Wet and Dry Deposition of NHX to Tampa Bay

1
Wet and Dry Deposition of NHX to Tampa Bay
Are these wet and dry ammonia deposition rates
high?
Why are we interested in NHx (ammoniaammonium)
atmospheric deposition?
Noreen D. Poor University of South Florida,
College of Public Health Department of
Environmental and Occupational Health, Tampa,
Florida
The ammonium deposition rate measured at the
Gandy Bridge site is 20 to 50 higher that rates
measured at the closest (but rural) NADP National
Trends Network sites that are to the north
(Chassahowitzka), east (Kennedy Space Center) and
south (Sarasota) of Tampa. If the 1.4 ratio of
dry to wet NHx deposition holds at these sites as
well as the Gandy Bridge site, and if the Gandy
Bridge site is representative of the entire Tampa
Bay, then the total NHx nitrogen deposition is
also 20 to 50 higher than at the nearest
background sites. We can also compare the
total atmospheric NHx flux of 450 metric tons
yr-1 with the 30 metric tons of ammonium stored
in the Tampa Bay 3.2 km3 water volume with a
typical ammonium nitrogen concentration of 0.01
mg l-1 (Wang, et al.,1999). We know that ammonia
is cycled through nitrate and organic nitrogen,
so a better comparison may be with the 630 metric
tons of organic plus inorganic nitrogen stored in
the Tampa Bay water volume. Either way, the NHx
flux is a significant external load of nitrogen
to Tampa Bay.
Photograph by J. Earls, 2000
Ammonia is a biologically-active nitrogen
compound that at elevated concentrations
contributes to the eutrophication of surface
waters. The microbial oxidation of ammonia to
nitrate fertilizes the production of algae. In
shallow waters, algal blooms threaten sea grass
beds that serve as a vital nursery for Tampa Bay.
The estimated annual atmospheric NHx nitrogen
flux to Tampa Bay is 450 metric tons, more than
10 of the Tampa Bay 3,500 metric tons total
annual nitrogen load (Wang, et al., 1999)
How is the dry deposition of ammonia calculated?
The NOAA inferential model calculates the gas and
particle dry deposition velocities (vdg, vdp)
with over-water 30-min wind speed, air
temperature, water temperature, and relative
humidity as inputs (Valigura, 1995). These
deposition velocities are averaged for each
month. Ambient air concentrations (Cair) of
ammonia and ammonium measured at the Gandy Bridge
site are also averaged for each month. Ammonia
equilibrium concentrations (Ceq) are estimated
from monthly water quality observations at a
station just west of the Gandy Bridge site. These
calculations are described by Asman, et al.
(1994). In the below equations, Henrys Law
constant (HNH3) has units of M atm-1, the
ammonium disassociation constant (KNH4) has units
of M, and salinity (S) has units of ppt.
Graphic courtesy of TBEP (www.tbep.org)
How are ambient air concentrations of NHx
measured?
What are the sources of atmospheric ammonia?
To pump and mass flow controller
Nylon filter
Intensive agriculture and related industries
contribute significantly to anthropogenic
atmospheric ammonia, as is demonstrated by a
recently completed United Kingdom ammonia
emissions inventory summarized in the table
below. On a global scale, biogenic emissions
still exceed anthropogenic emissions, with 45 Mt
versus 30 Mt of ammonia nitrogen.
A URG, Inc.?, annular denuder system (ADS)
located at a monitoring site on the eastern end
of the Gandy Bridge traps ambient air gases of
sulfur dioxide, nitric acid and ammonia, and
aerosols of sulfate, nitrate and ammonium.
Co-located with an identical unit at a height of
5 m, the ADS collects samples at an airflow rate
of 10 l min-1 for 24 hours every 6 days. Each ADS
consists of a 2.5 ?m particle cutpoint cyclone
inlet, two 150-mm long gas denuders and a filter
pack in series.
