Measurement of ambient NO₃ reactivity: design, characterization and first deployment of a new instrument

Abstract. We describe the first instrument for measurement of the rate constant (s⁻¹) for reactive loss (i.e., the total reactivity) of NO₃ in ambient air. Cavity-ring-down spectroscopy is used to monitor the mixing ratio of synthetically generated NO₃ ( ≈  30–50 pptv) after passing through a flow-tube reactor with variable residence time (generally 10.5 s). The change in concentration of NO₃ upon modulation of the bath gas between zero air and ambient air is used to derive its loss rate constant, which is then corrected for formation and decomposition of N₂O₅ via numerical simulation. The instrument is calibrated and characterized using known amounts of NO and NO₂ and tested in the laboratory with an isoprene standard. The lowest reactivity that can be detected (defined by the stability of the NO₃ source, instrumental parameters and NO₂ mixing ratios) is 0.005 s⁻¹. An automated dilution procedure enables measurement of NO₃ reactivities up to 45 s⁻¹, this upper limit being defined mainly by the dilution accuracy. The typical total uncertainty associated with the reactivity measurement at the center of its dynamic range is 16 %, though this is dependent on ambient NO₂ levels. Results from the first successful deployment of the instrument at a forested mountain site with urban influence are shown and future developments outlined.