Understanding Atmospheric Deposition in Tampa Bay
A public summary document of the Bay Region Atmospheric Chemistry Experiment (BRACE)
What goes up, must come
down!
This phrase doesn’t apply only to gravity,
but to many pollutants and materials
emitted to the air that return to the earth
over the land and water and contribute to
water quality problems.
NOx
NH3
NH3
Atmospheric deposition is the process by
which pollutants and other small substances
are transferred from the atmosphere to
the earth’s surface. Air pollutants such as
nitrogen oxides or sulfur dioxides can return
to the land and water as wet deposition (for
example, in rainfall) or as dry deposition,
which includes both particle dryfall and
surface gas exchange.
Harmful substances such as pesticides and
chemicals can be emitted into the air, often
through aerial spraying, the burning of fossil
fuels, or evaporation from the surface. Once
in the air, these substances can move with
the wind and deposit back onto the earth.
Compounds such as sulfur dioxide and
nitrogen oxides can react with water in the
atmosphere, causing “acid rain.” In extreme
cases the “acid rain” can cause property
damage and harm ecosystems. In the Tampa
Bay region, scientists and managers are
concerned about water pollution caused by
atmospheric deposition.
There are various forms of
nitrogen in the environment
Nitrogen is a chemical element with
the symbol N. It is found mainly in an
inert gaseous state as N2 and makes up
78% of the air we breathe. Only under
special conditions is N2 converted to a
biologically active form. Once converted
or “fixed,” nitrogen readily combines with
other elements to form new compounds.
“Fixed” nitrogen is a building block of
living organisms. Nitrogen compounds
containing oxygen such as Nitrogen Oxide
(NOx) are called “oxidized” forms, while those
without oxygen such as Ammonia (NH3) are
“reduced” forms. The most common forms of
nitrogen discussed in this document are:
Nitrogen Oxide: Nitrogen
x combined with 1 or 2 Oxygen
molecules
Ammonia: Nitrogen combined
3 with 3 Hydrogen molecules
NO
NH
NOx
NOx
NOx
NOx
NH3
NH3
Fertilizer
32-10-10
NOx
NOx
NH3
NH3
NOx
NOx
NOx
NH3
There are numerous sources of nitrogen emissions. The following are the major sources:
Sources of ammonia NH3 :
Fertilizer
32-10-10
Agricultural operations and
livestock production
Cars and trucks
Power plants and industry
Residential wastewater
Sources of nitrogen oxides NOx :
Power plants and industry
Residential and commercial
buildings
All mobile sources
Compressors and forklifts
Nitrogen emissions
are carried upwards. Once in the atmosphere, nitrogen returns
to earth through wet deposition
or dry deposition
. All of these sources
contribute to nitrogen pollution
in Tampa Bay.
Conceptual diagram from Cross, L.M. 2012. Understanding atmospheric deposition in Tampa Bay. Symbols courtesy of the Integration and Application Network.
Atmospheric nitrogen emissions come from natural and
anthropogenic sources
Sources of NOx emissions to
Atmospheric nitrogen comes from emissions
from both natural and anthropogenic
(human-derived) sources. Natural sources
include fecal matter from wild animals, forest
fires, lightning, oceans, soils, and vegetation.
In a densely populated watershed such as
Tampa Bay’s, natural sources make up a
relatively small component of all atmospheric
deposition. Human-derived sources include
emissions from fertilizer production and use,
human and farm animal waste, and fossil fuel
combustion. Of these, the burning of fossil
fuels in power plants, industry, and in mobile
sources such as cars, trucks, planes, and boats
is the largest contributor.
Tampa Bay watershed
Power Plants (50%)
Mobile Sources (25%)
Other Sources (25%)
Source: Poor et al., 2012
BRACE helps scientists better understand atmospheric deposition
Results show atmospheric deposition is a major source of nitrogen
Nitrogen is a pollutant in
Tampa Bay
Modeling allowed scientists to
calculate the total quantity of
nitrogen entering Tampa Bay from
atmospheric deposition. They
began with model-derived average
deposition rates across the land and
bay portions of the watershed and
then calculated the total loading
from direct and indirect atmospheric
deposition. Loading from point
sources such as domestic wastewater
and non-point sources including
stormwater was calculated from water
quality monitoring data collected at
facilities or at long-term monitoring
stations throughout Tampa Bay.
