Interferences in photolytic NO₂ 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.