0 7.2 5.3 19.3 2.5 14.7 49.6

2020 Maximum Feasible 0.5 0.0 6.4 4.7 16.9 2.2 13.3 44.1

2030 Scenario 0.5 0.0 6.2 4.6 16.7 2.2 13.0 43.3

Wet NH3 Deposition 

1985 Scenario 0.9 0.1 12.0 8.7 30.9 3.9 22.0 78.6

1985-2000 Calibration 1.0 0.1 11.8 8.2 30.3 3.7 22.3 77.4

2002 Scenario 1.0 0.1 11.7 7.8 29.7 3.6 22.5 76.4

2010 Scenario 1.0 0.1 11.3 7.3 28.3 3.5 21.7 73.0

2020 Scenario 1.0 0.1 12.0 7.4 29.2 3.6 22.7 76.1

2020 Maximum Feasible 1.0 0.1 11.8 7.4 28.9 3.6 22.4 75.1

2030 Scenario 1.1 0.1 13.0 8.1 32.2 3.9 24.1 82.4

Source: Phase 5.2-August 2009 Version of the Chesapeake Bay Watershed Model. Note: This table does not include 

the 15 percent decrease in wet and dry ammonia deposition for the Maximum Feasible scenario due to ammonia 

emission
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-18 September 24, 2010

Table L-4. Total nitrogen delivered to the Bay (millions pounds per year) from the nine major river 

basins under different key CMAQ atmospheric deposition scenarios applied to a 2002 Base 

condition of land use, BMPs, and point source discharges in order to show the relative effect of 

changing atmospheric deposition.

Basins

CMAQ

Atmo. 

Deposition

1985

Scenario

CMAQ

Atmo. 

Deposition

2002

Scenario

CMAQ 

Atmo. 

Deposition

2010

Scenario

CMAQ 

Atmo. 

Deposition

2020

Scenario

CMAQ

Atmo. 

Deposition

2020

Maximum

Feasible

Scenario

CMAQ

Atmo. 

Deposition

2030

Scenario

Susquehanna 160.4 148.1 141.4 138.7 137.6 139.3

West Shore 15.7 15.3 15.07 15.0 14.9 15.0

Potomac 77.0 72.2 69.4 68.3 67.9 68.6

Patuxent 4.8 4.5 4.4 4.3 4.3 4.3

Rappahannock 11.0 9.8 10.0 9.8 9.8 9.8

James 37.9 36.7 35.6 35.2 35. 35.1

York 9.3 8.9 8.6 8.4 8.4 8.4

East Shore MD-DE 31.6 29.8 29.2 29.2 29.1 29.7

East Shore VA 3.0 2.9 2.8 2.8 2.8 2.8

Total 350.7 328.1 316.5 311.7 309.7 313.0

Atmospheric Deposition of Nitrogen to the Tidal Chesapeake Bay

The regression and CMAQ models provide estimates of direct atmospheric deposition to the tidal 

waters of the Chesapeake Bay. Table L-5 lists the estimates of direct atmospheric deposition to 

the Bay’s tidal surfaces for seven key scenarios.

A key factor in the relative increase in the estimated reduced nitrogen deposition over time is the 

downward pressure on oxidized nitrogen emissions and the lack of controls on ammonia 

emissions. It is notable that changes in atmospheric chemistry of SOX and NOX in the seven key 

scenarios also affect ammonia dry deposition. In the scenarios with decreased SOX and NOX

emissions, the dry deposition of ammonia increases, even though the total nitrogen deposition is 

decreasing. The interplay of how decreased SOX and NOX emissions affect an increase of NH3

dry deposition is seen in Figure L-9.

How the percentage of ammonia, or reduced atmospheric deposition, to total nitrogen deposition 

is changing can be seen in Table L-5. For the 1985 Scenario, the percent ammonia deposition 

compared to the total DIN deposition was estimated to be 21 percent. For the 2010 and 2030 

scenarios, the percentage of ammonia deposition to the tidal Chesapeake was estimated to 

increase to 38 percent for the 2010 scenario and 55 percent for the 2030 scenario. The respective 

estimated ammonia deposition on the watershed for these same three scenarios—1985, 2010, and 

2030—are 24 percent, 44 percent, and 64 percent.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-19 September 24, 2010

Table L-5. Direct atmospheric deposition loads of nitrogen (millions of pounds as nitrogen) to Chesapeake 

