The partitioning of total nitrate (TNO
3) and total ammonium (TNH 4) between gas and aerosol phases is studied with two thermodynamic equilibrium models, ISORROPIA and the aerosol inorganics model (AIM), and three data sets: high time resolution measurement data from the 1999 Atlanta Supersite Experiment (summer case) and the 2002 Pittsburgh Air Quality Study (PAQS) Supersite Experiment (winter case), and 12-hour measurement data from the Clinton site, North Carolina, in 1999. At the Atlanta site, both models reproduced a large percentage of the observed aerosol NHj and HNO 3 (NH 4: >94% and HNO 3: >86%) within a factor of 1.5, whereas neither model reproduced a majority of observed aerosol NO 3- and NH 3 (NO 3-: <48% and NH 3: <51%) within a factor of 2. At the Pittsburgh site, both models reproduced more than 76% of observed NO 3 - within a factor of 2. At the Clinton site, both models performed a little better on aerosol NO 3- (47-58% within a factor of 1.5) than at the Atlanta site but worse than at the Pittsburgh site. Sensitivity test of thermodynamic models with Gaussian random errors indicates that in many cases, measurement errors in SO 42- and TNH 4 can explain a major fraction of the discrepancies between the equilibrium model predictions and observations in partitioning of TNO 3. Comparison of predictions of the three-dimensional (3-D) Community Multiscale Air Quality (CMAQ) model with the observations over the continental United States indicates that the performance of the 3-D model for NO 3-, HNO 3, NH 4-, and NH 3 strongly depends on its performance for TNO 3, TNH 4, and SO 42-. Tests show that errors associated with SO 42- and TNH 4 predictions of the 3-D model can result in the thermodynamic model calculation replicating only 47% and 60% of base case NO 3- within a factor of 2 for summer and winter cases, respectively. It was found that errors in TNH 4 are more critical than errors in SO 42- to prediction of NO 3-. Copyright 2005 by the American Geophysical Union.