Interference from HONO in the measurement of ambient air NO2 via photolytic conversion and quantification of NO.

The reference method to quantify mixing ratios of the criteria air pollutant nitrogen dioxide (NO2) is NO-O3 chemiluminescence (CL), in which mixing ratios of nitric oxide (NO) are measured by sampling ambient air directly, and mixing ratios of NOx (= sum of NO and NO2) are measured by converting NO2 to NO using, for example, heated molybdenum catalyst or, more selectively, photolytic conversion (P-CL). In this work, the nitrous acid (HONO) interference in the measurement of NO2 by P-CL was investigated. Results with two photolytic NO2 converters are presented. The first used radiation centered at 395 nm, a wavelength region commonly utilized in P-CL. The second used light at 415 nm, where the overlap with the HONO absorption spectrum and hence its photolysis rate are less. Mixing ratios of NO2, NOx and HONO entering and exiting the converters were quantified by Thermal Dissociation Cavity Ring-down Spectroscopy (TD-CRDS). Both converters exhibited high NO2 conversion efficiency (CFNO2; > 90%) and partial conversion of HONO. Plots of CF against flow rate were consistent with photolysis frequencies of 4.2 s-1 and 2.9 s-1 for NO2 and 0.25 s-1 and 0.10 s-1 for HONO at 395 nm and 415 nm, respectively. CFHONO was larger than predicted from the overlap of the emission and HONO absorption spectra. The results imply that measurements of NO2 by P-CL marginally but systematically overestimate true NO2 concentrations, and that this interference should be considered in environments with high HONO:NO2 ratios such as the marine boundary layer or in biomass burning plumes.

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

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. IMPLICATIONS: A new near-UV photolytic converter for measurement of the criteria pollutant nitrogen dioxide (NO2) in ambient air by NO-O3 chemiluminescence (CL) was characterized. Four mechanisms that generate interferences were identified and investigated experimentally: direct photolysis of nitrous acid, which occurred at a rate constant 6% that of NO2, thermal decomposition of PAN and N2O5, surface promoted chemistry involving nitric acid, and secondary chemistry involving NO in the tubing connecting the converter and CL analyzer. These interferences are predicted to occur in all NO2 P-CL systems but can be avoided by appropriate thermal management and operating at high flow rates.

A compact diode laser cavity ring-down spectrometer for atmospheric measurements of NO3 and N2O5 with automated zeroing and calibration.

A compact rack-mounted cavity ring-down spectrometer (CRDS) for simultaneous measurements of the nocturnal nitrogen oxides NO(3) and N(2)O(5) in ambient air is described. The instrument uses a red diode laser to quantify mixing ratios of NO(3) (at its absorption maximum at 662 nm) and of N(2)O(5) following its thermal dissociation to NO(3) in a second detection channel. The spectrometer is equipped with an automated zeroing and calibration setup to determine effective NO(3) absorption cross-sections and NO(3) and N(2)O(5) inlet transmission efficiencies. The instrument response was calibrated using simultaneous measurements of NO(2), generated by thermal dissociation of N(2)O(5) and/or by titration of NO(3) with excess NO, using blue diode laser CRDS at 405 nm. When measuring ambient air, the (2σ, 10 s) precision of the red diode CRDS varied between 5 and 8 parts-per-trillion by volume (pptv), which sufficed to quantify N(2)O(5) concentrations under moderately polluted conditions. Sample N(2)O(5) measurements made on a rooftop on the University of Calgary campus in August 2010 are presented. A maximum N(2)O(5) mixing ratio of 130 pptv was observed, corresponding to a steady-state lifetime of less than 50 min. The NO(3) mixing ratios were below the detection limit, consistent with their predicted values based on equilibrium calculations. During the measurement period, the instrument response for N(2)O(5) was 70% of the theoretical maximum, rationalized by a slight mismatch of the laser diode output with the NO(3) absorption line and a N(2)O(5) inlet transmission efficiency less than unity. Advantages and limitations of the instrument's compact design are discussed.