Bay’s tidal surface waters for seven key scenarios

Total Wet Wet

Inorganic Organic Total Organic Total

Wet NOx Dry NOx Wet NH3 Dry NH3 Nitrogen Nitrogen Nitrogen Wet PO4 PhosphorusPhosphorus

SCENARIO Deposition Deposition Deposition Deposition Deposition Deposition Deposition Deposition Deposition Deposition

1985 Scenario 6.57 13.15 3.34 1.97 25.03 1.05 26.08 0.33 0.98 1.31

2002 Scenario 4.81 10.04 3.57 2.12 20.54 1.05 21.59 0.33 0.98 1.31

2010 Scenario 3.27 6.85 3.49 2.76 16.37 1.05 17.42 0.33 0.98 1.31

2020 Scenario 2.56 5.11 3.72 3.24 14.63 1.05 15.68 0.33 0.98 1.31

2020 Maximum Feasible Scenario 2.30 4.48 3.64 3.41 13.83 1.05 14.88 0.33 0.98 1.31

2020 Max Fes w/ 15% NH4 Drop 2.30 4.48 3.09 2.90 12.77 1.05 13.82 0.33 0.98 1.31

2030 Scenario 2.22 4.30 3.96 4.08 14.56 1.05 15.61 0.33 0.98 1.31

Note: This table includes two entries for the Maximum Feasible Scenario. The 2020 Max Fes w/15% NH4 Drop 

scenario includes the 15% decrease in wet and dry ammonia deposition for the Maximum Feasible Scenario due to 

ammonia emission control management practices in the Bay watershed jurisdictions described in CMAQ 2020 

Maximum Feasible Scenario; the 2020 Maximum Feasible Scenario does not.

Figure L-9. Decreased SOX and NOX emissions cause increased NH3 dry deposition.

Atmospheric Deposition of Nitrogen to the Coastal Ocean

The CMAQ Model allows us to estimate atmospheric deposition loads to the coastal ocean at the 

mouth of the Chesapeake Bay, which contributes to the coastal ocean nutrient budgets made by 

others (Fennel et al. 2006; Howarth et al. 1995; Howarth 1998). The estimated distribution of 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-20 September 24, 2010

2001 atmospheric deposition loads to North America and adjacent coastal ocean is shown in 

Figure L-10. Howarth (1998) reported that atmospheric deposition loads are roughly equivalent 

to watershed loads in the northeast United States (Maine to Virginia). Howarth (1998) estimated

that the watershed inputs of nitrogen to the northeast coastal waters to be 0.27 teragram. Inputs 

from direct atmospheric deposition to coastal waters are 0.21 teragram, and inputs from deep 

ocean upwelling are 1.54 teragrams for a total input to the coastal ocean of 2.02 teragrams

(Howarth 1998).

Figure L-10. Estimated 2001 annual total deposition of nitrogen (kg-N/ha) to North America and 

adjacent coastal ocean based on outputs from the CMAQ Air Quality Model, 36 km x 36 km grid.

That has implications for the fixed-ocean boundary condition used in the Chesapeake Bay Water 

Quality Sediment Transport Model (WQSTM). Atmospheric deposition total nitrogen loads to 

the coastal ocean are estimated to be about 6.63 kg/ha in the Base Case 2002 scenario (Table L6). That correlates to 43.8 million kilograms of total nitrogen deposition to a region of the ocean 

that can exchange waters with the Chesapeake (Table L-6). In the case of the 2020 Maximum 

Feasible scenario, the nitrogen atmospheric deposition to the same region is estimated to be 29.4 

million pounds, a reduction of 32 percent. If that same reduction is extrapolated to the coastal 

ocean, the direct atmospheric inputs to the coastal ocean would decrease to 0.14 teragram. 

Assuming the watershed loads discharged to the ocean and the deep upwelling pelagic loads are 

constant, that would give a combined watershed, direct deposition, and uncontrollable deep 

upwelling load of 1.95 teragrams, a decrease of 3 percent relative to the estimated current ocean 

boundary condition. Table L-6 lists the estimated reductions of the ocean boundary for the five 

key CMAQ scenarios

To determine CMAQ estimates of atmospheric deposition to the coastal ocean region affecting 

nitrogen loads through the ocean boundary EPA assigned boundaries as shown in Figure L-11 

that correspond to the proximate region of the coastal ocean exchanging waters with the 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-21 September 24, 2010

Chesapeake Bay. The boundary is adjacent to the shore, and is inside, or west, of the Gulf 

Stream. To account for the prevailing north to south current along the coast, the coastal ocean 

boundary includes more of the coastal waters north of the Chesapeake Bay mouth.

