Gas phase ammonia (NH3) measurements were made in July and August 2002 during the Atlant Aerosol Nucleation and Real-Time Characterization Experiment with two different chemical ionization mass spectrometry techniques. Correlations between the 1 min data from both instruments yielded a slope of 1.17 and an intercept of -0.295 ppbv, with a linear correlation coefficient (r2) of 0.71. Ambient NH
3 mixing ratios ranged from 0.4 to 13 ppbv. NH 3 observations were compared to the Community Multiscale Air Quality (CMAQ) modeling system as well as a thermodynamic equilibrium model, ISORROPIA, used by CMAQ to predict NH 3 partitioning. A morning rise in both observed and modeled NH 3 mixing ratios strongly suggests a regional influence due to automobile emissions. However, at midday the predicted NH 3 decreased to less than 0.5 ppbv, while the observations remained around 3 ppbv. Both observed and modeled ammonium nitrate levels were too low to support the observed midday NH 3 mixing ratios. ISORROPIA calculations of NH 3 constrained by the total measured ammonia mass (NH 3 + ammonium (NH 4+) agreed well with the observations (slope of 1.25 and r2 of 0.75). For times when the net aerosol charge was near zero the agreement was excellent (slope of 1.22 and r2 of 0.88). These results indicate that for most of the observed conditions, ISORROPIA could accurately predict NH 3 partitioning. The observations suggest that local sunlight- or temperature-driven NH 3 sources, such as soil emissions, may be responsible for the discrepancy between the model results and measured values. Copyright 2006 by the American Geophysical Union.