Significant uncertainty exists in the magnitude and variability of ammonia (NH3) emissions. NH3 emissions are needed as input for air quality modeling of aerosols and deposition of nitrogen compounds. Approximately 85% of NH3 emissions are estimated to come from agricultural nonpoint sources, which are suspected to have a strong seasonal pattern. Because no seasonal information is available in current NH3 emission inventories for air quality modeling, the emissions are often distributed evenly over the year by default. Doing so can adversely affect air quality model-predicted concentrations of nitrogen-containing compounds, as shown here. We apply a Kalman filter inverse modeling technique to deduce monthly 1990 NH3 emissions for the eastern United States. The U.S. Environmental Protection Agency (USEPA) Community Multiscale Air Quality (CMAQ) model and ammonium (NH4+) wet concentration data from the National Atmospheric Deposition Program network are used. The results illustrate the strong seasonal differences in NH3 emissions that were anticipated, where NH3 emissions are more than 75% lower during the colder seasons fall and winter as compared to peak emissions during summer. The results also suggest that the current USEPA 1990 National Emission Inventory for NH3 is too high by at least 20%. This is supported by a recent USEPA study of emission factors that proposes lower emission factors for cattle and swine, which are two of the largest sources of NH3 emissions in the inventory.