Abstract
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.
Highlights
The catalytic cycling of nitrogen oxides (NOx = NO + NO2) plays a key role in the formation of tropospheric ozone (O3) from the photooxidation of volatile organic compounds (VOCs)
At a flow rate of 1.9 slpm, we observe 100 % conversion of HNO3 at oven temperatures above 600 ◦C, whereas the thermograms obtained at 1 and 3 slpm reach a maximum conversion of 100 % at 550 and 650 ◦C, respectively
Using a thermal dissociation cavity ring-down spectrometer, we have quantitatively added reactive nitrogen species to the Thermal dissociation (TD) inlet in order to test the efficiency of the thermal conversion of each species to NO2 and the effect of any interferences from other trace gases which may be present in the ambient troposphere
Summary
Womack et al.: Evaluation of the accuracy of thermal dissociation CRDS and LIF techniques chain termination that determines the efficiency of the O3 production cycle and can transport NOx far from the original emission source. For this reason, total reactive nitrogen (NOy = NO + NO2+ RONO2+ RO2NO2+ HNO3+ HONO + NO3 + 2× N2O5+ aerosol nitrates) is an important tracer in monitoring tropospheric O3 production. Measured total reactive nitrogen has in some cases deviated significantly from the sum of the measured individual components, NOy,i (see Fahey et al, 1986; Bradshaw et al, 1998; Neuman et al, 2012; and others referenced within). This unmeasured NOy, sometimes referred to as “missing NOy”, indicates the need for a more complete understanding of total and speciated reactive nitrogen and for accurate analytical instrumentation for NOy measurement (Crosley, 1996; Williams et al, 1998; Day et al, 2003)
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