Bay area scientists were involved with an
air quality modeling and measurement
project called the Bay Region Atmospheric
Chemistry Experiment (BRACE) from 2002 to
2007. The goals of the project were to:
Improve estimates of atmospheric
nitrogen deposition to Tampa Bay.
Determine the sources of
atmospheric deposition in the local Tampa
Bay area and beyond.
Although useful, models are not exact.
Depending on the measurement techniques
or the computational tools used, as well as
when measurements were taken, estimates
may vary. That is why it is beneficial to use a
variety of tools, models, and measurements.
In this project, model results were compared
to actual air pollutant measurements taken
at the ground level and from aircraft as a
way to ”ground-truth” the results.
Assess the impact of air quality
regulations on nitrogen deposition to Tampa
Bay.
Seagrass requires clear water, free from excess pollutants
such as nitrogen.
Waterbodies with heavy algal growth cannot support a
healthy community of aquatic plants and animals.
Atmospheric deposition can
enter Tampa Bay directly or
indirectly
Atmospheric deposition of nitrogen to
Tampa Bay can occur in two forms - direct or
indirect - as discussed below:
Indirect Atmospheric Deposition
occurs when air emissions fall first onto the
land or watershed surface and then enter
the bay with stormwater. Indirect deposition
is included in the stormwater category.
This graphic depicts Tampa Bay and its watershed,
indicated by the darker brown colors. Rain falling
anywhere within the watershed, which is approximately
six times as large as the bay, can eventually drain into
Tampa Bay. Monitoring stations are also noted.
Domestic
Point Sources
(11%)
Industrial Point
Sources (3%)
Groundwater
and Springs (3%)
Material
Losses(1%)
Non-point
Source
(Stormwater)
from Other
(25%)
Indirect
Atmospheric
Deposition
through
Stormwater
(40%)
Atmospheric deposition, both direct and in-direct via
stormwater, accounted for 57% of the total nitrogen
loading to Tampa Bay in 2002 as shown in the graphic to
the left. Non-point sources, including indirect atmospheric
deposition, currently contribute more nitrogen to the bay
than point sources. Source: Poor et al., 2012
Mobile sources have a disproportionate impact
Scientists installed air quality
monitors at six Tampa Bay
locations. Results were
analyzed in a laboratory and
compared to sophisticated
computer model simulations.
Photos: N Poor
Scientists used high tech instruments to
measure meteorology and air pollutant
concentrations at six temporary sites across
the watershed. These measurements were
used to validate CMAQ model performance.
Two additional sites, Gandy Bridge Wellfield
and Verna Wellfield, hosted long-term
atmospheric deposition measurements.
The Gandy Bridge site is located in Tampa
directly across the bay from the Progress
Energy Bartow Plant in northern St.
Petersburg. The Verna Wellfield monitoring
site is located in Sarasota County, just
outside the Tampa Bay watershed. Both
locations are noted on the watershed map
at the left.
Atmospheric deposition is
predominately made up
of emissions from mobile
sources such as cars, trucks,
diesel vehicles, and boats; and
stationary sources such as
power plants. Mobile emissions
are released closer to the
surface of the land or water
and, therefore, have a much
larger influence locally than
stationary sources that release
emissions from tall stacks higher
into the atmosphere. Emissions
from power plants and other
stationary sources may travel
longer distances throughout
the airshed, with deposition
occurring outside the watershed.
Mobile sources have a disproportionately higher
contribution than power plants to atmospheric
nitrogen deposition to Tampa Bay.
NOx
NOx
NOx
NOx
NOx
NOx
Over the watershed,
NOx emissions from
mobile sources were
responsible for 4 times
more NOx deposition
than power plants.
Gulf of
Mexico
Research shows that approximately 2/3 of
nitrogen loading to Tampa Bay through both
direct and indirect deposition is from dry
deposition, despite the watershed’s average
annual rainfall rate of more than 50 inches.
Relative contributions of wet and dry
deposition to total atmospheric deposition
of nitrogen to Tampa Bay
Over the bay, NOx
emissions from mobile
sources were responsible
for twice the NOx
deposition than power
plants.
