Measurement of NOY during the SCOS97-NARSTO

Dual converter NOY analyzers configured to also measure nitric acid were evaluated under laboratory conditions, smog chamber simulations, and during ambient monitoring. Commercial molybdenum metal converters in the NOY configuration were found to be capable of converting all major NOY species to NO under laboratory testing conditions. Some converters required higher-temperature set points than received from the manufacturer and several converters did not achieve 95% efficiency at the highest temperature allowed by the manufacturer (380°C). They also exhibited varying degrees of efficiency for oxidizing ammonia to NO depending on previous usage, but not operating temperature. After the field measurement program, the efficiency for converting nitric acid was found to be variable during smog chamber measurements. This was ascribed to degradation of the converter under field measurement conditions. The ambient measurement program showed a significant memory effect that contributed to a positive NOY measurement bias. The magnitude of this memory effect appeared to be related to the degree of ambient concentrations of pollutants being sampled. Ambient nitric acid measurements determined from the dual converter approach were related to those from a tunable diode laser absorption spectrometer although the correlation was not high. The variable converter efficiency for ammonia may lead to negative biases when measuring nitric acid.