Browsing by Author "Matchett, Laura C"

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    Potential interferences in photolytic nitrogen dioxide converters for ambient air monitoring: Evaluation of a prototype
    (2020-08-03) Jordan, Nick; Garner, Natasha M; Matchett, Laura C; Tokarek, Travis W; Osthoff, Hans D; Odame-Ankrah, Charles A; Grimm, Charles E; Pickrell, Kelly N; Swainson, Christopher; Rosentreter, Brian W
    Mixing ratios of the criteria air contaminant nitrogen dioxide (NO2) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O3 chemiluminescence (CL). In this work, the performance of a photolytic NO2 converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO2 and NOx (= NO + NO2) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO2 photolysis frequency was determined by measuring the rate of conversion to NO as a function of converter residence time and found to be 4.2 s-1. A maximum 96% conversion of NO2 to NO over a large dynamic range was achieved at a residence time of (1.5 ± 0.3) s, independent of relative humidity. Interferences from odd nitrogen (NOy) species such as peroxyacyl nitrates (PAN; RC(O)O2NO2), alkyl nitrates (AN; RONO2), nitrous acid (HONO), and nitric acid (HNO3) were evaluated by operating the prototype converter outside its optimum operating range (i.e., at higher pressure and longer residence time) for easier quantification of interferences. Four mechanisms that generate artifacts and interferences were identified as follows: direct photolysis, foremost of HONO at a rate constant of 6% that of NO2; thermal decomposition, primarily of PAN; surface promoted photochemistry; and secondary chemistry in the connecting tubing. These interferences are likely present to a certain degree in all photolytic converters currently in use but are rarely evaluated or reported. Recommendations for improved performance of photolytic converters include operating at lower cell pressure and higher flow rates, thermal management that ideally results in a match of photolysis cell temperature with ambient conditions, and minimization of connecting tubing length. When properly implemented, these interferences can be made negligibly small when measuring NO2 in ambient air.
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    Quantification of Non-refractory Aerosol Nitrate in Ambient Air by Thermal Dissociation Cavity Ring-Down Spectroscopy
    (2020-07-08) Garner, Natasha M; Matchett, Laura C; Osthoff, Hans D
    A thermal dissociation cavity ring-down spectrometer (TD-CRDS) for real-time quantification of non-refractory aerosol nitrate in ambient air is described. The instrument uses four parallel detection channels and heated quartz inlets to convert particulate organic nitrate (pON) (at 350 °C) and ammonium nitrate (NH4NO3) aerosol (at 540 °C) to nitrogen dioxide (NO2), whose mixing ratio is monitored via its absorption at 405 nm. Concentrations of aerosol nitrate are determined by difference relative to a parallel TD-CRDS channel in which aerosol is removed by in-line filtering. The method was validated by sampling gas streams containing laboratory-generated NH4NO3 aerosol in parallel to a scanning mobility particle sizer (SMPS). Scatter plots of TD-CRDS and SMPS data correlated (r2 > 0.9) with slopes near unity, confirming quantitative TD-CRDS response to NH4NO3 aerosol. In contrast, no response was observed when sampling (NH4)2SO4 aerosol. Instrument limits of detection (LOD; 2σ, 10 s) were 120 parts per trillion by volume (10-12, pptv) for NO2 and 148 pptv for ammonium nitrate. Partial and unsustained conversion of refractory sodium nitrate (NaNO3) was observed at the inlet temperature used for complete dissociation of HNO3 and NH4NO3, suggesting that this channel may not constitute a robust measurement of total odd nitrogen (NOy ) in environments where NaNO3 particles may be present (e.g., the polluted marine boundary layer). A potential application of the TD-CRDS is the calibration of particle counters, for which convenient methods are not currently available. Sample ambient air measurements of pON and total aerosol nitrate in Calgary are presented.