The California Acid Deposition Monitoring Program (CADMP) sampler was evaluated for nitric acid measurement accuracy under both laboratory and field conditions. The key issues of this denuder difference-based sampling method were nitric acid penetration through the perfluoroalkoxy (PFA) Teflon-Coated cyclone assembly and sampling plenum, and the efficiency of the denuder in removing nitric acid.
One aspect of the field evaluation involved sampling particle-free ambient air containing nitric acid, and monitoring concentrations before and after the major components. This was done for the Azusa sampler that had been used for over one year in the field after reconditioning (five years of total use). Nitric acid penetration was approximately 85%, while the denuder efficiency was over 90%. These values are similar with those determined by the developers of the sampler. Another aspect was to compare ambient nitric acid measurements with a tunable diode laser absorption spectrometer (TDLAS). For this comparison, the Azusa sampler (now with two years of use since reconditioning) was employed in a one-month field study in Claremont, California. The correlation of concentrations between these two methods was less than 0.36 and the CADMP was, on the average, four to eight times lower. These results indicated that the denuders had failed during the second year of sampling after reconditioning. A third aspect of the field evaluation involved operating two nylon filters in series in both Riverside and Claremont, California. In Riverside (unlike Claremont) a mean of 25% of the nitrate was found on the back filter, indicating either collection inefficiency for nitric acid under these conditions or adsorption of an interfering nitrogenous species such as nitrous acid. Laboratory testing showed that peroxyacetyl nitrate (PAN) was not such an interferent.
The laboratory study of nitric acid penetration was conducted by introducing synthetically generated nitric acid into the sampler and monitoring the concentrations at various points of the sampler using either a commercial NO-NOx analyzer or the nylon filters normally used for nitric acid collection. The NO-NOx analyzer allowed real-time concentration measurement. This feature led to the discovery of temporally changing sorptive properties of nitric acid to the PFA Teflon-coated aluminum surfaces as a function of temperature, relative humidity, and previous nitric acid sampling history. The penetration of nitric acid through the sampler was evaluated by allowing the nitric acid concentration to stabilize prior to filter sampling. The results, under these conditions, were the same as those for filtered ambient air.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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