Evaluation of the accuracy of thermal dissociation CRDS and LIF techniques for atmospheric measurement of reactive nitrogen species

Abstract. The sum of all reactive nitrogen species (NO_y) includes NO_x (NO₂ + NO) and all of its oxidized forms, and the accurate detection of NO_y is critical to understanding atmospheric nitrogen chemistry. Thermal dissociation (TD) inlets, which convert NO_y to NO₂ followed by NO₂ detection, are frequently used in conjunction with techniques such as laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) to measure total NO_y when set at > 600 °C or speciated NO_y when set at intermediate temperatures. We report the conversion efficiency of known amounts of several representative NO_y species to NO₂ in our TD-CRDS instrument, under a variety of experimental conditions. We find that the conversion efficiency of HNO₃ is highly sensitive to the flow rate and the residence time through the TD inlet as well as the presence of other species that may be present during ambient sampling, such as ozone (O₃). Conversion of HNO₃ at 400 °C, nominally the set point used to selectively convert organic nitrates, can range from 2 to 6 % and may represent an interference in measurement of organic nitrates under some conditions. The conversion efficiency is strongly dependent on the operating characteristics of individual quartz ovens and should be well calibrated prior to use in field sampling. We demonstrate quantitative conversion of both gas-phase N₂O₅ and particulate ammonium nitrate in the TD inlet at 650 °C, which is the temperature normally used for conversion of HNO₃. N₂O₅ has two thermal dissociation steps, one at low temperature representing dissociation to NO₂ and NO₃ and one at high temperature representing dissociation of NO₃, which produces exclusively NO₂ and not NO. We also find a significant interference from partial conversion (5–10 %) of NH₃ to NO at 650 °C in the presence of representative (50 ppbv) levels of O₃ in dry zero air. Although this interference appears to be suppressed when sampling ambient air, we nevertheless recommend regular characterization of this interference using standard additions of NH₃ to TD instruments that convert reactive nitrogen to NO or NO₂.