Figure L-11. Boundaries of the coastal ocean region used to adjust the ocean boundary 

conditions in the Chesapeake Bay WQSTM.

Estimated atmospheric deposition loads to the coastal waters are listed in Table L- 7 for key 

scenarios. The loads to the coastal ocean in kilograms per hectare for the CMAQ Base 2002 

scenario are shown in Figure L-12. TableM-8 lists the relative reduction of atmospheric 

deposition of nitrogen in coastal waters versus the Base Calibration scenario.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-22 September 24, 2010

Table L-6. Atmospheric deposition loads of nitrogen (kg per hectare) to the coastal water area

shown in Figure L-11 for key scenarios

Scenario Dry deposition Wet deposition Total deposition

Base 2002 Scenario 3.32 3.31 6.63

2010 Scenario 2.59 2.68 5.27

2020 Scenario 2.26 2.49 4.75

2020 Maximum Feasible 2.10 2.35 4.45

2030 Scenario 2.13 2.40 4.53

Table L-7. Total atmospheric deposition loads of nitrogen (millions of kg) to coastal waters for key 

scenarios

Scenario Dry deposition Wet deposition Total deposition

Base 2002 Scenario 21.90 21.89 43.80

2010 Scenario 17.12 17.71 34.82

2020 Scenario 14.94 16.45 31.39

2020 Maximum Feasible 13.87 15.50 29.37

2030 Scenario 14.06 15.88 29.95

Table L-8. Adjustment of the ocean boundary load for all nitrogen species for key CMAQ Model 

scenarios’ deposition to coastal waters adjacent to the Chesapeake Bay mouth

Scenario % Reduction of ocean boundary

Base 2002 Scenario 0%

2010 Scenario 2.1%

2020 Scenario 2.9%

2020 Maximum Feasible 3.5%

2030 Scenario 3.3%
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-23 September 24, 2010

Figure L-12. Nitrogen atmospheric deposition loads (kg/ha) to the coastal ocean region for the 

Base 2002 scenario.

Adjustment of Ocean Boundary Concentrations in the WQSTM from

Reductions in Atmospheric Deposition to Coastal Waters and 

Internal Bay Load Changes

Ocean boundary concentrations of the Bay Water Quality and Sediment Transport Model statevariables are set based on monthly observations at the Bay mouth water quality monitoring 

stations. The exchange of materials at the Bay mouth/ocean boundary follows the two layer 

flows of the estuary. Net outflow occurs predominantly at the upper and southern boundaries 

with the ebb tides, while net inflow occurs predominantly at the lower and northern boundaries. 

The ocean boundary values govern the inflowing flux of ocean nutrients and sediment to the 

Bay. Specifically, two adjustments are made to the ocean boundary conditions to adjust for 

changes in loads in the Chesapeake and for changes in atmospheric deposition.

 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-24 September 24, 2010

Adjustment of Nutrient Boundary Conditions Due to Load Reductions in the 

Chesapeake

Previous versions of the Bay Water Quality Model (8k grid version) found that a 90 percent 

reduction in nitrogen load from the watershed produced a 10 percent reduction in inflowing 

nitrogen concentration at the Bay mouth. Likewise, a 90 percent phosphorus load reduction 

produced a 5 percent reduction in inflowing phosphorus.

Accordingly, for each load reduction scenario, the percent reduction (or increase) of total 

nitrogen and total phosphorus loads in the entire Bay versus the Base Calibration scenario is 

calculated

TN reduction = 100 × (TN Base Calibration scenario – TN scenario) / TN Base Calibration 

scenario

TP reduction = 100 × (TP Base Calibration scenario – TP scenario) / TP Base Calibration 

scenario

 

EPA further calculates the following factors:

 TN Factor = 1 – 0.1 × TN reduction/90

 TP Factor = 1 – 0.05 × TP reduction/90

EPA then uses the TN factor and TP factor to multiply the Base Calibration ocean boundary 

concentrations of all the nitrogen and phosphorus nutrient species in each boundary cell, with the 

only exception of the cells in the southern boundary, because the southern Bay cells have 

predominantly outflows. No adjustments are made to ocean boundary sediment because it 

responds do different dynamics, and the source of the ocean input is primarily from courser 

particles entrained in the southbound long-shore current.

