NITRIC-ACID NITRATE AEROSOL MEASUREMENTS BY A DIFFUSION DENUDER - A PERFORMANCE EVALUATION

A nitric acid diffusion denuder made of nylon was operated in Riverside, CA, Houston, TX and Claremont, CA [USA]. The pre-exponential and diffusion coefficients for the first term of the Gormley-Kennedy equation were estimated by regressing the log (nitrate 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 hypothesis that the means of the diffusion coefficients are equal may be rejected at the <5% level of significance. 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 this analysis. Candidate sources are deposition of gaseous N-compounds (analyzed as nitrate) other than nitric acid and release of nitric acid vapor from particles during transit in the denuder tube.