Articles | Volume 10, issue 5
Atmos. Meas. Tech., 10, 1911–1926, 2017
Atmos. Meas. Tech., 10, 1911–1926, 2017

Research article 31 May 2017

Research article | 31 May 2017

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

Caroline C. Womack1, J. Andrew Neuman1,2, Patrick R. Veres1,2, Scott J. Eilerman1,2, Charles A. Brock1, Zachary C. J. Decker3, Kyle J. Zarzana1,2, William P. Dube1,2, Robert J. Wild1,2, Paul J. Wooldridge4, Ronald C. Cohen4,5, and Steven S. Brown1,3 Caroline C. Womack et al.
  • 1Chemical Sciences Division, Earth Science Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, CO 80309, USA
  • 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, Boulder, CO 80309, USA
  • 4Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA
  • 5Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720, USA

Abstract. The sum of all reactive nitrogen species (NOy) includes NOx (NO2 + NO) and all of its oxidized forms, and the accurate detection of NOy is critical to understanding atmospheric nitrogen chemistry. Thermal dissociation (TD) inlets, which convert NOy 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 NOy when set at > 600 °C or speciated NOy when set at intermediate temperatures. We report the conversion efficiency of known amounts of several representative NOy 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.

Short summary
The accurate detection of reactive nitrogen species (NOy) is key to understanding tropospheric ozone production. Typically, NOy is detected by thermal conversion to NO2, followed by NO2 detection. Here, we assess the conversion efficiency of several NOy species to NO2 in a thermal dissociation cavity ring-down spectrometer and discuss how this conversion efficiency is affected by certain experimental conditions, such as oven residence time, and interferences from non-NOy species.