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

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.