Interferences in photolytic NO<sub>2</sub> measurements: explanation for an apparent missing oxidant?

Abstract

Abstract. Measurement of NO2 at low concentrations is non-trivial. A variety of techniques exist, with the conversion of NO2 into NO followed by chemiluminescent detection of NO being prevalent. Historically this conversion has used a catalytic approach (Molybdenum); however this has been plagued with interferences. More recently, photolytic conversion based on UV-LED irradiation of a reaction cell has been used. Although this appears to be robust there have been a range of observations in low NOx environments which have measured higher NO2 concentrations than might be expected from steady state analysis of simultaneously measured NO, O3, JNO2 etc. A range of explanations exist in the literature most of which focus on an unknown and unmeasured "compound X" that is able to convert NO to NO2 selectively. Here we explore in the laboratory the interference on the photolytic NO2 measurements from the thermal decomposition of peroxyacetyl nitrate (PAN) within the photolysis cell. We find that approximately 5 % of the PAN decomposes within the instrument providing a potentially significant interference. We parameterize the decomposition in terms of the temperature of the light source, the ambient temperature and a mixing timescale (∼ 0.4 s for our instrument) and expand the parametric analysis to other atmospheric compounds that decompose readily to NO2 (HO2NO2, N2O5, CH3O2NO2, IONO2, BrONO2, Higher PANs). We apply these parameters to the output of a global atmospheric model (GEOS-Chem) to investigate the global impact of this interference on (1) the NO2 measurements and (2) the NO2 : NO ratio i.e. the Leighton relationship. We find that there are significant interferences in cold regions with low NOx concentrations such as Antarctic, the remote Southern Hemisphere and the upper troposphere. Although this interference is likely instrument specific, it appears that the thermal decomposition of NO2 within the instrument's photolysis cell may give an explanation for the anomalously high NO2 that has been reported in remote regions, and would reconcile measured and modelled NO2 to NO ratios without having to invoke novel chemistry. Better instrument characterization, coupled to instrumental designs which reduce the heating within the cell seem likely to minimize the interference in the future, thus simplifying interpretation of data from remote locations.

US COVID‐19 Shutdown Demonstrates Importance of Background NO2 in Inferring NOx Emissions From Satellite NO2 Observations

“…We use hourly surface measurements of NO 2 concentrations from the EPA Air Quality System (AQS) accessed through the application programming interface (API, aqs.epa.gov/aqsweb/documents/data_api.html). The measurements are made by a chemiluminescence analyzer with a molybdenum converter, which has been reported to have interferences from NO x oxidation products (Dunlea et al., 2007; Reed et al., 2016), but we assume here that these do not affect the relative long‐term trends. For the 2019–2020 analysis, we only include AQS sites with data for each month in both 2019 and 2020, for a total of 328 sites.…”

“…html). The measurements are made by a chemiluminescence analyzer with a molybdenum converter, which has been reported to have interferences from NO x oxidation products (Dunlea et al, 2007;Reed et al, 2016), but we assume here that these do not affect the relative long-term trends. For the 2019-2020 analysis, we only include AQS sites with data for each month in both 2019 and 2020, for a total of 328 sites.…”

Seasonal and geographical variability of nitryl chloride and its precursors in Northern Europe

“…NO 2 was measured at Leicester and in Penlee Point with a Teledyne Model T200 NO/NO 2 /NO x analyser, using chemiluminescence detection with a molybdenum converter (Hama et al, 2017). This type of instrument is known to be subject to interference from PAN and other NO y species (Reed et al, 2016): although this is unlikely to be an issue for Leicester, where fresh NO x emissions dominate, it may be significant for Penlee Point. Concentrations of PAN in clean oceanic air masses are typically lower than 300-400 ppt (Roberts et al, 1996;Heard et al, 2006); with a conversion factor of PAN on a molybdenum converter of 80-95% (Williams et al, 1998;Steinbacher et al, 2007), it can be estimated that the interference was of the order of 300 ppt.…”

“…Concentrations of PAN in clean oceanic air masses are typically lower than 300-400 ppt (Roberts et al, 1996;Heard et al, 2006); with a conversion factor of PAN on a molybdenum converter of 80-95% (Williams et al, 1998;Steinbacher et al, 2007), it can be estimated that the interference was of the order of 300 ppt. In Weybourne, NO 2 was measured with a dualchannel chemiluminescent instrument using a UV-LED photolytic converter (Air Quality Design Inc., Golden, CO, Reed et al, 2016).…”

Isotopic characterization of nitrogen oxides (NO<sub>x</sub>), nitrous acid (HONO), and nitrate (NO<sub>3</sub><sup>−</sup>(p)) from laboratory biomass burning during FIREX

“…Following the ADS, to avoid scrubbing of HONO, are a flow meter (Omega) and the 189 NO x collection system for analysis of δ 15 N-NO x (Fibiger et al, 2014;Fibiger and 190 Hastings, 2016;Wojtal et al, 2016 (Reed et al, 2016). In this study, only the HONO 236…”