Measuring Concentrations of Nitrous Acid Gas by the Filter-Pack Sampling Method.

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The filter-pack method is generally used for the measurement of soluble ion components in aerosol, sulfur dioxide, nitric acid gas, hydrocloric acid gas, and ammonia gas in the air. In this study, the simultaneous measurement of nitrous acid gas using double alkaline impregnated filters of the filter-pack method is being considered for the first time. The filter pack contains five filters, i. e., a Teflon filter, a nylon filter, a firstalkalineimpregnated filter, a second alkaline-impregnated filter, and an acid-impregnated filter. In a laboratory experiment, it was found that artifact formation of nitrate and nitrite ions, representing below 1% of the concentration of nitrogen dioxides, occurs in the alkaline-impregnated filter. Thus, the interference on nitrogen dioxides can be estimated from the NO<SUB>2</SUB><SUP>-</SUP> and NO<SUB>3</SUB><SUP>-</SUP> content of the second alkaline-impregnated filter. This principle is similar to that of an annular denuder method. In addition, a parallel survey using the filter-pack method and the annular denuder method, which is generally used for the measurement of nitrous acid gas, was carried out in the field for three and half years beginning in October 2003. This was done in order to measure the weekly concentrations of nitrous acid gas in Sapporo. The results showed that nitrous acid concentrations obtained using the two methods were in good agreement because both concentrations were highly correlated (r=0.95); moreover the slope of the correlation curve was close to 1.0, and the intercept was shown to be small: i. e., y=1.09 x+2.28. Thus, it was concluded that the filter-pack method is useful for the monitoring of air pollutants including nitrous acid gas like an annular denuder method.
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