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

The reference method to quantify mixing ratios of the criteria air pollutant nitrogen dioxide (NO) is NO-O chemiluminescence (CL), in which mixing ratios of nitric oxide (NO) are measured by sampling ambient air directly, and mixing ratios of NO (= sum of NO and NO) are measured by converting NO 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 NO by P-CL was investigated. Results with two photolytic NO 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 NO, NO and HONO entering and exiting the converters were quantified by Thermal Dissociation Cavity Ring-down Spectroscopy (TD-CRDS). Both converters exhibited high NO conversion efficiency (CF; > 90%) and partial conversion of HONO. Plots of CF against flow rate were consistent with photolysis frequencies of 4.2 s and 2.9 s for NO and 0.25 s and 0.10 s for HONO at 395 nm and 415 nm, respectively. CF was larger than predicted from the overlap of the emission and HONO absorption spectra. The results imply that measurements of NO by P-CL marginally but systematically overestimate true NO concentrations, and that this interference should be considered in environments with high HONO:NO ratios such as the marine boundary layer or in biomass burning plumes.