HNO3 analyzer by scrubber difference and the NO–ozone chemiluminescence method

A fast response analyzer for HNO3 in highly polluted air is described. The time resolution attainable was 12 s. The method is based on the difference in a technique for HNO3-scrubbed and non-scrubbed air and the reduction of HNO3 to NO with the use of a line of catalytic converters and a method for the subsequent NO-ozone chemiluminescence. A sample air stream, in which particulates are removed with a Teflon filter, is divided into two channels. CH-1 is directly connected to the converter line, and CH-2 contains a HNO3 scrubber packed with a nylon fiber that goes to another converter line. Each converter line is composed of a hot quartz-bead converter (QBC) and a molybdenum converter (MC) in a series. A QBC reduces HNO3 to (NO+NO2), which is called NOx. The MC reduces the NOx to NO. For CH-1, the analyzer detects most compounds that typically comprise NOy (J. Geophys. Res. 91 (1986) 9781). These CH-1 compounds are called NOy′ hereafter (NOy-particulate nitrate) because the particulates are removed by the filter. A difference in the detector signal for the two channels indicates HNO3. For a blank test, atmospheric air in which HNO3 was pre-scrubbed by an extra nylon fiber was introduced to the analyzer. Variations in the blank value were 0.38±0.42 and 0.34±0.55 ppb during the high readings (NOy′-HNO3 ) (called NOy* hereafter) (111±12 ppb, N=180), and low NOy* readings (62±8 ppb, N=180), respectively, indicating that the lowest detection limit of the analyzer is 1.1 ppb (2σ). When the data obtained with the analyzer is compared to the data using the denuder method, a linear correlation with the regression of Y=0.973X+0.077 (r2=0.916 (N=20)) in the range of 0–6.5 ppb HNO3 is obtained, which is an excellent agreement. Atmospheric monitoring was carried out at Kobe. Although the average concentration of HNO3 was 2.6±1.3 ppb, ca.10 ppb for a HNO3 concentration was occasionally observed when the NOy* concentration was high, i.e., more than 100 ppb.