A nitric acid diffusion denuder made of nylon was operated in Riverside, CA, Houston, TX, and Claremont, CA. The pre-exponential and diffusion coefficients for the first term of the Gormley-Kennedy equation were estimated by regressing the log (mass deposited) against the axial distance from the entrance of the denuder. The field study average values of the two coefficients were compared by the two-tail t-test to the average values for the HNO3-Air system determined in the laboratory; the test revealed that the means of the diffusion coefficients have probabilities less than or equal to 0.03 of being equal. The nitrate ion mass patterns in the denuder for the field cases do not adhere to the Gormley-Kennedy model applied to the simple HNO3-Air system. The source of excess nitrate deposited in the nylon tube could not be identified by the analysis. Candidate sources are deposition of gaseous N-compounds (analyzed as nitrate) other than nitric acid and evaporation of nitric acid from particles during transit in the denuder tube.