Evaluation of the accuracy of
atmospheric measurement of
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. Brown1,31Chemical 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, CO 80309 USA 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 USA 4Department of Chemistry, University of California, Berkeley, CA 94720 USA 5Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 USA
Received: 06 Dec 2016 – Accepted for review: 12 Dec 2016 – Discussion started: 20 Dec 2016
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 ringdown 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 2–6 % 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, 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.
Womack, C. C., Neuman, J. A., Veres, P. R., Eilerman, S. J., Brock, C. A., Decker, Z. C. J., Zarzana, K. J., Dube, W. P., Wild, R. J., Wooldridge, P. J., Cohen, R. C., and Brown, S. S.: Evaluation of the accuracy of
atmospheric measurement of
nitrogen species, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2016-398, in review, 2016.