Data obtained during the 1999 Atlanta Supersite Experiment are used to test the validity of the assumption of thermodynamic equilibrium between fine particulate (PM2.5) nitrate (NO3−) and ammonium (NH4+) and gas-phase nitric acid (HNO3(g)) and ammonia (NH3(g)). Equilibrium is tested by first calculating the equilibrium concentrations of HNO3(g) and NH3(g) implied by the PM2.5 inorganic composition (i.e, Na+, NH4+, Cl−, NO3−, and SO42−), temperature, and relative humidity observed at the site. These calculated equilibrium concentrations are then compared to the corresponding observed gas-phase concentrations. The observed PM2.5 composition is based on the 5-min averaged measurements of the Georgia Tech PILS [Weber et al., 2001], while the observed HNO3(g) and NH3(g) concentrations are based on the measurements of Edgerton et al. [2000a] and Slanina et al. , respectively. The equilibrium gas-phase concentrations are calculated using the ISORROPIA model of Nenes et al. . Out of the entire Atlanta Supersite database, we were able to identify 272 five-minute intervals with overlapping measurements of PM2.5 composition, HNO3(g) and NH3(g). Initial calculations using these 272 data points suggest an absence of thermodynamic equilibrium with the calculated equilibrium NH3(g) generally less than its observed concentration and predicted HNO3(g) generally greater than the observed concentration. However, relatively small downward adjustments in the measured PM2.5 SO42− (or apparent acidity) bring the calculated and measured NH3(g) and HNO3(g) into agreement. Moreover, with the exception of 31 of the 272 data points with either anomalously low observed concentrations of SO42− or NH3(g), there is a close correspondence between the SO42− (or acidity) correction needed for HNO3 and that needed for NH3 (slope of 1.04, intercept of ∼0, and r2 = 0.96). The average relative corrections required for equilibrium with HNO3 and NH3 are −14.1% and −13.7%, respectively; significantly larger than the estimated uncertainty arising from random errors in the measurement. One interpretation of our results is that thermodynamic equilibrium does in fact apply to the inorganic PM2.5 composition during the Atlanta Supersite Experiment and either (1) the PM2.5 SO42− concentration measured by the PILS was systematically overestimated by ∼15% or (2) the PM2.5 PILS systematically underestimated the concentration of the alkaline components by ∼15%; and/or 3. The ISORROPIA model systematically underestimated the pH of the PM2.5 encountered during the experiment.