Ammonia denuder
Example For an air temperature of 23 oC, a water
temperature of 25 oC, a salinity of 25 ppt, an
ammonium concentration of 0.01 mg l-1 and a pH
7.9, the estimated Ceq is 0.15 ?g m-3. With an
average monthly vdg of 0.007 m s-1 and an average
monthly ammonia Cair of 1.7 ?g m-3, the FNH3 is
0.011 ?g m-2 s-1. With an average monthly vdp of
0.001 m s-1 and an average monthly ammonium Cair
of 0.8 ?g m-3, the FNH4 is 0.0008 ?g m-2 s-1.
Thus the average monthly NHx nitrogen flux is
0.0097 ?g m-2 s-1 (0.26 kg ha-1). As can be seen
in the above cumulative area plot, NHx nitrogen
dominates the total dry nitrogen deposition. In
July 1998, the combination of 28 oC waters and an
ammonium concentration of 0.03 mg l-1 raised Ceq
above Cair, thus reversing the direction of the
ammonia flux. Coupled air and water ammonia
measurements are needed to see if the flux
reversal actually occurs.
Airflow
Acid gas denuder
Cyclone inlet
The first of two denuders is coated with sodium
carbonate to absorb the acid gases, and the
second denuder is coated with citric acid to
absorb the ammonia. A single 47-mm diameter 1-?m
pore size nylon filter retains the aerosols. The
denuders and filter are extracted with an aqueous
solution and the extract analyzed for ammonium by
automated colorimetry.
URG, Inc.?, Annular Denuder System (ADS).
How is the wet deposition of ammonia calculated?
Table compiled from Misselbrook, et al. (2000)
and Sutton, et al. (2000)
How are rainfall concentrations of ammonia
measured?
Wet deposition is calculated as the product of
ammonium concentration (Crain) in daily rainfall
and the depth of the rainfall (L), with units of
mg l-1 and mm, respectively. Multiplying by 10-2
converts these units to kg-N ha-1. The daily
rainfall fluxes are then summed for a month to
get the monthly ammonium wet deposition. The
rainy months in Tampa are typically June, July,
August and September but the 1997-1998 El Niño
phenomenon brought unseasonable record rainfalls,
and ammonium wet deposition reached 0.6 kg-N ha-1
or 60 metric tons to Tampa Bay in April 1997.
References
A wet-only precipitation bucket is located at the
Gandy Bridge monitoring site and is part of the
National Atmospheric Deposition Program AirMon
Network (http//nadp.sws.uiuc.edu)
Asman, W. A. H., R. M. Harrison, and C.J. Ottley
(1994) Estimation of the net air-sea flux of
ammonia over the southern bight of the North Sea.
Atmospheric Environment 28 3647-3654. Misselbroo
k, T.S., T. J. Van Der Weerden, B. F. Pain, S.C.
Jarvis, B.J. Chambers, K. A. Smith, V. R.
Phillips, T.G.M. Demmers (2000) Ammonia emission
factors for UK agriculture. Atmospheric
Environment 34 871-880. Sutton, M. A., U.
Dragosits, Y.S. Tang, and D. Fowler (2000)
Ammonia emissions from non-agricultural sources
in the UK. Atmospheric Environment 34
855-869. Wang, P.F., J. Martin, and G. Morrison
(1999) Water quality and eutrophication in Tampa
Bay, Florida. Estuarine, Coastal and Shelf
Science 491-20.
Wet-only precipitation bucket (left) and rain
gauges (right).
During rainfall, a lid covering the rain bucket
lifts off and water collects in the bucket. The
lid covers the bucket once the rain stops. Total
daily rainfall is measured with a co-located
National Weather Service rain gauge, and the
weight of water collected per rainfall is
recorded by a Belfort rain gauge. The rain
bucket is inspected daily, and if water is
present, the sample is decanted and stored
refrigerated for up to one week. Each week, the
collected samples are shipped to the Illinois
State Water Survey for analysis of the water pH,
conductivity, and inorganic ions including
ammonia nitrogen.
Acknowledgements
The author thanks the Tampa Bay Estuary Program
for funding this research, and the Florida
Department of Environmental Protection,
Environmental Protection Commission of
Hillsborough County, USEPA Region IV,
ESE-Harding, Inc., Lee Chapin and Ray Pribble for
their assistance with data collection and
analysis.