Ammonia exchange between the atmosphere and the surface waters at two locations in the Chesapeake Bay

Excess phytoplankton production, which contributes to hypoxic conditions, is nitrogen limited in the Chesapeake Bay during the summer months. Therefore, understanding the flux of ammonia by direct deposition to the biologically active surface layer is critical to understanding the nutrient dynamics of the bay. This paper presents the results of a 2-yr study measuring gaseous ammonia (NH3) and aerosol ammonium (NH4+) in Baltimore and Solomons, MD,from which direct atmospheric loading of total ammonia (N-t = NH3 + NH4+) to the Chesapeake Bay is estimated. Mean atmospheric concentrations of total ammonia for Baltimore and Solomons were 2.7 +/- 1.7 and 1.0 +/- 0.8 mug of N m(-3), respectively. Monte Carlo estimates of gross dry deposition ranged from < 100 to 4900 mug of N m(-2) d(-1). However, based upon water quality parameters, Monte Carlo estimates of gross volatilization of NH3 were calculated to range from < 100 to 7700 mug of N m(-2) d(-1). The resulting net air-sea exchange flux varied seasonally from a net deposition into the water during the winter to a net volatilizing into the atmosphere during the summer. A total of 60% of the paired air-water samples had flux estimates that were not significantly different than equilibrium at the 90% confidence interval. The gross deposition, gross volatilization, and not air-sea fluxes were greater and more variable in Baltimore relative to the rural site. Atmospheric ammonia concentrations decrease during the winter at the rural site. However, the not exchange is still into the water due to an exponential decrease in [NH3](eq) with temperature. These results indicate that the nitrogen-limited Chesapeake Bay can act as a source of ammonia to the local atmosphere.