Source: Poor et al., 2012
Half of all NOx emissions come from
outside the Tampa Bay watershed
NOx
emissions
from within
Tampa Bay
watershed
(50%)
Dry deposition a larger
source than wet deposition
NOx
Tampa Bay
Airshed
Air monitors were
also attached to
a mobile trailer,
“Sydney,” at a
Tampa Bay BRACE
site.
Photos: N Poor
NOx
Emissions from outside the watershed are significant
Monitors at the
Gandy Bridge site
measured emissions
from the Bartow
power plant before
and after conversion
to natural gas.
Direct Atmospheric Deposition
occurs when air pollutants such as NOx and
NH3 fall directly onto Tampa Bay, interacting
with the bay’s water surface. The nitrogen
is then incorporated into the water and
becomes a source of water pollution.
Direct
Atmospheric
Deposition
(17%)
Results suggest atmospheric deposition is
a large source of nitrogen loading to Tampa
Bay. In 2002, for example, CMAQ-supported
estimate was that 57% of all nitrogen
entering Tampa Bay came from atmospheric
sources. Of that, 17% was direct deposition
to the bay; 40% returned indirectly to
the bay via stormwater runoff from the
watershed. In 2002, nitrogen coming from
the air made up 1,973 tons of the 3,485 total
tons of nitrogen entering the bay.
Source: Poor et al., 2012
Project used air monitors
and computer models
This study used a combination of computer
models, national emissions data, and
local air quality monitoring. The computer
modeling system, the Community
Multiscale Air Quality (CMAQ) v4.4,
simulates how air emissions behave in the
atmosphere, including interaction with
other pollutants, sea salt (spray), and water.
For example, in a cloud, molecules of NOx or
NH3 may change from gas to solid particles,
then to water-dissolved particles in raindrops
that return to the surface as wet deposition.
CMAQ simulations of air pollution emissions
for the entire continental US provided a
broader perspective on the impact to Tampa
Bay of nitrogen sources from outside the
watershed.
Total Nitrogen Loading to Tampa Bay in 2002
(3,485 Tons)
NOx
emissions
from outside
watershed
(50%)
Atlantic
Ocean
Lake
Okeechobee
Source: R. Dennis,
US EPA
Just like there is a Tampa Bay watershed,
there is also a Tampa Bay airshed - an area
where air pollutant sources contribute to
atmospheric deposition in Tampa Bay. The
airshed is much larger than the watershed,
encompassing almost the entire state and
extending into the southeastern US. This
means that emissions from places as far
away as Atlanta may drift into the Tampa Bay
area. Results of this study suggest that half
of the NOx emissions measured in Tampa
Bay comes from outside the watershed. This
means that reductions in emissions on a
state-wide or region-wide level could have
a positive impact on our local air and water
quality.
Wet Deposition (33%)
Dry Deposition (67%)
Source: Poor et al., 2012
Nitrogen is an important nutrient that is
necessary for plant growth. However, too
much nitrogen can cause excessive algae
growth that can block sunlight necessary
for seagrass growth. It can also deplete
dissolved oxygen levels, leading to fish
kills. Nitrogen enters Tampa Bay through
point sources such as direct discharge of
wastewater, as well as non-point sources.
Stormwater, a non-point source, includes
runoff from urban, residential, agricultural,
mining, and natural areas.
Goals of BRACE project
Oxidized forms of nitrogen such as
nitrogen oxides (NOx) made up 60% of the
total atmospheric loading to Tampa Bay,
compared to 40% for reduced forms such as
ammonia (NH3). Although more research
has focused on NOx, NH3 is still an important
contributor to nitrogen loading.
Relative contributions of oxidized and
reduced forms of nitrogen in Tampa Bay
Reduced N,
such as NH3
(40%)
Oxidized N,
such as NOx
(60%)
Source: Poor et al., 2012
Tampa Bay community committed to managing nitrogen inputs
Partnerships formed to
manage nitrogen pollution
Managing nitrogen in Tampa Bay is a key
goal of the Tampa Bay Estuary Program
(TBEP), an intergovernmental partnership
devoted to protecting and restoring
Tampa Bay. TBEP and its partners in the
Nitrogen Management Consortium (NMC)
have developed a “nitrogen management
strategy” to encourage industries,
governments, and
citizens in the Tampa
Bay area to minimize
their nitrogen loading
to the bay.
