Development of a measurement system for nitrate radical and dinitrogen pentoxide using a thermal conversion/laser-induced fluorescence technique

An instrument for measuring atmospheric nitrate radical (N O3) and dinitrogen pentoxide (N2 O5) has been developed by a thermal conversion/laser-induced fluorescence (TC/LIF) technique. N2 O5 is thermally decomposed and converted to N O3, which is measured by laser-induced fluorescence. In situ, fast-response, sensitive measurement of N O3 N2 O5 is expected by use of LIF. In detecting N O3, dual-wavelength excitation at 622.96 and 618.81 nm was adopted to remove potential interference and to guarantee high selectivity. A high-power dye laser system was used as the source of excitation light. To measure ambient air directly, the N O3 detection cell was placed on the rooftop. The laser beam was guided by an optical fiber into the excitation cell. Transmittance of the laser beam was 80% for a 10 m long fiber. To calibrate the instrument, the series of thermal decomposition of N2 O5 and the gas phase titration of N O3 by NO were conducted. N O3 reduction by adding NO was also applied to determine accurately the zero points of the detector. After optimization of conditions such as gate timing in photon counting and the settings of the N2 O5 converter, the present detection limits of N O3 and N2 O5 were determined to be 4 and 6 pptv, respectively, for the integration time of 10 min (signal-to-noise ratio=1). It was confirmed that the interference of N O2 on N2 O5 detection is negligible, but can be significant for N O3 measurement when N O2 concentration is extremely high (>100 ppbv). Measurement of N2 O5 in ambient air was made in the urban area of Tokyo, Japan. Observed data demonstrated the capacity of the TC/LIF instrument with a powerful dye laser and a single-path excitation cell for ambient measurements. In this article, we focus on the instrumentation and characterization.