Adjustment of Nutrient Boundary Conditions from Atmospheric Deposition 

Load Reductions in the coastal Shelf

If a load reduction scenario involves reducing nitrogen load from the atmosphere, a further 

adjustment in the boundary conditions is done. A reduction of nitrogen atmospheric deposition 

on the coastal ocean adjacent to the Chesapeake Bay causes reductions of nitrogen 

concentrations in the shelf waters and thereby, reduction to inputs of nitrogen to the Bay.

For example, with the 2020 Clean-Air scenario, the reduction of atmospheric deposition of 

nitrogen versus the Base Calibration scenario in the shelf waters is 0.029 (Table L-8). In that 

case, the ocean boundary TN factor is further reduced by the third term on the right-hand side of 

the following equation:

 

TN Factor = 1 – 0.1 × TN reduction/90 – 0.029 × 26/32

In the above formula, the 0.029 is multiplied with a ratio of 26/32. That is based on the average 

salinity at the boundary to be 26 ppt, and the average salinity of shelf waters to be 32 ppt. The 
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-25 September 24, 2010

ratio 26/32 represents the ratio of the incoming ocean water over the sum of the incoming water 

and the freshwater going out the boundary (i.e., the mixing water at the boundary).

Allocation of Atmospheric Deposition of Nitrogen to Tidal Waters

In determining the allowable loading from air deposition, EPA separated the nitrogen deposition 

into two discreet parcels: (1) deposition occurring on the land and non-tidal waters which is 

subsequently transported to the Bay, also called indirect deposition; and (2) atmospheric 

deposition occurring directly onto the Bay tidal surface waters also called direct deposition 

(Figure O-13).

Figure L-13. EPA’s reference allocation of nitrogen atmospheric deposition to the Bay watershed 

and the allocation of nitrogen atmospheric deposition direct to tidal waters.

The deposition on the land becomes part of the allocated load to the jurisdictions because the air 

deposition on the land becomes mixed with the nitrogen loadings from the land-based sources 

EPA Referenced Allocation of

Deposition to the Watershed

EPA Allocation of Deposition

to Tidal Waters
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-26 September 24, 2010

and, therefore, becomes indistinguishable from land based sources. Furthermore, once the 

nitrogen is deposited on the land it, would be managed and controlled along with other sources 

of nitrogen that are present on that parcel of land. That is also called the referenced allocation as 

Clean Air Act mandates nationwide reductions, as estimated in the CMAQ 2020 scenario, are 

required to reduce the air deposition to the watershed and are assumed to be in place as the Bay 

watershed jurisdictions finalize and implement their Watershed Implementation Plans to reduce 

nitrogen loads further with land-based Best Management Practices (BMPs). In contrast, the 

nitrogen deposition directly to the Bay’s tidal surface waters is a direct loading with no landbased management controls and, therefore, needs to be linked directly back to the air sources and 

air controls as EPA’s allocation of atmospheric nitrogen deposition.

EPA included an explicit basinwide nitrogen allocation, which was determined to be 15.7 million 

pounds of atmospheric deposition loads direct to Chesapeake Bay and tidal tributary surface 

waters. Activities associated with implementation of federal Clean Air Act regulations by EPA 

and the jurisdictions through 2020 will ensure achievement of this allocation. This nitrogen 

atmospheric deposition allocation is already accounted for within the jurisdiction and major river 

basin nitrogen allocations. Any additional nitrogen reductions realized through more stringent air 

pollution controls at the jurisdictional level, beyond federal requirements to meet air quality 

standards, may be credited to the individual jurisdictions through future revisions to the 

jurisdictions’ Watershed Implementation Plans s, 2-year milestones, and the Chesapeake Bay 

TMDL tracking and accounting framework.

In determining the amount of air controls to be used as a basis for the air allocation, EPA relied 

on current laws and regulations under the Clean Air Act. These requirements, together with 

national air modeling analysis, provided the resulting allocated load to air from direct deposition 

to the tidal waters of the Bay and its tidal tributaries.

The air allocation scenario represents emission reductions due to regulations implemented 

through the Clean Air Act authority to meet National Ambient Air Quality Standards for criteria 

pollutants in 2020. The air allocation scenario includes

The CAMR.

The BART used for reducing regional haze, and the off-road diesel and heavy duty diesel 

regulations.

On-Road mobile sources: For On-Road Light Duty Mobile Sources this includes Tier 2 

vehicle emissions standards and the Gasoline Sulfur Program, which affects SUVs pickups, 

and vans, which are now subject to same national emission standards as cars.

