Potential interferences in photolytic nitrogen dioxide converters for ambient air monitoring: Evaluation of a prototype

• Published in: Journal of the Air & Waste Management Association (1995) 2020-08, Vol.70 (8), p.753-764
• Main Authors: 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.
• Format: Article
• Language: English
• Subjects: Acids; Air monitoring; Air Pollutants - analysis; Air Pollutants - chemistry; Air pollution; Atmospheric chemistry; Cavity ringdown; Chemiluminescence; Contaminants; Conversion; Converters; Decomposition; Emission analysis; Environmental Monitoring - instrumentation; Flow velocity; High flow; Mixing ratio; Nitrates; Nitric acid; Nitric oxide; Nitrogen dioxide; Nitrogen Dioxide - analysis; Nitrogen Dioxide - chemistry; Nitrogen oxides; Nitrous acid; Optimization; Photochemistry; Photolysis; Pollutants; Pollution monitoring; Prototypes; Relative humidity; Residence time distribution; Semiconductor lasers; Spectroscopy; Thermal decomposition; Thermal management
• ISSN: 1096-2247; 2162-2906
• DOI: 10.1080/10962247.2020.1769770

Mixing ratios of the criteria air contaminant nitrogen dioxide (NO 2 ) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O 3 chemiluminescence (CL). In this work, the performance of a photolytic NO 2 converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO 2 and NO x (= NO + NO 2 ) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO 2 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 NO 2 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 (NO y ) species such as peroxyacyl nitrates (PAN; RC(O)O 2 NO 2 ), alkyl nitrates (AN; RONO 2 ), nitrous acid (HONO), and nitric acid (HNO 3 ) 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 NO 2 ; 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 NO 2 in ambient air. Implications: A new near-UV photolytic converter for measurement of the criteria pollutant nitrogen dioxide (NO 2 ) in ambient air by CL was characterized. Four mechanisms that generate interferences were identified and investigated experimentally: direct photolysis of HONO which occurred at a rate constant 6% that of NO 2 , thermal decomposition of PAN and N 2 O 5 , surface promoted chemistry involving HNO 3 , and secondary chemistry involving NO in the tubing connecting the con