Wet and Dry Deposition of NHX to Tampa Bay Are these wet and dry ammonia deposition rates high? Why are we interested in NHx (ammonia+ammonium) 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, Henry’s 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). Cair= concentration of NHx nitrogen in air (g m-3) Cextract= concentration of NH4 nitrogen in extract (g l-1) Vextract = volume of extract (l) Vair = volume of air through sampler (14.4 m3) 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. Misselbrook, 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 49:1-20. 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.