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

Abstract

Abstract. Measurement of NO 2 at low concentrations (tens of ppts) is non-trivial. A variety of techniques exist, with the conversion of NO 2 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-NO x environments which have measured higher NO 2 concentrations than might be expected from steady-state analysis of simultaneously measured NO, O 3 , j NO 2 , 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 NO 2 selectively. Here we explore in the laboratory the interference on the photolytic NO 2 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 NO 2 (HO 2 NO 2 , N 2 O 5 , CH 3 O 2 NO 2 , IONO 2 , BrONO 2 , 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 NO 2 measurements and (2) the NO 2 : NO ratio, i.e. the Leighton relationship. We find that there are significant interferences in cold regions with low NO x concentrations such as the Antarctic, the remote Southern Hemisphere, and the upper troposphere. Although this interference is likely instrument-specific, the thermal decomposition to NO 2 within the instrument's photolysis cell could give an at least partial explanation for the anomalously high NO 2 that has been reported in remote regions. The interference can be minimized by better instrument characterization, coupled to instrumental designs which reduce the heating within the cell, thus simplifying interpretation of data from remote locations.

The IAGOS NO<sub><i>x</i></sub> instrument – design, operation and first results from deployment aboard passenger aircraft

“…As a result, thermal decomposition of reservoir species containing NO 2 can lead to erroneously enhanced NO 2 measurements. Reed et al (2016) showed that the PAN interference could be up to 8 and 25 % when using an actively cooled and a not actively cooled photolytic converter, respectively. In the laboratory, we found NO 2 enhancements of 30 % by mixing PAN with the sample flow (at 35 • C instrument temperature and pressure level of 250 hPa), which was quantitatively generated from a NO calibration gas by photolysis of acetone (100 ppbv) in a flow system (Pätz et al, 2002;Volz-Thomas et al, 2002).…”

“…However, previous studies identify an unexplained imbalance between the measured and calculated NO 2 in low-NO x regions, which was explained by interferences of NO 2 -containing species and the large uncertainty of the calculations (e.g., Crawford et al, 1996;Reed et al, 2016). Thus, the impact of interference from NO 2 -containing species on the IAGOS measurements requires further investigations, which will be performed once a larger data set is available.…”

“…CLD instruments have often been coupled to a photolytic or catalytic converter to measure NO 2 and NO x by using a xenon lamp, blue-light converter or catalytic conversion of NO 2 into NO, prior to the CLD unit Ryerson et al, 2000;Nakamura et al, 2003;Pollack et al, 2010;Villena et al, 2012;Reed et al, 2016). NO 2 measurements at low-NO x conditions (below 0.1 ppbv) are close to the limit of detection (Yang et al, 2004), and depending on the installed converter each instrument might show interferences with other nitrogen-oxide-containing species (e.g., Reed et al, 2016).…”

The IAGOS NO<sub>x</sub> Instrument – Design, Operation and First Results from Deployment aboard Passenger Aircraft

“…Therefore the knowledge of the spatial distribution of NO x is important to identify the sources, sinks and its partitioning between NO and NO 2 in the atmosphere (Monks et al, 2009). 10 Most relevant natural sources of NO x are lightning (LNO x ), biomass burning, soil emissions, and anthropogenic sources, such as power generation, road transportation and aviation. The current knowledge of the global distribution of NO x and its emission estimates is based mostly on specific aircraft missions (Emmons et al, 1997;Rohrer et al, 1997;Schumann and Huntrieser, 2007;Ziereis et al, 2000;Gressent et al, 2016), on surface 15 monitoring stations (Aerosols, Clouds and Trace gases Research Infrastructure (ACTRIS); www.actris.eu), satellite measurements (Fishman et al, 2008;de Laat et al, 2014;Duncan et al, 2015) and model simulations (Ehhalt et al, 1992;Emmons et al, 1997).…”

Laser induced fluorescence based detection of atmospheric nitrogen dioxide and comparison of different techniques during the PARADE 2011 field campaign

“…However, in the case of NO 2 to NO conversion, a potential interference from NO z species cannot be fully excluded for the NO 2 measurement, e.g. (Villena et al, 2012;Reed et al, 2016). Therefore, a direct detection of NO 2 is advantageous.…”