On-Road Heavy Duty Diesel Rule – Tier 4: New emission standards on diesel engines 

starting with the 2010 model year for NOx, plus some diesel engine retrofits.

Clean Air Non-Road Diesel Rule: Off-road diesel engine vehicle rule, commercial marine 

diesels, and locomotive diesels (phased in by 2014) require controls on new engines.

EGUs: CAIR second phase in place (in coordination with earlier NOx SIP call).

Non-EGUs: Solid Waste Rules (Hospital/Medical Waste Incinerator Regulations).
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-27 September 24, 2010

The controls described above were modeled using the national air models (CMAQ) and the 

amount of deposition direct to the Chesapeake Bay’s tidal surface waters was determined. On the 

basis of the air allocation scenario as described above, the nitrogen deposition direct to tidal 

surface waters is 15.7 million pounds per year. Therefore, the air allocation for the Chesapeake 

Bay TMDL is 15.7 million pounds per year of nitrogen.

EGU

mobile

off road diesel

combined

2010 2025

New Rule - New CMAQ Run

EGU

mobile

off road diesel

combined

2010 2025

New Rule - New CMAQ Run

EPA anticipates that the loading cap of 15.7 million pounds of atmospheric deposition loads 

direct to Chesapeake Bay and tidal tributary surface waters will be achieved through 

implementation of federal Clean Air Act regulations by EPA and the states through 2020. 

Projected reductions in atmospheric deposition loads to the surrounding watershed over this 

same period are already accounted for within the individual jurisdiction and major river basin 

nitrogen draft allocations. Any additional nitrogen reductions realized through more stringent air 

pollution controls at the jurisdiction level, beyond minimum federal requirements, as for example 

in ammonia deposition reductions, may be credited to the individual jurisdictions through future 

revisions to the jurisdictions’ Watershed Implementation Plans s, two-year milestones and the 

Bay TMDL tracking and accounting framework.
Draft Appendix L – Chesapeake Bay TMDL for Nutrients and Sediment

L-28 September 24, 2010

References

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Sulfur and nitrogen deposition on ecosystems in the United States. Journal of the Air and 

Waste Management Association. December 2007.

Fennel et al. 2006

Grimm, J.W., and Lynch, J.A., 2000, Enhanced wet deposition estimates for the Chesapeake Bay 

watershed using modeled precipitation inputs: DNR Chesapeake Bay and Tidewater 

Programs CBWP-MANTA-AD-99-2.

Grimm, J.W., and Lynch, J.A., 2005, Improved daily precipitation nitrate and ammonium 

concentration models for the Chesapeake Bay Watershed: Environmental Pollution, v. 135, 

no. 2005, p. 445-455.

Hameedi, J., Paerl, H., Kennish, M., and Whitall, D., 2007. Nitrogen deposition in U.S. coastal 

bays and estuaries. Journal of the Air and Waste Management Association. December 2007.

Howarth, R.W. 1998. An assessment of human influences on fluxes of nitrogen from the 

terrestrial landscape to the estuaries and continental shelves of the North Atlantic Ocean: 

Nutrient Cycling in Agroecosystems, v. 52, p. 213-223.

Howarth, R. W.; Billen, G.; Swaney, D.; Townsend, A.; Jaworski, N.; Lajtha, K.; Downing, J. 

A.; Elmgren, ER.; Caraco, N.; Jordan, T.; Berendse, F.; Freney, J.; Kudeyarov, V.; Murdoch, 

P.; Zhao-liang, HZhu, 1995. Regional nitrogen budgets and riverine N & P fluxes for the 

drainages to the North Atlantic Ocean: Natural and human influences. Biogeochemistry 35:1 

pp 75-139.

Keene et al. 2006.

Knap et al. 1986.

Linker, L.C., G.W. Shenk, R.L. Dennis, and J.S. Sweeney. 2000. Cross-Media Models of the 

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Lynch, J.A., and J.W. Grimm. 2003. Improved Daily Nitrate and Ammonium Concentration 

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Moker and Zika Year?

STAC (Scientific and Technical Advisory Committee). 2007. Workshop on Atmospheric

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on the Landscape. May 30, 2007 SUNY Binghamton, Binghamton, New York.

Scudlark, J. R. and T. M. Church. 1993. ―Atmospheric Input of Inorganic Nitrogen to Delaware 

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Resources, Annapolis, MD.

Smullen, J. T., J. L. Taft, and J. Macknis. 1982. Nutrient and sediment loads to the tidal 

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