Performance of the Annular Denuder System with Different Arrangements for HNO3 and HNO2 Measurements in Taiwan

Abstract

ABSTRACT Experiments on different annular denuder system (ADS)arrangements for sampling nitrous acid (HNO2) and ni-tric acid (HNO3) gases were conducted in this study toevaluate their sampling artifacts. The evaluation basis isthe one that employed one sodium chloride denuder forsampling HNO3 gas and two sodium carbonate (Na2CO3)denuders for sampling HNO2 gas, which is a commonlyemployed ADS arrangement in many field applicationsin the United States. A field study was conducted inHsinchu, Taiwan, and the results indicated that this ADSarrangement may yield over 80% relative errors for HNO3gas. It also showed that the relative errors for HNO2 gascan be less than 10% as sampled with only one Na2CO3denuder. This is attributed to the fact that the ambientHNO3 concentration measured in this study was relativelylow while the HNO2 concentration was high, as comparedto typical concentrations of these two gases measured inthe United States. The sampling error of HNO3 gas may be due to highconcentrations of N-containing interfering speciespresent in Taiwan’s atmosphere. Because the relative sam-pling errors of HNO3 and HNO2 gases depend mainly ontheir concentrations in the atmosphere as well as con-centrations caused by interfering species, the risk for higherror while measuring low HNO2 concentrations by onlyone Na2CO3 denuder is also possible. As a result, it is sug-gested that pretests are necessary to evaluate possiblesources and degrees of sampling errors before fieldsampling of HNO2 and HNO3 gases. The sampling errorsof these two gases can, therefore, be minimized with abetter arrangement of the ADS.