Tampa Bay
NMC
Cities and counties have improved
wastewater treatment and expanded
use of reclaimed water. Industries have
reduced discharges and expanded onsite stormwater treatment. Agricultural
operations have installed water-conserving
irrigation that reduces fertilizer runoff. And
homeowners are helping out by properly
disposing of pet waste and by not applying
nitrogen fertilizers during summer rains.
Upgrades at Tampa Bay
power plants result in better
air and water quality
Reducing nitrogen in air pollution is an
important element of this strategy. Since
2002, power plants in the Tampa Bay have
reduced nitrogen emissions through
various initiatives. The Gannon, Big Bend,
and Bartow plants installed NOx reduction
equipment on their smoke stacks. The
Gannon and Bartow plants also converted
their facilities from coal-fired, which
produces more sulfur dioxide, carbon
dioxide, and nitrogen oxides, to natural gas.
Between 2002 and
2010, these power
plant upgrades
resulted in a
reduction of 95 tons
of nitrogen through
deposition. Air
quality monitors
in two Tampa Bay
counties showed
a decrease in NOx
concentrations
during this period.
National air pollution
regulations target fixed and
mobile sources
National air quality regulations such as
the Clean Air Interstate Rule (CAIR) seek
to improve air quality through phased
reductions in emissions from both fixed
and mobile sources. CAIR regulations
address sulfur dioxide and nitrogen oxide
pollution in 27 eastern states and the
District of Columbia to improve human and
ecosystem health. CAIR focuses primarily
on power plants that can reduce emissions
by installing pollution control equipment,
switching fuels, or buying excess allowances
from other sources that have reduced their
emissions through a cap- and-trade system.
Management
recommendations for better
air and water quality
This study showed that atmospheric
deposition is a major source of nitrogen
loading to Tampa Bay. Therefore, reducing
nitrogen deposition may have a significant
impact on water quality in Tampa Bay. Many
projects already have been implemented
to reduce nitrogen loading from point and
non-point sources, but to protect human
health and to maintain water quality in
Tampa Bay, additional reductions will be
necessary to offset new nitrogen loading
from population growth in the watershed.
Management recommendations from this
study include the following:
Encourage use of low-emission
personal vehicles and public transportation.
US EPA cites that driving a car is a typical
citizen’s most polluting daily activity.
Support partner actions (e.g., Tampa
Bay Nitrogen Management Consortium)
related to air quality improvements.
If fully implemented by 2020, CAIR and
related federal regulations would remove 7.4
million tons of NOx emissions from all states
in the continental US, with 5.7 million tons
from states in the eastern US. About 75%
of this reduction is from a modernized fleet
of on-road motor vehicles. Current federal
regulations do not address NH3 emissions,
which are expected to grow with increased
agricultural demand.
BRACE scientists used computer models
to estimate the potential impact of these
regulations on atmospheric nitrogen
deposition to Tampa Bay and its watershed.
Using 2002 meteorology, implementation
of CAIR and related federal regulations
through 2010 would lead to a 24% reduction
of atmospheric nitrogen to the bay. This
corresponds to a removal of approximately
430 tons of nitrogen, more than 12% of the
total nitrogen loading to Tampa Bay in 2002.
Continued implementation of national air
quality regulations may ultimately improve
both air and water quality within the Tampa
Bay watershed.
Re-establish atmospheric
deposition monitoring stations within the
Tampa Bay watershed to track changes in
emissions.
Develop better monitoring and
measurements of direct dry deposition.
Citizens, industries, and governments
all play an important role in reducing air
pollution and keeping Tampa Bay healthy
and vibrant!
Acknowledgements
The BRACE project was funded by the Florida
Department of Environmental Protection.
Technical synthesis was completed by Noreen
Poor, Kivmetrics, LLC. [email protected]
Poor, N.D., Cross, L.M., Dennis, R.L. 2012. Lessons learned from
the Bay Region Atmospheric Chemistry Experiment (BRACE)
and Implications for Nitrogen Management of Tampa Bay.
Atmospheric Environment, in review.
Public summary document prepared by Lindsay
M. Cross, TBEP. [email protected]
Cross, L.M. 2012. Understanding atomspheric
deposition in Tampa Bay. Technical report #04-12 of the Tampa
Bay Estuary Program. Available at www.tbeptech.org